llvm dev - Sep 2020 - [MTE] Globals Tagging - Discussion

Hi David,

Does the tagging of these hidden symbols only protect against
RW
> primitives without a similar ldg? If I knew the address of the hidden
> symbol I could presumably use the same sequence, but I think I'm
> stretching what memory tagging is supposed to protect against.

I might be missing your point here - but don't forget that the local
globals are always PC-relative direct loads/stores. The `ldg` sequence in
that example can only be used to get `&g` (and nothing else). There
shouldn't be any `ldg`'s of arbitrary addresses (unless an attacker
already
has control of the instruction pointer, which means they've already
bypassed MTE).

Does this mean that the value of array_end must have the same tag
as
> array[]. Then &array_end would have a different tag since it's a
> different global?
>
Yes, exactly.

For example you might assign tag 1 to array, then tag 2 to
array_end.
> Which means that array_end has a tag of 2 and so does array[16].
> (assuming they're sequential)
> |            array            | array_end/array[16] |
> | < 1> <1> <1> <1>  |            <2>         
|
>

So if we just did a RELATIVE relocation then array_end's value
would
> have a tag of 2, so you couldn't do:
> for (int* ptr=array; ptr != array_end; ++ptr)
> Since it's always != due to the tags.
> Do I have that right?

 Yep - you've got it right, this is why we need TAGGED_RELATIVE. For
clarity, here's the memory layout where array_end is relocated using
TAGGED_RELATIVE{*r_offset = &array[16], r_addend = &array[0]}:
array array_end (padding)
Memory Tag 0x1 0x1 0x2 0x2
Value 0 0 0 0 (0x1 << 56) | &array[16] 0 0
So the address tag of `array` and `array_end` are the same (only
`&array_end` has an memory/address tag of 0x2), and thus `for (int*
ptr=array; ptr != array_end; ++ptr)` works normally.

Also, if you have this same example but the array got rounded up
to
> the nearest granule e.g. (4 byte ints, 16 byte granules)
> int array[3]; // rounded up to be array[4]
> int* array_end = array[3];
> Would you emit a normal RELATIVE relocation for array_end, because
> it's within the bounds of the rounded up array. Or a TAGGED_RELATIVE
> relocation because it's out of bounds of the original size of the
> array?
> (I don't think doing the former is a problem but I'm not a linker
expert)

At this stage, this would generate a TAGGED_RELATIVE. We expect
TAGGED_RELATIVE to be relatively scarce, and coming up with a more complex
scheme for the linker to optimise this edge case where it's in bounds of
the granule padding (but not the symbol itself) seems over-the-top. In
saying that, it's a possibility for later revisions.

On Fri, Sep 18, 2020 at 4:10 AM David Spickett <david.spickett at
linaro.org>
wrote:
> Hi Mitch,
>
> In the intro you say:
> > It would also allow attackers with a semilinear RW primitive to
> trivially attack global variables if the offset is controllable. Dynamic
> global tags are required to provide the same MTE mitigation guarantees that
> are afforded to stack and heap memory.
>
> Then later:
> > b) Hidden Symbols (static int g; or -fvisibility=hidden)
> > Materialization of hidden symbols now fetch and insert the memory tag
> via. `ldg`. On aarch64, this means non PC-relative
> loads/stores/address-taken (*g = 7;) generates:
> >  adrp x0, g;
> >  ldg x0, [x0, :lo12:g]; // new instruction
> >  mov x1, #7;
> >  str x1, [x0, :lo12:g];
>
> Does the tagging of these hidden symbols only protect against RW
> primitives without a similar ldg? If I knew the address of the hidden
> symbol I could presumably use the same sequence, but I think I'm
> stretching what memory tagging is supposed to protect against. Mostly
> wanted to check I understood.
>
> Speaking of understanding...
>
> > Introduce a TAGGED_RELATIVE relocation - in order to solve the problem
> where the tag derivation shouldn't be from the relocation result, e.g.
> > static int array[16] = {};
> > // array_end must have the same tag as array[]. array_end is out of
> > // bounds w.r.t. array, and may point to a completely different
global.
> > int *array_end = &array[16];
>
> Does this mean that the value of array_end must have the same tag as
> array[]. Then &array_end would have a different tag since it's a
> different global?
>
> For example you might assign tag 1 to array, then tag 2 to array_end.
> Which means that array_end has a tag of 2 and so does array[16].
> (assuming they're sequential)
> |            array            | array_end/array[16] |
> | < 1> <1> <1> <1>  |            <2>         
|
>
> So if we just did a RELATIVE relocation then array_end's value would
> have a tag of 2, so you couldn't do:
> for (int* ptr=array; ptr != array_end; ++ptr)
> Since it's always != due to the tags.
>
> Do I have that right?
>
> Also, if you have this same example but the array got rounded up to
> the nearest granule e.g. (4 byte ints, 16 byte granules)
> int array[3]; // rounded up to be array[4]
> int* array_end = array[3];
>
> Would you emit a normal RELATIVE relocation for array_end, because
> it's within the bounds of the rounded up array. Or a TAGGED_RELATIVE
> relocation because it's out of bounds of the original size of the
> array?
> (I don't think doing the former is a problem but I'm not a linker
expert)
>
> Thanks,
> David Spickett.
>
> On Thu, 17 Sep 2020 at 23:05, Mitch Phillips via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi folks,
> >
> >
> > ARM v8.5 introduces the Memory Tagging Extension (MTE), a hardware
that
> allows for detection of memory safety bugs (buffer overflows,
> use-after-free, etc) with low overhead. So far, MTE support is implemented
> in the Scudo hardened allocator (compiler-rt/lib/scudo/standalone) for
> heap, and stack allocation is implemented in LLVM/Clang behind
> -fsanitize=memtag.
> >
> >
> > As part of a holistic MTE implementation, global memory should also be
> properly tagged. HWASan (a software-only implementation of MTE) has a
> schema that uses static tags, however these can be trivially determined by
> an attacker with access to the ELF file. This would allow attackers with
> arbitrary read/write to trivially attack global variables. It would also
> allow attackers with a semilinear RW primitive to trivially attack global
> variables if the offset is controllable. Dynamic global tags are required
> to provide the same MTE mitigation guarantees that are afforded to stack
> and heap memory.
> >
> >
> > We've got a plan in mind about how to do MTE globals with fully
dynamic
> tags, but we'd love to get feedback from the community. In particular -
> we'd like to try and align implementation details with GCC as the
scheme
> requires cooperation from the compiler, linker, and loader.
> >
> >
> > Our current ideas are outlined below. All the compiler features
> (including realignment, etc.) would be guarded behind -fsanitize=memtag.
> Protection of read-only globals would be enabled-by-default, but can be
> disabled at compile time behind a flag (likely
> -f(no)sanitize-memtag-ro-globals).
> >
> >
> > a) Dynamic symbols (int f; extern int f;)
> >
> > Mark all tagged global data symbols in the dynamic symbol table as
> st_other.STO_TAGGED.
> >
> > Teach the loader to read the symbol table at load time (and dlopen())
> prior to relocations, and apply random memory tags (via. `irg -> stg`)
to
> each STO_TAGGED carrying global.
> >
> > b) Hidden Symbols (static int g; or -fvisibility=hidden)
> >
> > Have the compiler mark hidden tagged globals in the symbol table as
> st_other.STO_TAGGED.
> >
> > Have the linker read the symbol table and create a table of {
> unrelocated virtual address, size } pairs for each STO_TAGGED carrying
> hidden global, storing this in a new section (.mteglobtab).
> >
> > Create a new dynamic entry "DT_MTEGLOBTAB" that points to
this segment,
> along with "DT_MTEGLOBENT" for the size of each entry and
"DT_MTEGLOBSZ"
> for the size (in bytes) of the table.
> >
> > Similar to dynamic symbols, teach the loader to read this table and
> apply random memory tags to each global prior to relocations.
> >
> > Materialization of hidden symbols now fetch and insert the memory tag
> via. `ldg`. On aarch64, this means non PC-relative
> loads/stores/address-taken (*g = 7;) generates:
> >   adrp x0, g;
> >   ldg x0, [x0, :lo12:g]; // new instruction
> >   mov x1, #7;
> >   str x1, [x0, :lo12:g];
> >
> > Note that this materialization sequence means that executables built
> with MTE globals are not able to run on non-MTE hardware.
> >
> > Note: Some dynamic symbols can be transformed at link time into hidden
> symbols if:
> >
> > The symbol is in an object file that is statically linked into an
> executable and is not referenced in any shared libraries, or
> >
> > The symbol has its visibility changed with a version script.
> >
> > These globals always have their addresses derived from a GOT entry,
and
> thus have their address tag materialized through the RELATIVE relocation of
> the GOT entry. Due to the lack of dynamic symbol table entry however, the
> memory would go untagged. The linker must ensure it creates an MTEGLOBTAB
> entry for all hidden MTE-globals, including those that are transformed from
> external to hidden. DSO's linked with -Bsymbolic retain their dynamic
> symbol table entries, and thus require no special handling.
> >
> >
> > c) All symbols
> >
> > Realign to granule size (16 bytes), resize to multiple of granule size
> (e.g. 40B -> 48B).
> >
> > Ban data folding (except where contents and size are same, no tail
> merging).
> >
> > In the loader, ensure writable segments (and possibly .rodata, see
next
> dot point) are mapped MAP_ANONYMOUS and PROT_MTE (with the contents of the
> mappings filled from the file), as file-based mappings aren't
necessarily
> backed by tag-capable memory. It also requires in-place remapping of data
> segments from the program image (as they're already mapped by the
kernel
> before PT_INTERP invokes the loader).
> >
> > Make .rodata protection optional. When read-only protection is in use,
> the .rodata section should be moved into a separate segment. For Bionic
> libc, the rodata section takes up 20% of its ALLOC | READ segment, and
we'd
> like to be able to maintain page sharing for the remaining 189KiB of other
> read-only data in this segment.
> >
> > d) Relocations
> >
> > GLOB_DAT, ABS64, and RELATIVE relocations change semantics - they
would
> be required to retrieve and insert the memory tag of the symbol into the
> relocated value. For example, the ABS64 relocation becomes:
> >   sym_addr = get_symbol_address() // sym_addr = 0x1008
> >   sym_addr |= get_tag(sym_addr & 0xf) // get_tag(0x1008 & 0xf
== 0x1000)
> >   *r_offset = sym_addr + r_addend;
> >
> > Introduce a TAGGED_RELATIVE relocation - in order to solve the problem
> where the tag derivation shouldn't be from the relocation result, e.g.
> > static int array[16] = {};
> > // array_end must have the same tag as array[]. array_end is out of
> > // bounds w.r.t. array, and may point to a completely different
global.
> > int *array_end = &array[16];
> >
> > TAGGED_RELATIVE stores the untagged symbol value in the place
(*r_offset
> == &array[16]), and keeps the address where the tag should be derived
in
> the addend (RELA-only r_addend == &array[0]).
> >
> > For derived symbols where the granule-aligned address is in-bounds of
> the tag (e.g. array_end = &array[7] implies the tag can be derived from
> (&array[0] & 0xf)), we can use a normal RELATIVE relocation.
> >
> > The TAGGED_RELATIVE operand looks like:
> >   *r_offset |= get_tag(r_addend & ~0xf);
> >
> > ABS64, RELATIVE, and TAGGED_RELATIVE relocations need a slight tweak
to
> grab the place's memory tag before use, as the place itself may be
tagged.
> So, for example, the TAGGED_RELATIVE operation above actually becomes:
> >   r_offset = ldg(r_offset);
> >   *r_offset |= get_tag(r_addend & ~0xf);
> >
> > Introduce an R_AARCH64_LDG_LO9_SHORT_NC relocation for relocating the
> 9-bit immediate for the LDG instruction. This isn't MTE-globals
specific,
> we just seem to be missing the relocation to encode the 9-bit immediate for
> LDG at bits [12..20]. This would save us an additional ADD instruction in
> the inline-LDG sequence for hidden symbols.
> >
> > We considered a few other schemes, including:
> >
> > Creating a dynamic symbol table entry for all hidden globals and
giving
> them the same st_other.STO_TAGGED treatment. These entries would not
> require symbol names, but Elf(Sym) entries are 24 bytes (in comparison to 8
> bytes for the MTEGLOBTAB schema under the small code model). For an AOSP
> build, using dynamic symbol entries instead of MTEGLOBTAB results in a
> 2.3MiB code size increase across all DSO's.
> >
> > Making all hidden symbol accesses go through a local-GOT. Requires an
> extra indirection for all local symbols - resulting in increased cache
> pressure (and thus decreased performance) over a simple `ldg` of the tag
> (as the dcache and tag-cache are going to be warmed anyway for the
> load/store). Unlike the MTEGLOBTAG scheme however, this scheme is backwards
> compatible, allowing MTE-globals built binaries to run on old ARM64
> hardware (as no incompatible instructions are emitted), the same as heap
> tagging. Stack tagging requires a new ABI - and we expect the MTE globals
> scheme to be enabled in partnership with stack tagging, thus we are
> unconcerned about the ABI requirement for the MTEGLOBTAG scheme.
> >
> >
> > Please let us know any feedback you have. We're currently working
on an
> experimental version and will update with any more details as they arise.
> >
> >
> > Thanks,
> >
> > Mitch.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Fri, Sep 18, 2020 at 12:18 PM Mitch Phillips via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi David,
>
> Does the tagging of these hidden symbols only protect against RW
>> primitives without a similar ldg? If I knew the address of the hidden
>> symbol I could presumably use the same sequence, but I think I'm
>> stretching what memory tagging is supposed to protect against.
>
>
> I might be missing your point here - but don't forget that the local
> globals are always PC-relative direct loads/stores. The `ldg` sequence in
> that example can only be used to get `&g` (and nothing else). There
> shouldn't be any `ldg`'s of arbitrary addresses (unless an attacker
already
> has control of the instruction pointer, which means they've already
> bypassed MTE).
>
> Does this mean that the value of array_end must have the same tag as
>> array[]. Then &array_end would have a different tag since it's
a
>> different global?
>>
>
> Yes, exactly.
>
> For example you might assign tag 1 to array, then tag 2 to array_end.
>> Which means that array_end has a tag of 2 and so does array[16].
>> (assuming they're sequential)
>> |            array            | array_end/array[16] |
>> | < 1> <1> <1> <1>  |            <2>     
|
>>
>
>
> So if we just did a RELATIVE relocation then array_end's value would
>> have a tag of 2, so you couldn't do:
>> for (int* ptr=array; ptr != array_end; ++ptr)
>> Since it's always != due to the tags.
>> Do I have that right?
>
>
>  Yep - you've got it right, this is why we need TAGGED_RELATIVE. For
> clarity, here's the memory layout where array_end is relocated using
> TAGGED_RELATIVE{*r_offset = &array[16], r_addend = &array[0]}:
> array array_end (padding)
> Memory Tag 0x1 0x1 0x2 0x2
> Value 0 0 0 0 (0x1 << 56) | &array[16] 0 0
> So the address tag of `array` and `array_end` are the same (only
> `&array_end` has an memory/address tag of 0x2), and thus `for (int*
> ptr=array; ptr != array_end; ++ptr)` works normally.
>
> Also, if you have this same example but the array got rounded up to
>> the nearest granule e.g. (4 byte ints, 16 byte granules)
>> int array[3]; // rounded up to be array[4]
>> int* array_end = array[3];
>> Would you emit a normal RELATIVE relocation for array_end, because
>> it's within the bounds of the rounded up array. Or a
TAGGED_RELATIVE
>> relocation because it's out of bounds of the original size of the
>> array?
>> (I don't think doing the former is a problem but I'm not a
linker expert)
>
>
> At this stage, this would generate a TAGGED_RELATIVE. We expect
> TAGGED_RELATIVE to be relatively scarce, and coming up with a more complex
> scheme for the linker to optimise this edge case where it's in bounds
of
> the granule padding (but not the symbol itself) seems over-the-top. In
> saying that, it's a possibility for later revisions.
>
The plan calls to
> Realign to granule size (16 bytes), resize to multiple of granule size
(e.g. 40B -> 48B).
so this would never happen.

The symbols are resized in order to prevent smaller untagged symbols from
getting into the padding of the 16-byte aligned tagged ones.
I'm not sure if it's desirable to change the symbol size just for this
reason. The linker could always suppress such packing for STO_TAGGED
symbols.

In any case, since all sizes and alignments are known, the compiler should
be allowed to emit RELATIVE in the rounded-up array case.

>
> On Fri, Sep 18, 2020 at 4:10 AM David Spickett <david.spickett at
linaro.org>
> wrote:
>
>> Hi Mitch,
>>
>> In the intro you say:
>> > It would also allow attackers with a semilinear RW primitive to
>> trivially attack global variables if the offset is controllable.
Dynamic
>> global tags are required to provide the same MTE mitigation guarantees
that
>> are afforded to stack and heap memory.
>>
>> Then later:
>> > b) Hidden Symbols (static int g; or -fvisibility=hidden)
>> > Materialization of hidden symbols now fetch and insert the memory
tag
>> via. `ldg`. On aarch64, this means non PC-relative
>> loads/stores/address-taken (*g = 7;) generates:
>> >  adrp x0, g;
>> >  ldg x0, [x0, :lo12:g]; // new instruction
>> >  mov x1, #7;
>> >  str x1, [x0, :lo12:g];
>>
>> Does the tagging of these hidden symbols only protect against RW
>> primitives without a similar ldg? If I knew the address of the hidden
>> symbol I could presumably use the same sequence, but I think I'm
>> stretching what memory tagging is supposed to protect against. Mostly
>> wanted to check I understood.
>>
>> Speaking of understanding...
>>
>> > Introduce a TAGGED_RELATIVE relocation - in order to solve the
problem
>> where the tag derivation shouldn't be from the relocation result,
e.g.
>> > static int array[16] = {};
>> > // array_end must have the same tag as array[]. array_end is out
of
>> > // bounds w.r.t. array, and may point to a completely different
global.
>> > int *array_end = &array[16];
>>
>> Does this mean that the value of array_end must have the same tag as
>> array[]. Then &array_end would have a different tag since it's
a
>> different global?
>>
>> For example you might assign tag 1 to array, then tag 2 to array_end.
>> Which means that array_end has a tag of 2 and so does array[16].
>> (assuming they're sequential)
>> |            array            | array_end/array[16] |
>> | < 1> <1> <1> <1>  |            <2>     
|
>>
>> So if we just did a RELATIVE relocation then array_end's value
would
>> have a tag of 2, so you couldn't do:
>> for (int* ptr=array; ptr != array_end; ++ptr)
>> Since it's always != due to the tags.
>>
>> Do I have that right?
>>
>> Also, if you have this same example but the array got rounded up to
>> the nearest granule e.g. (4 byte ints, 16 byte granules)
>> int array[3]; // rounded up to be array[4]
>> int* array_end = array[3];
>>
>> Would you emit a normal RELATIVE relocation for array_end, because
>> it's within the bounds of the rounded up array. Or a
TAGGED_RELATIVE
>> relocation because it's out of bounds of the original size of the
>> array?
>> (I don't think doing the former is a problem but I'm not a
linker expert)
>>
>> Thanks,
>> David Spickett.
>>
>> On Thu, 17 Sep 2020 at 23:05, Mitch Phillips via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hi folks,
>> >
>> >
>> > ARM v8.5 introduces the Memory Tagging Extension (MTE), a hardware
that
>> allows for detection of memory safety bugs (buffer overflows,
>> use-after-free, etc) with low overhead. So far, MTE support is
implemented
>> in the Scudo hardened allocator (compiler-rt/lib/scudo/standalone) for
>> heap, and stack allocation is implemented in LLVM/Clang behind
>> -fsanitize=memtag.
>> >
>> >
>> > As part of a holistic MTE implementation, global memory should
also be
>> properly tagged. HWASan (a software-only implementation of MTE) has a
>> schema that uses static tags, however these can be trivially determined
by
>> an attacker with access to the ELF file. This would allow attackers
with
>> arbitrary read/write to trivially attack global variables. It would
also
>> allow attackers with a semilinear RW primitive to trivially attack
global
>> variables if the offset is controllable. Dynamic global tags are
required
>> to provide the same MTE mitigation guarantees that are afforded to
stack
>> and heap memory.
>> >
>> >
>> > We've got a plan in mind about how to do MTE globals with
fully dynamic
>> tags, but we'd love to get feedback from the community. In
particular -
>> we'd like to try and align implementation details with GCC as the
scheme
>> requires cooperation from the compiler, linker, and loader.
>> >
>> >
>> > Our current ideas are outlined below. All the compiler features
>> (including realignment, etc.) would be guarded behind
-fsanitize=memtag.
>> Protection of read-only globals would be enabled-by-default, but can be
>> disabled at compile time behind a flag (likely
>> -f(no)sanitize-memtag-ro-globals).
>> >
>> >
>> > a) Dynamic symbols (int f; extern int f;)
>> >
>> > Mark all tagged global data symbols in the dynamic symbol table as
>> st_other.STO_TAGGED.
>> >
>> > Teach the loader to read the symbol table at load time (and
dlopen())
>> prior to relocations, and apply random memory tags (via. `irg ->
stg`) to
>> each STO_TAGGED carrying global.
>> >
>> > b) Hidden Symbols (static int g; or -fvisibility=hidden)
>> >
>> > Have the compiler mark hidden tagged globals in the symbol table
as
>> st_other.STO_TAGGED.
>> >
>> > Have the linker read the symbol table and create a table of {
>> unrelocated virtual address, size } pairs for each STO_TAGGED carrying
>> hidden global, storing this in a new section (.mteglobtab).
>> >
>> > Create a new dynamic entry "DT_MTEGLOBTAB" that points
to this segment,
>> along with "DT_MTEGLOBENT" for the size of each entry and
"DT_MTEGLOBSZ"
>> for the size (in bytes) of the table.
>> >
>> > Similar to dynamic symbols, teach the loader to read this table
and
>> apply random memory tags to each global prior to relocations.
>> >
>> > Materialization of hidden symbols now fetch and insert the memory
tag
>> via. `ldg`. On aarch64, this means non PC-relative
>> loads/stores/address-taken (*g = 7;) generates:
>> >   adrp x0, g;
>> >   ldg x0, [x0, :lo12:g]; // new instruction
>> >   mov x1, #7;
>> >   str x1, [x0, :lo12:g];
>> >
>> > Note that this materialization sequence means that executables
built
>> with MTE globals are not able to run on non-MTE hardware.
>> >
>> > Note: Some dynamic symbols can be transformed at link time into
hidden
>> symbols if:
>> >
>> > The symbol is in an object file that is statically linked into an
>> executable and is not referenced in any shared libraries, or
>> >
>> > The symbol has its visibility changed with a version script.
>> >
>> > These globals always have their addresses derived from a GOT
entry, and
>> thus have their address tag materialized through the RELATIVE
relocation of
>> the GOT entry. Due to the lack of dynamic symbol table entry however,
the
>> memory would go untagged. The linker must ensure it creates an
MTEGLOBTAB
>> entry for all hidden MTE-globals, including those that are transformed
from
>> external to hidden. DSO's linked with -Bsymbolic retain their
dynamic
>> symbol table entries, and thus require no special handling.
>> >
>> >
>> > c) All symbols
>> >
>> > Realign to granule size (16 bytes), resize to multiple of granule
size
>> (e.g. 40B -> 48B).
>> >
>> > Ban data folding (except where contents and size are same, no tail
>> merging).
>> >
>> > In the loader, ensure writable segments (and possibly .rodata, see
next
>> dot point) are mapped MAP_ANONYMOUS and PROT_MTE (with the contents of
the
>> mappings filled from the file), as file-based mappings aren't
necessarily
>> backed by tag-capable memory. It also requires in-place remapping of
data
>> segments from the program image (as they're already mapped by the
kernel
>> before PT_INTERP invokes the loader).
>> >
>> > Make .rodata protection optional. When read-only protection is in
use,
>> the .rodata section should be moved into a separate segment. For Bionic
>> libc, the rodata section takes up 20% of its ALLOC | READ segment, and
we'd
>> like to be able to maintain page sharing for the remaining 189KiB of
other
>> read-only data in this segment.
>> >
>> > d) Relocations
>> >
>> > GLOB_DAT, ABS64, and RELATIVE relocations change semantics - they
would
>> be required to retrieve and insert the memory tag of the symbol into
the
>> relocated value. For example, the ABS64 relocation becomes:
>> >   sym_addr = get_symbol_address() // sym_addr = 0x1008
>> >   sym_addr |= get_tag(sym_addr & 0xf) // get_tag(0x1008 &
0xf == 0x1000)
>> >   *r_offset = sym_addr + r_addend;
>> >
>> > Introduce a TAGGED_RELATIVE relocation - in order to solve the
problem
>> where the tag derivation shouldn't be from the relocation result,
e.g.
>> > static int array[16] = {};
>> > // array_end must have the same tag as array[]. array_end is out
of
>> > // bounds w.r.t. array, and may point to a completely different
global.
>> > int *array_end = &array[16];
>> >
>> > TAGGED_RELATIVE stores the untagged symbol value in the place
>> (*r_offset == &array[16]), and keeps the address where the tag
should be
>> derived in the addend (RELA-only r_addend == &array[0]).
>> >
>> > For derived symbols where the granule-aligned address is in-bounds
of
>> the tag (e.g. array_end = &array[7] implies the tag can be derived
from
>> (&array[0] & 0xf)), we can use a normal RELATIVE relocation.
>> >
>> > The TAGGED_RELATIVE operand looks like:
>> >   *r_offset |= get_tag(r_addend & ~0xf);
>> >
>> > ABS64, RELATIVE, and TAGGED_RELATIVE relocations need a slight
tweak to
>> grab the place's memory tag before use, as the place itself may be
tagged.
>> So, for example, the TAGGED_RELATIVE operation above actually becomes:
>> >   r_offset = ldg(r_offset);
>> >   *r_offset |= get_tag(r_addend & ~0xf);
>> >
>> > Introduce an R_AARCH64_LDG_LO9_SHORT_NC relocation for relocating
the
>> 9-bit immediate for the LDG instruction. This isn't MTE-globals
specific,
>> we just seem to be missing the relocation to encode the 9-bit immediate
for
>> LDG at bits [12..20]. This would save us an additional ADD instruction
in
>> the inline-LDG sequence for hidden symbols.
>> >
>> > We considered a few other schemes, including:
>> >
>> > Creating a dynamic symbol table entry for all hidden globals and
giving
>> them the same st_other.STO_TAGGED treatment. These entries would not
>> require symbol names, but Elf(Sym) entries are 24 bytes (in comparison
to 8
>> bytes for the MTEGLOBTAB schema under the small code model). For an
AOSP
>> build, using dynamic symbol entries instead of MTEGLOBTAB results in a
>> 2.3MiB code size increase across all DSO's.
>> >
>> > Making all hidden symbol accesses go through a local-GOT. Requires
an
>> extra indirection for all local symbols - resulting in increased cache
>> pressure (and thus decreased performance) over a simple `ldg` of the
tag
>> (as the dcache and tag-cache are going to be warmed anyway for the
>> load/store). Unlike the MTEGLOBTAG scheme however, this scheme is
backwards
>> compatible, allowing MTE-globals built binaries to run on old ARM64
>> hardware (as no incompatible instructions are emitted), the same as
heap
>> tagging. Stack tagging requires a new ABI - and we expect the MTE
globals
>> scheme to be enabled in partnership with stack tagging, thus we are
>> unconcerned about the ABI requirement for the MTEGLOBTAG scheme.
>> >
>> >
>> > Please let us know any feedback you have. We're currently
working on an
>> experimental version and will update with any more details as they
arise.
>> >
>> >
>> > Thanks,
>> >
>> > Mitch.
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> I might be missing your point here - but don't forget that the local
globals are always PC-relative direct loads/stores.
I did forget! Thanks for clarifying, now I understand.

On Fri, 18 Sep 2020 at 20:51, Evgenii Stepanov <eugenis at google.com>
wrote:
>
>
>
> On Fri, Sep 18, 2020 at 12:18 PM Mitch Phillips via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
>>
>> Hi David,
>>
>>> Does the tagging of these hidden symbols only protect against RW
>>> primitives without a similar ldg? If I knew the address of the
hidden
>>> symbol I could presumably use the same sequence, but I think
I'm
>>> stretching what memory tagging is supposed to protect against.
>>
>>
>> I might be missing your point here - but don't forget that the
local globals are always PC-relative direct loads/stores. The `ldg` sequence in
that example can only be used to get `&g` (and nothing else). There
shouldn't be any `ldg`'s of arbitrary addresses (unless an attacker
already has control of the instruction pointer, which means they've already
bypassed MTE).
>>
>>> Does this mean that the value of array_end must have the same tag
as
>>> array[]. Then &array_end would have a different tag since
it's a
>>> different global?
>>
>>
>> Yes, exactly.
>>
>>> For example you might assign tag 1 to array, then tag 2 to
array_end.
>>> Which means that array_end has a tag of 2 and so does array[16].
>>> (assuming they're sequential)
>>> |            array            | array_end/array[16] |
>>> | < 1> <1> <1> <1>  |            <2> 
|
>>>
>>>
>>>
>>> So if we just did a RELATIVE relocation then array_end's value
would
>>> have a tag of 2, so you couldn't do:
>>> for (int* ptr=array; ptr != array_end; ++ptr)
>>> Since it's always != due to the tags.
>>> Do I have that right?
>>
>>
>>  Yep - you've got it right, this is why we need TAGGED_RELATIVE.
For clarity, here's the memory layout where array_end is relocated using
TAGGED_RELATIVE{*r_offset = &array[16], r_addend = &array[0]}:
>> arrayarray_end(padding)
>> Memory Tag0x10x10x20x2
>> Value0000(0x1 << 56) | &array[16]00
>>
>> So the address tag of `array` and `array_end` are the same (only
`&array_end` has an memory/address tag of 0x2), and thus `for (int*
ptr=array; ptr != array_end; ++ptr)` works normally.
>>
>>> Also, if you have this same example but the array got rounded up to
>>> the nearest granule e.g. (4 byte ints, 16 byte granules)
>>> int array[3]; // rounded up to be array[4]
>>> int* array_end = array[3];
>>> Would you emit a normal RELATIVE relocation for array_end, because
>>> it's within the bounds of the rounded up array. Or a
TAGGED_RELATIVE
>>> relocation because it's out of bounds of the original size of
the
>>> array?
>>> (I don't think doing the former is a problem but I'm not a
linker expert)
>>
>>
>> At this stage, this would generate a TAGGED_RELATIVE. We expect
TAGGED_RELATIVE to be relatively scarce, and coming up with a more complex
scheme for the linker to optimise this edge case where it's in bounds of the
granule padding (but not the symbol itself) seems over-the-top. In saying that,
it's a possibility for later revisions.
>
>
> The plan calls to
> > Realign to granule size (16 bytes), resize to multiple of granule size
(e.g. 40B -> 48B).
> so this would never happen.
>
> The symbols are resized in order to prevent smaller untagged symbols from
getting into the padding of the 16-byte aligned tagged ones.
> I'm not sure if it's desirable to change the symbol size just for
this reason. The linker could always suppress such packing for STO_TAGGED
symbols.
>
> In any case, since all sizes and alignments are known, the compiler should
be allowed to emit RELATIVE in the rounded-up array case.
>
>>
>>
>> On Fri, Sep 18, 2020 at 4:10 AM David Spickett <david.spickett at
linaro.org> wrote:
>>>
>>> Hi Mitch,
>>>
>>> In the intro you say:
>>> > It would also allow attackers with a semilinear RW primitive
to trivially attack global variables if the offset is controllable. Dynamic
global tags are required to provide the same MTE mitigation guarantees that are
afforded to stack and heap memory.
>>>
>>> Then later:
>>> > b) Hidden Symbols (static int g; or -fvisibility=hidden)
>>> > Materialization of hidden symbols now fetch and insert the
memory tag via. `ldg`. On aarch64, this means non PC-relative
loads/stores/address-taken (*g = 7;) generates:
>>> >  adrp x0, g;
>>> >  ldg x0, [x0, :lo12:g]; // new instruction
>>> >  mov x1, #7;
>>> >  str x1, [x0, :lo12:g];
>>>
>>> Does the tagging of these hidden symbols only protect against RW
>>> primitives without a similar ldg? If I knew the address of the
hidden
>>> symbol I could presumably use the same sequence, but I think
I'm
>>> stretching what memory tagging is supposed to protect against.
Mostly
>>> wanted to check I understood.
>>>
>>> Speaking of understanding...
>>>
>>> > Introduce a TAGGED_RELATIVE relocation - in order to solve the
problem where the tag derivation shouldn't be from the relocation result,
e.g.
>>> > static int array[16] = {};
>>> > // array_end must have the same tag as array[]. array_end is
out of
>>> > // bounds w.r.t. array, and may point to a completely
different global.
>>> > int *array_end = &array[16];
>>>
>>> Does this mean that the value of array_end must have the same tag
as
>>> array[]. Then &array_end would have a different tag since
it's a
>>> different global?
>>>
>>> For example you might assign tag 1 to array, then tag 2 to
array_end.
>>> Which means that array_end has a tag of 2 and so does array[16].
>>> (assuming they're sequential)
>>> |            array            | array_end/array[16] |
>>> | < 1> <1> <1> <1>  |            <2> 
|
>>>
>>> So if we just did a RELATIVE relocation then array_end's value
would
>>> have a tag of 2, so you couldn't do:
>>> for (int* ptr=array; ptr != array_end; ++ptr)
>>> Since it's always != due to the tags.
>>>
>>> Do I have that right?
>>>
>>> Also, if you have this same example but the array got rounded up to
>>> the nearest granule e.g. (4 byte ints, 16 byte granules)
>>> int array[3]; // rounded up to be array[4]
>>> int* array_end = array[3];
>>>
>>> Would you emit a normal RELATIVE relocation for array_end, because
>>> it's within the bounds of the rounded up array. Or a
TAGGED_RELATIVE
>>> relocation because it's out of bounds of the original size of
the
>>> array?
>>> (I don't think doing the former is a problem but I'm not a
linker expert)
>>>
>>> Thanks,
>>> David Spickett.
>>>
>>> On Thu, 17 Sep 2020 at 23:05, Mitch Phillips via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> >
>>> > Hi folks,
>>> >
>>> >
>>> > ARM v8.5 introduces the Memory Tagging Extension (MTE), a
hardware that allows for detection of memory safety bugs (buffer overflows,
use-after-free, etc) with low overhead. So far, MTE support is implemented in
the Scudo hardened allocator (compiler-rt/lib/scudo/standalone) for heap, and
stack allocation is implemented in LLVM/Clang behind -fsanitize=memtag.
>>> >
>>> >
>>> > As part of a holistic MTE implementation, global memory should
also be properly tagged. HWASan (a software-only implementation of MTE) has a
schema that uses static tags, however these can be trivially determined by an
attacker with access to the ELF file. This would allow attackers with arbitrary
read/write to trivially attack global variables. It would also allow attackers
with a semilinear RW primitive to trivially attack global variables if the
offset is controllable. Dynamic global tags are required to provide the same MTE
mitigation guarantees that are afforded to stack and heap memory.
>>> >
>>> >
>>> > We've got a plan in mind about how to do MTE globals with
fully dynamic tags, but we'd love to get feedback from the community. In
particular - we'd like to try and align implementation details with GCC as
the scheme requires cooperation from the compiler, linker, and loader.
>>> >
>>> >
>>> > Our current ideas are outlined below. All the compiler
features (including realignment, etc.) would be guarded behind
-fsanitize=memtag. Protection of read-only globals would be enabled-by-default,
but can be disabled at compile time behind a flag (likely
-f(no)sanitize-memtag-ro-globals).
>>> >
>>> >
>>> > a) Dynamic symbols (int f; extern int f;)
>>> >
>>> > Mark all tagged global data symbols in the dynamic symbol
table as st_other.STO_TAGGED.
>>> >
>>> > Teach the loader to read the symbol table at load time (and
dlopen()) prior to relocations, and apply random memory tags (via. `irg ->
stg`) to each STO_TAGGED carrying global.
>>> >
>>> > b) Hidden Symbols (static int g; or -fvisibility=hidden)
>>> >
>>> > Have the compiler mark hidden tagged globals in the symbol
table as st_other.STO_TAGGED.
>>> >
>>> > Have the linker read the symbol table and create a table of {
unrelocated virtual address, size } pairs for each STO_TAGGED carrying hidden
global, storing this in a new section (.mteglobtab).
>>> >
>>> > Create a new dynamic entry "DT_MTEGLOBTAB" that
points to this segment, along with "DT_MTEGLOBENT" for the size of
each entry and "DT_MTEGLOBSZ" for the size (in bytes) of the table.
>>> >
>>> > Similar to dynamic symbols, teach the loader to read this
table and apply random memory tags to each global prior to relocations.
>>> >
>>> > Materialization of hidden symbols now fetch and insert the
memory tag via. `ldg`. On aarch64, this means non PC-relative
loads/stores/address-taken (*g = 7;) generates:
>>> >   adrp x0, g;
>>> >   ldg x0, [x0, :lo12:g]; // new instruction
>>> >   mov x1, #7;
>>> >   str x1, [x0, :lo12:g];
>>> >
>>> > Note that this materialization sequence means that executables
built with MTE globals are not able to run on non-MTE hardware.
>>> >
>>> > Note: Some dynamic symbols can be transformed at link time
into hidden symbols if:
>>> >
>>> > The symbol is in an object file that is statically linked into
an executable and is not referenced in any shared libraries, or
>>> >
>>> > The symbol has its visibility changed with a version script.
>>> >
>>> > These globals always have their addresses derived from a GOT
entry, and thus have their address tag materialized through the RELATIVE
relocation of the GOT entry. Due to the lack of dynamic symbol table entry
however, the memory would go untagged. The linker must ensure it creates an
MTEGLOBTAB entry for all hidden MTE-globals, including those that are
transformed from external to hidden. DSO's linked with -Bsymbolic retain
their dynamic symbol table entries, and thus require no special handling.
>>> >
>>> >
>>> > c) All symbols
>>> >
>>> > Realign to granule size (16 bytes), resize to multiple of
granule size (e.g. 40B -> 48B).
>>> >
>>> > Ban data folding (except where contents and size are same, no
tail merging).
>>> >
>>> > In the loader, ensure writable segments (and possibly .rodata,
see next dot point) are mapped MAP_ANONYMOUS and PROT_MTE (with the contents of
the mappings filled from the file), as file-based mappings aren't
necessarily backed by tag-capable memory. It also requires in-place remapping of
data segments from the program image (as they're already mapped by the
kernel before PT_INTERP invokes the loader).
>>> >
>>> > Make .rodata protection optional. When read-only protection is
in use, the .rodata section should be moved into a separate segment. For Bionic
libc, the rodata section takes up 20% of its ALLOC | READ segment, and we'd
like to be able to maintain page sharing for the remaining 189KiB of other
read-only data in this segment.
>>> >
>>> > d) Relocations
>>> >
>>> > GLOB_DAT, ABS64, and RELATIVE relocations change semantics -
they would be required to retrieve and insert the memory tag of the symbol into
the relocated value. For example, the ABS64 relocation becomes:
>>> >   sym_addr = get_symbol_address() // sym_addr = 0x1008
>>> >   sym_addr |= get_tag(sym_addr & 0xf) // get_tag(0x1008
& 0xf == 0x1000)
>>> >   *r_offset = sym_addr + r_addend;
>>> >
>>> > Introduce a TAGGED_RELATIVE relocation - in order to solve the
problem where the tag derivation shouldn't be from the relocation result,
e.g.
>>> > static int array[16] = {};
>>> > // array_end must have the same tag as array[]. array_end is
out of
>>> > // bounds w.r.t. array, and may point to a completely
different global.
>>> > int *array_end = &array[16];
>>> >
>>> > TAGGED_RELATIVE stores the untagged symbol value in the place
(*r_offset == &array[16]), and keeps the address where the tag should be
derived in the addend (RELA-only r_addend == &array[0]).
>>> >
>>> > For derived symbols where the granule-aligned address is
in-bounds of the tag (e.g. array_end = &array[7] implies the tag can be
derived from (&array[0] & 0xf)), we can use a normal RELATIVE
relocation.
>>> >
>>> > The TAGGED_RELATIVE operand looks like:
>>> >   *r_offset |= get_tag(r_addend & ~0xf);
>>> >
>>> > ABS64, RELATIVE, and TAGGED_RELATIVE relocations need a slight
tweak to grab the place's memory tag before use, as the place itself may be
tagged. So, for example, the TAGGED_RELATIVE operation above actually becomes:
>>> >   r_offset = ldg(r_offset);
>>> >   *r_offset |= get_tag(r_addend & ~0xf);
>>> >
>>> > Introduce an R_AARCH64_LDG_LO9_SHORT_NC relocation for
relocating the 9-bit immediate for the LDG instruction. This isn't
MTE-globals specific, we just seem to be missing the relocation to encode the
9-bit immediate for LDG at bits [12..20]. This would save us an additional ADD
instruction in the inline-LDG sequence for hidden symbols.
>>> >
>>> > We considered a few other schemes, including:
>>> >
>>> > Creating a dynamic symbol table entry for all hidden globals
and giving them the same st_other.STO_TAGGED treatment. These entries would not
require symbol names, but Elf(Sym) entries are 24 bytes (in comparison to 8
bytes for the MTEGLOBTAB schema under the small code model). For an AOSP build,
using dynamic symbol entries instead of MTEGLOBTAB results in a 2.3MiB code size
increase across all DSO's.
>>> >
>>> > Making all hidden symbol accesses go through a local-GOT.
Requires an extra indirection for all local symbols - resulting in increased
cache pressure (and thus decreased performance) over a simple `ldg` of the tag
(as the dcache and tag-cache are going to be warmed anyway for the load/store).
Unlike the MTEGLOBTAG scheme however, this scheme is backwards compatible,
allowing MTE-globals built binaries to run on old ARM64 hardware (as no
incompatible instructions are emitted), the same as heap tagging. Stack tagging
requires a new ABI - and we expect the MTE globals scheme to be enabled in
partnership with stack tagging, thus we are unconcerned about the ABI
requirement for the MTEGLOBTAG scheme.
>>> >
>>> >
>>> > Please let us know any feedback you have. We're currently
working on an experimental version and will update with any more details as they
arise.
>>> >
>>> >
>>> > Thanks,
>>> >
>>> > Mitch.
>>> >
>>> >
>>> > _______________________________________________
>>> > LLVM Developers mailing list
>>> > llvm-dev at lists.llvm.org
>>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev