llvm dev - May 2017 - [lld][ELF] Add option to make .dynamic read only

The motivation is not only memory savings but also security:
can-never-be-written is strictly better than RELRO in all cases. The
biggest win is when .dynamic is the sole reason for having a writable
segment at all. The distinction is fairly small for exploitability, but not
negligible.

LLD already has several command-line options that are not supported by or
are different from ld or gold, so this wouldn't be the first one (and
probably not the last). If LLD supported per-OS configuration, we would
make this default for Fuchsia and wouldn't need the new option, but since
these don't exists, it's something we would handle through the Clang
driver.

On Tue, May 16, 2017 at 7:12 PM Rui Ueyama via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Thank you for confirming. But one page is too small, no? I think we are
> not that picky about memory footprint. For example, LLD and gold's
outputs
> are usually slightly different in size due to various reasons such as the
> differences how they order sections, common symbols, etc., but no one seems
> to care about that kind of negligible differences. Merging DT_MIPS_RLD_MAP
> into DT_DEBUG_INDIRECT seems like a good change, but adding a new command
> option to save one page at runtime seems overkill to me. (But I don't
know
> much about Fuchsia, so correct me if I'm wrong.)
>
> On Tue, May 16, 2017 at 6:08 PM, Jake Ehrlich via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Your understanding is correct. It saves a small number of physical
pages.
>> It's strictly better even if it's only a little bit better.
>>
>>
>> On Tue, May 16, 2017, 5:14 PM Rui Ueyama <ruiu at google.com>
wrote:
>>
>>> Hi Jake,
>>>
>>> Thank you for sending this to llvm-dev!
>>>
>>> If I understand correctly, your motivation to add an option to make
>>> .dynamic sections read-only is to share .dynamic sections in memory
so that
>>> each process executing the same file doesn't have to have a
separate copy
>>> of a .dynamic section. But you would usually save just one page or
two by
>>> doing that because .dynamic sections are usually very small. Even
if they
>>> are large, they could be shared between processes by copy-on-write.
What am
>>> I missing?
>>>
>>> On Tue, May 16, 2017 at 4:31 PM, Jake Ehrlich via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> This is a proposal to add an option to lld that forces .dynamic
>>>> sections to be read-only.
>>>>
>>>> The .dynamic section is almost read-only except for the
DT_DEBUG entry
>>>> which requires the dynamic linker to modify a word. MIPS has
long since had
>>>> a solution to this using the DT_MIPS_RLD_MAP entry to give a
pointer to
>>>> another section which is writable. It would be nice to have
this
>>>> functionality on other targets as well however. Right now many
dynamic
>>>> linkers do not support this layer of indirection so this
can't happen by
>>>> default, it must be an option. Currently .dynamic gets mapped
to a
>>>> PT_GNU_RELRO section. PT_GNU_RELRO sections are not shared
between
>>>> instances of a executable however. It would be a nice
optimization to share
>>>> the .dynamic section of an application where possible. In
particular we
>>>> would like to have this option for Fuchsia. Other dynamic
linkers, like
>>>> musl, already have some support for this on non-MIPS targets as
seen here:
>>>> http://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n1629.
In
>>>> particular the DT_DEBUG_INDIRECT entry is recognized.
>>>>
>>>> This change would refactor code that already exists for MIPS
specific
>>>> .dynamic sections and make them available to other targets.
Mostly this
>>>> would just require renaming some things and moving a few lines
around. The
>>>> biggest part of this change would be to add the flag. I propose
it be a -z
>>>> keyword called "rodynamic".
>>>>
>>>> I have already made a patch for this change which can be seen
here:
>>>> https://reviews.llvm.org/D33251
>>>>
>>>> Best,
>>>> Jake
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Wed, May 17, 2017 at 1:11 PM, Petr Hosek <phosek at chromium.org>
wrote:
> The motivation is not only memory savings but also security:
> can-never-be-written is strictly better than RELRO in all cases. The
> biggest win is when .dynamic is the sole reason for having a writable
> segment at all. The distinction is fairly small for exploitability, but not
> negligible.
>
I'm not even sure if it is strictly better to make .dynamic read-only by
default is better than relro. It seems almost the same to me. What are
attack scenario you can think of in which you can exploit only when
.dynamic sections are read-only from beginning?

Maybe Fuchsia OS does something special for executable that contain
read-only sections only?

LLD already has several command-line options that are not supported by
or
> are different from ld or gold, so this wouldn't be the first one (and
> probably not the last). If LLD supported per-OS configuration, we would
> make this default for Fuchsia and wouldn't need the new option, but
since
> these don't exists, it's something we would handle through the
Clang driver.
>
Yes, we have bunch of options that are not supported by GNU linkers,
particularly for LTO. So adding new options is fine as long as doing it
makes sense. But the bar should be higher than implementing compatible
options.

Having no host OS-specific default options in the linker is more like a
feature than a lack of a feature. We rely on compilers to give appropriate
options to us. That makes the linker's behavior more predictable, and it
also makes it very easy to support cross-linking.

On Tue, May 16, 2017 at 7:12 PM Rui Ueyama via llvm-dev
<
> llvm-dev at lists.llvm.org> wrote:
>
>> Thank you for confirming. But one page is too small, no? I think we are
>> not that picky about memory footprint. For example, LLD and gold's
outputs
>> are usually slightly different in size due to various reasons such as
the
>> differences how they order sections, common symbols, etc., but no one
seems
>> to care about that kind of negligible differences. Merging
DT_MIPS_RLD_MAP
>> into DT_DEBUG_INDIRECT seems like a good change, but adding a new
command
>> option to save one page at runtime seems overkill to me. (But I
don't know
>> much about Fuchsia, so correct me if I'm wrong.)
>>
>> On Tue, May 16, 2017 at 6:08 PM, Jake Ehrlich via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Your understanding is correct. It saves a small number of physical
>>> pages. It's strictly better even if it's only a little bit
better.
>>>
>>>
>>> On Tue, May 16, 2017, 5:14 PM Rui Ueyama <ruiu at google.com>
wrote:
>>>
>>>> Hi Jake,
>>>>
>>>> Thank you for sending this to llvm-dev!
>>>>
>>>> If I understand correctly, your motivation to add an option to
make
>>>> .dynamic sections read-only is to share .dynamic sections in
memory so that
>>>> each process executing the same file doesn't have to have a
separate copy
>>>> of a .dynamic section. But you would usually save just one page
or two by
>>>> doing that because .dynamic sections are usually very small.
Even if they
>>>> are large, they could be shared between processes by
copy-on-write. What am
>>>> I missing?
>>>>
>>>> On Tue, May 16, 2017 at 4:31 PM, Jake Ehrlich via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> This is a proposal to add an option to lld that forces
.dynamic
>>>>> sections to be read-only.
>>>>>
>>>>> The .dynamic section is almost read-only except for the
DT_DEBUG entry
>>>>> which requires the dynamic linker to modify a word. MIPS
has long since had
>>>>> a solution to this using the DT_MIPS_RLD_MAP entry to give
a pointer to
>>>>> another section which is writable. It would be nice to have
this
>>>>> functionality on other targets as well however. Right now
many dynamic
>>>>> linkers do not support this layer of indirection so this
can't happen by
>>>>> default, it must be an option. Currently .dynamic gets
mapped to a
>>>>> PT_GNU_RELRO section. PT_GNU_RELRO sections are not shared
between
>>>>> instances of a executable however. It would be a nice
optimization to share
>>>>> the .dynamic section of an application where possible. In
particular we
>>>>> would like to have this option for Fuchsia. Other dynamic
linkers, like
>>>>> musl, already have some support for this on non-MIPS
targets as seen here:
>>>>>
http://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n1629. In
>>>>> particular the DT_DEBUG_INDIRECT entry is recognized.
>>>>>
>>>>> This change would refactor code that already exists for
MIPS specific
>>>>> .dynamic sections and make them available to other targets.
Mostly this
>>>>> would just require renaming some things and moving a few
lines around. The
>>>>> biggest part of this change would be to add the flag. I
propose it be a -z
>>>>> keyword called "rodynamic".
>>>>>
>>>>> I have already made a patch for this change which can be
seen here:
>>>>> https://reviews.llvm.org/D33251
>>>>>
>>>>> Best,
>>>>> Jake
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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On Wed, May 17, 2017 at 1:32 PM, Rui Ueyama via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Wed, May 17, 2017 at 1:11 PM, Petr Hosek <phosek at chromium.org>
wrote:
>
>> The motivation is not only memory savings but also security:
>> can-never-be-written is strictly better than RELRO in all cases. The
>> biggest win is when .dynamic is the sole reason for having a writable
>> segment at all. The distinction is fairly small for exploitability, but
not
>> negligible.
>>
>
> I'm not even sure if it is strictly better to make .dynamic read-only
by
> default is better than relro. It seems almost the same to me. What are
> attack scenario you can think of in which you can exploit only when
> .dynamic sections are read-only from beginning?
>
Maybe Fuchsia OS does something special for executable that
contain
> read-only sections only?
>
 I'd be interested in this as well, CFI relies on relro for its vtable
protection to work. Is the issue just the window of opportunity between
mapping the pages and protecting them?
>
> LLD already has several command-line options that are not supported by or
>> are different from ld or gold, so this wouldn't be the first one
(and
>> probably not the last). If LLD supported per-OS configuration, we would
>> make this default for Fuchsia and wouldn't need the new option, but
since
>> these don't exists, it's something we would handle through the
Clang driver.
>>
>
> Yes, we have bunch of options that are not supported by GNU linkers,
> particularly for LTO. So adding new options is fine as long as doing it
> makes sense. But the bar should be higher than implementing compatible
> options.
>
> Having no host OS-specific default options in the linker is more like a
> feature than a lack of a feature. We rely on compilers to give appropriate
> options to us. That makes the linker's behavior more predictable, and
it
> also makes it very easy to support cross-linking.
>
Note that you could avoid needing to add a new flag by defining a new
ELFOSABI for Fuchsia and making this behaviour conditional on it.

Peter
>
> On Tue, May 16, 2017 at 7:12 PM Rui Ueyama via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Thank you for confirming. But one page is too small, no? I think we
are
>>> not that picky about memory footprint. For example, LLD and
gold's outputs
>>> are usually slightly different in size due to various reasons such
as the
>>> differences how they order sections, common symbols, etc., but no
one seems
>>> to care about that kind of negligible differences. Merging
DT_MIPS_RLD_MAP
>>> into DT_DEBUG_INDIRECT seems like a good change, but adding a new
command
>>> option to save one page at runtime seems overkill to me. (But I
don't know
>>> much about Fuchsia, so correct me if I'm wrong.)
>>>
>>> On Tue, May 16, 2017 at 6:08 PM, Jake Ehrlich via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Your understanding is correct. It saves a small number of
physical
>>>> pages. It's strictly better even if it's only a little
bit better.
>>>>
>>>>
>>>> On Tue, May 16, 2017, 5:14 PM Rui Ueyama <ruiu at
google.com> wrote:
>>>>
>>>>> Hi Jake,
>>>>>
>>>>> Thank you for sending this to llvm-dev!
>>>>>
>>>>> If I understand correctly, your motivation to add an option
to make
>>>>> .dynamic sections read-only is to share .dynamic sections
in memory so that
>>>>> each process executing the same file doesn't have to
have a separate copy
>>>>> of a .dynamic section. But you would usually save just one
page or two by
>>>>> doing that because .dynamic sections are usually very
small. Even if they
>>>>> are large, they could be shared between processes by
copy-on-write. What am
>>>>> I missing?
>>>>>
>>>>> On Tue, May 16, 2017 at 4:31 PM, Jake Ehrlich via llvm-dev
<
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> This is a proposal to add an option to lld that forces
.dynamic
>>>>>> sections to be read-only.
>>>>>>
>>>>>> The .dynamic section is almost read-only except for the
DT_DEBUG
>>>>>> entry which requires the dynamic linker to modify a
word. MIPS has long
>>>>>> since had a solution to this using the DT_MIPS_RLD_MAP
entry to give a
>>>>>> pointer to another section which is writable. It would
be nice to have this
>>>>>> functionality on other targets as well however. Right
now many dynamic
>>>>>> linkers do not support this layer of indirection so
this can't happen by
>>>>>> default, it must be an option. Currently .dynamic gets
mapped to a
>>>>>> PT_GNU_RELRO section. PT_GNU_RELRO sections are not
shared between
>>>>>> instances of a executable however. It would be a nice
optimization to share
>>>>>> the .dynamic section of an application where possible.
In particular we
>>>>>> would like to have this option for Fuchsia. Other
dynamic linkers, like
>>>>>> musl, already have some support for this on non-MIPS
targets as seen here:
>>>>>>
http://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n1629. In
>>>>>> particular the DT_DEBUG_INDIRECT entry is recognized.
>>>>>>
>>>>>> This change would refactor code that already exists for
MIPS specific
>>>>>> .dynamic sections and make them available to other
targets. Mostly this
>>>>>> would just require renaming some things and moving a
few lines around. The
>>>>>> biggest part of this change would be to add the flag. I
propose it be a -z
>>>>>> keyword called "rodynamic".
>>>>>>
>>>>>> I have already made a patch for this change which can
be seen here:
>>>>>> https://reviews.llvm.org/D33251
>>>>>>
>>>>>> Best,
>>>>>> Jake
>>>>>>
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>>>
>>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>

-- 
-- 
Peter
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