Andreas F. Borchert
2013-Aug-21 16:32 UTC
[Dovecot] Bug in dovecot 2.2.5: segfault due to bad alignment
Take a look at the sources, hmac.h declares struct hmac_context:
struct hmac_context {
char ctx[HMAC_MAX_CONTEXT_SIZE];
char ctxo[HMAC_MAX_CONTEXT_SIZE];
const struct hash_method *hash;
};
If compiled for a 32 bit virtual address space, this has an alignment
requirement of 4 due to the hash pointer.
In line 171 of auth-token.c, we have following declaration of ctx
as a local variable in auth_token_get():
struct hmac_context ctx;
This is put on an address with an alignment requirement of 4. In
lines 174 and 175 hmac_init is invoked with hash_method_sha1:
hmac_init(&ctx, (const unsigned char*)username, strlen(username),
&hash_method_sha1);
In hmac.c, lines 43 and following, ctx->ctx with an alignment of
4 is passed to meth->init and meth->loop where meth refers to
hash_method_sha1:
meth->init(ctx->ctx);
meth->loop(ctx->ctx, k_ipad, 64);
These functions refer now to sha1_init and sha1_loop where the
first parameter is expected to be a pointer to struct sha1_ctxt,
a data structure which is declared in sha1.h:
struct sha1_ctxt {
union {
uint8_t b8[20];
uint32_t b32[5];
} h;
union {
uint8_t b8[8];
uint64_t b64[1];
} c;
union {
uint8_t b8[64];
uint32_t b32[16];
} m;
uint8_t count;
};
Here we have with b64 one uint64_t which has on a SPARC platform
an alignment requirement of 8. In consequence, struct sha1_ctxt
has an alignment requirement of 8. With the invocations of
meth->init and meth->loop above we pass a pointer to a data
structure of alignment 4 to a function expecting a pointer to
a data structure of alignment 8. Chances are that the alignment
requirement is not met, causing a segmentation violation.
This must be solved by declaring struct hmac_context such that
is not just big enough but respects also the highest alignment
required for one of the hashing data structures.
There are several options to do this:
* Beginning with C11, you are free to use an alignment specifier,
i.e. add _Alignas ( uint64_t )
(see section 6.7.5 in ISO 9899-2011)
* GCC supports alignment attributes,
i.e. add __attribute__ ((aligned (8)))
or whatever is required instead of 8,
see http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html
* Do not use a local variable for it, allocate the data structure
using malloc instead.
If you want to see a live crash, here is the relevant output of gdb
that debugs ${prefix}/libexec/dovecot/auth.
Program received signal SIGSEGV, Segmentation fault.
sha1_loop (ctxt=0xffbff63c, input=0xffbff548, len=64) at sha1.c:224
224 sha1.c: No such file or directory.
(gdb) where
#0 sha1_loop (ctxt=0xffbff63c, input=0xffbff548, len=64) at sha1.c:224
#1 0xff2e218c in hmac_init (ctx=ctx at entry=0xffbff63c,
key=key at entry=0x6a698 "borchert", key_len=8,
meth=0x555d0 <hash_method_sha1>) at hmac.c:44
#2 0x00023310 in auth_token_get (service=service at entry=0x6a648
"imap",
session_pid=0x56071 "26272", username=0x6a698
"borchert",
session_id=0x6a650 "AErv6nbk6gB/AAAB") at auth-token.c:174
#3 0x00021708 in userdb_callback (result=USERDB_RESULT_OK, request=0x6a530)
at auth-request-handler.c:668
#4 0x0001f144 in auth_request_userdb_callback (result=<optimized out>,
result at entry=USERDB_RESULT_OK, request=request at entry=0x6a530)
at auth-request.c:1039
#5 0x000312c8 in prefetch_lookup (auth_request=0x6a530,
callback=0x1f058 <auth_request_userdb_callback>) at
userdb-prefetch.c:40
#6 0x0001f37c in auth_request_lookup_user (request=0x6a530,
callback=callback at entry=0x2150c <userdb_callback>) at
auth-request.c:1072
#7 0x00022034 in auth_request_handler_master_request (
handler=<optimized out>, master=master at entry=0x6b120,
id=1292369921,
client_id=1, params=0x55bfc) at auth-request-handler.c:758
#8 0x0001be98 in master_input_request (args=<optimized out>,
conn=0x6b120)
at auth-master-connection.c:127
#9 auth_master_input_line (line=<optimized out>, conn=0x6b120)
at auth-master-connection.c:598
#10 master_input (conn=0x6b120) at auth-master-connection.c:653
#11 0xff2ecca4 in io_loop_call_io (io=io at entry=0x6b398) at ioloop.c:387
#12 0xff2ed604 in io_loop_handler_run (ioloop=ioloop at entry=0x5e5e8)
at ioloop-poll.c:211
#13 0xff2ec7a8 in io_loop_run (ioloop=0x5e5e8) at ioloop.c:406
#14 0xff29ad7c in master_service_run (service=0x5e128,
callback=0x27b40 <client_connected>) at master-service.c:566
#15 0x0001852c in main (argc=1, argv=0xffbffd54) at main.c:393
(gdb) print ctxt
$1 = (struct sha1_ctxt *) 0xffbff63c
As you can see, ctxt is on a 4-byte boundary, not on an 8-byte boundary.
The crash happens at sha1.c:224 where the 8-byte-alignment is indeed
mandatory on a SPARC architecture:
ctxt->c.b64[0] += copysiz * 8;
The environment is Solaris 10 on SPARCv9. The sources have been compiled
using gcc 4.8.0 for 32 bit.
Andreas.
Timo Sirainen
2013-Aug-28 22:44 UTC
[Dovecot] Bug in dovecot 2.2.5: segfault due to bad alignment
Attached patch fixes this? If yes, I'll commit it. -------------- next part -------------- A non-text attachment was scrubbed... Name: diff Type: application/octet-stream Size: 2705 bytes Desc: not available URL: <http://dovecot.org/pipermail/dovecot/attachments/20130829/49e231b6/attachment-0001.obj> -------------- next part -------------- On 21.8.2013, at 19.32, Andreas F. Borchert <dovecot at andreas-borchert.de> wrote:> Take a look at the sources, hmac.h declares struct hmac_context: > > struct hmac_context { > char ctx[HMAC_MAX_CONTEXT_SIZE]; > char ctxo[HMAC_MAX_CONTEXT_SIZE]; > const struct hash_method *hash; > }; > > If compiled for a 32 bit virtual address space, this has an alignment > requirement of 4 due to the hash pointer. > > In line 171 of auth-token.c, we have following declaration of ctx > as a local variable in auth_token_get(): > > struct hmac_context ctx; > > This is put on an address with an alignment requirement of 4. In > lines 174 and 175 hmac_init is invoked with hash_method_sha1: > > hmac_init(&ctx, (const unsigned char*)username, strlen(username), > &hash_method_sha1); > > In hmac.c, lines 43 and following, ctx->ctx with an alignment of > 4 is passed to meth->init and meth->loop where meth refers to > hash_method_sha1: > > meth->init(ctx->ctx); > meth->loop(ctx->ctx, k_ipad, 64); > > These functions refer now to sha1_init and sha1_loop where the > first parameter is expected to be a pointer to struct sha1_ctxt, > a data structure which is declared in sha1.h: > > struct sha1_ctxt { > union { > uint8_t b8[20]; > uint32_t b32[5]; > } h; > union { > uint8_t b8[8]; > uint64_t b64[1]; > } c; > union { > uint8_t b8[64]; > uint32_t b32[16]; > } m; > uint8_t count; > }; > > Here we have with b64 one uint64_t which has on a SPARC platform > an alignment requirement of 8. In consequence, struct sha1_ctxt > has an alignment requirement of 8. With the invocations of > meth->init and meth->loop above we pass a pointer to a data > structure of alignment 4 to a function expecting a pointer to > a data structure of alignment 8. Chances are that the alignment > requirement is not met, causing a segmentation violation. > > This must be solved by declaring struct hmac_context such that > is not just big enough but respects also the highest alignment > required for one of the hashing data structures. > > There are several options to do this: > > * Beginning with C11, you are free to use an alignment specifier, > i.e. add _Alignas ( uint64_t ) > (see section 6.7.5 in ISO 9899-2011) > > * GCC supports alignment attributes, > i.e. add __attribute__ ((aligned (8))) > or whatever is required instead of 8, > see http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html > > * Do not use a local variable for it, allocate the data structure > using malloc instead. > > If you want to see a live crash, here is the relevant output of gdb > that debugs ${prefix}/libexec/dovecot/auth. > > Program received signal SIGSEGV, Segmentation fault. > sha1_loop (ctxt=0xffbff63c, input=0xffbff548, len=64) at sha1.c:224 > 224 sha1.c: No such file or directory. > (gdb) where > #0 sha1_loop (ctxt=0xffbff63c, input=0xffbff548, len=64) at sha1.c:224 > #1 0xff2e218c in hmac_init (ctx=ctx at entry=0xffbff63c, > key=key at entry=0x6a698 "borchert", key_len=8, > meth=0x555d0 <hash_method_sha1>) at hmac.c:44 > #2 0x00023310 in auth_token_get (service=service at entry=0x6a648 "imap", > session_pid=0x56071 "26272", username=0x6a698 "borchert", > session_id=0x6a650 "AErv6nbk6gB/AAAB") at auth-token.c:174 > #3 0x00021708 in userdb_callback (result=USERDB_RESULT_OK, request=0x6a530) > at auth-request-handler.c:668 > #4 0x0001f144 in auth_request_userdb_callback (result=<optimized out>, > result at entry=USERDB_RESULT_OK, request=request at entry=0x6a530) > at auth-request.c:1039 > #5 0x000312c8 in prefetch_lookup (auth_request=0x6a530, > callback=0x1f058 <auth_request_userdb_callback>) at userdb-prefetch.c:40 > #6 0x0001f37c in auth_request_lookup_user (request=0x6a530, > callback=callback at entry=0x2150c <userdb_callback>) at auth-request.c:1072 > #7 0x00022034 in auth_request_handler_master_request ( > handler=<optimized out>, master=master at entry=0x6b120, id=1292369921, > client_id=1, params=0x55bfc) at auth-request-handler.c:758 > #8 0x0001be98 in master_input_request (args=<optimized out>, conn=0x6b120) > at auth-master-connection.c:127 > #9 auth_master_input_line (line=<optimized out>, conn=0x6b120) > at auth-master-connection.c:598 > #10 master_input (conn=0x6b120) at auth-master-connection.c:653 > #11 0xff2ecca4 in io_loop_call_io (io=io at entry=0x6b398) at ioloop.c:387 > #12 0xff2ed604 in io_loop_handler_run (ioloop=ioloop at entry=0x5e5e8) > at ioloop-poll.c:211 > #13 0xff2ec7a8 in io_loop_run (ioloop=0x5e5e8) at ioloop.c:406 > #14 0xff29ad7c in master_service_run (service=0x5e128, > callback=0x27b40 <client_connected>) at master-service.c:566 > #15 0x0001852c in main (argc=1, argv=0xffbffd54) at main.c:393 > (gdb) print ctxt > $1 = (struct sha1_ctxt *) 0xffbff63c > > As you can see, ctxt is on a 4-byte boundary, not on an 8-byte boundary. > The crash happens at sha1.c:224 where the 8-byte-alignment is indeed > mandatory on a SPARC architecture: > > ctxt->c.b64[0] += copysiz * 8; > > The environment is Solaris 10 on SPARCv9. The sources have been compiled > using gcc 4.8.0 for 32 bit. > > Andreas. >