[pycrypto] Crypto.Random crashes due to unaligned access

Andrew Cooper andyhhp at gmail.com
Thu Oct 24 13:10:29 PDT 2013

On 24/10/2013 17:59, Dwayne Litzenberger wrote:
> Hi Greg!
> What version/build of GCC is this?  Does "setup.py test" crash for you as well?
> I'd rather figure out how to fix the problem than to start making copies of the key.
> Greg Price <gnprice at gmail.com> wrote:
>> I get the following crash in a PyCrypto built from the current master,
>> af058ee (aka v2.6.1-136-gaf058ee):
>>>>> import Crypto.Random
>>>>> Crypto.Random.new().read(1)
>> Segmentation fault (core dumped)
>> This is on i686.  I compiled with GCC 4.6.3 (or "Ubuntu/Linaro
>> 4.6.3-1ubuntu5".)
>> GDB shows the crash is here:
>> Program received signal SIGSEGV, Segmentation fault.
>> aes_key_setup_enc (keylen=32, cipherKey=
>> 0x841b1bc
>> "L\fB2\244\225\235\206^\242\305\305b\201\200\335ņ{d\240\343\262;m\361\243\276u~\337&",
>> rk=
>>    0x84900a8) at src/AESNI.c:122
>> 122            rk[0] = _mm_loadu_si128((const __m128i*) cipherKey);
>> at which the instruction is
>> (gdb) x/i $pc
>> => 0xb78f2600 <ALGnew+2160>: movdqa %xmm0,0x40(%esi)
>> This is an aligned store.  The documentation of MOVDQA says it should
>> be 16-byte aligned.  The value of rk (aka %esi + 0x40) is only 8-byte
>> aligned:
>> (gdb) p rk
>> $5 = (__m128i *) 0x84900a8
>> (gdb) p/x $esi
>> $9 = 0x8490068
>> It's not clear to me why GCC generated an aligned instruction here --
>> in fact, the definition of _mm_loadu_si128 in my emmintrin.h appears
>> to be
>> extern __inline __m128i __attribute__((__gnu_inline__,
>> __always_inline__, __artificial__))
>> _mm_loadu_si128 (__m128i const *__P)
>> {
>>  return (__m128i) __builtin_ia32_loaddqu ((char const *)__P);
>> }

http://gcc.gnu.org/onlinedocs/gcc/X86-Built_002din-Functions.html  would
suggest that __builtin_ia32_loaddqu is specified to generate movdqu
rather than movdqa.

There is certainly nothing I can see in the code which would allow GCC
to presume that the address is 16  byte aligned.


>> and the name of that builtin sure sounds more like MOVDQU than MOVDQA.
>> Perhaps GCC somehow decides that it can prove the pointer is aligned
>> here.
>> I don't know why GCC makes this mistake, or (since it's never the
>> compiler's fault) which code is lying to it about something being
>> aligned. Anyone know how to investigate this kind of question?
>> A workaround would be to make sure that the cipherKey argument to
>> aes_key_setup_enc() in src/AESNI.c is always 16-byte aligned.  At
>> present, that argument comes straight from the first Python-level
>> argument to _AESNI.new(); see the PyArg_ParseTupleAndKeywords() call
>> in src/block_template.c.  I guess to implement this workaround we'd
>> copy the key to a new, aligned buffer if it's not aligned.
>> I can send a patch for that workaround if it seems like the best
>> approach.  Happy to hear alternatives, and of course it'd be most
>> satisfying if we can understand why the compiler is emitting this
>> output in the first place.
>> Greg

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