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+ <H1>[pycrypto] Crypto.Random crashes due to unaligned access</H1>

+ Andrew Cooper

+ <A HREF="mailto:pycrypto%40lists.dlitz.net?Subject=Re%3A%20%5Bpycrypto%5D%20Crypto.Random%20crashes%20due%20to%20unaligned%20access&In-Reply-To=%3C52697EB5.7070106%40gmail.com%3E"

+ TITLE="[pycrypto] Crypto.Random crashes due to unaligned access">andyhhp at gmail.com

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+ Thu Oct 24 13:10:29 PDT 2013

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+<PRE>

+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 <<A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">gnprice at gmail.com</A>> 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);

+>> }

+

+<A HREF="http://gcc.gnu.org/onlinedocs/gcc/X86-Built_002din-Functions.html">http://gcc.gnu.org/onlinedocs/gcc/X86-Built_002din-Functions.html</A> 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.

+~Andrew

+>>

+>> 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

+

+</PRE>

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