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

+ Sebastian Ramacher

+ <A HREF="mailto:pycrypto%40lists.dlitz.net?Subject=Re%3A%20%5Bpycrypto%5D%20Crypto.Random%20crashes%20due%20to%20unaligned%20access&In-Reply-To=%3C20131027220710.GB11300%40ramacher.at%3E"

+ TITLE="[pycrypto] Crypto.Random crashes due to unaligned access">sebastian+lists at ramacher.at

+ </A>

+ Sun Oct 27 15:07:10 PDT 2013

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+<PRE>Hi,

+On 2013-10-24 09:59:12, 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);

+> >}

+> >

+> >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

+

+I debugged this for a while and the problem is not _mm_loadu_si128.

+That's fine. It generates the correct movdqu instruction for that. The

+problem is the rk[0] = ... part. On amd64, ek and dk from block_state

+get aligned at 16 byte boundaries and everything works out properly.

+However, on i386 this does not appear to be true.

+I have some ideas how to fix it and will hopefully come up with a patch

+next week.

+Regards

+--

+Sebastian Ramacher

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