• James Yonan's avatar
    crypto: crypto_memneq - add equality testing of memory regions w/o timing leaks · 6bf37e5a
    James Yonan authored
    When comparing MAC hashes, AEAD authentication tags, or other hash
    values in the context of authentication or integrity checking, it
    is important not to leak timing information to a potential attacker,
    i.e. when communication happens over a network.
    Bytewise memory comparisons (such as memcmp) are usually optimized so
    that they return a nonzero value as soon as a mismatch is found. E.g,
    on x86_64/i5 for 512 bytes this can be ~50 cyc for a full mismatch
    and up to ~850 cyc for a full match (cold). This early-return behavior
    can leak timing information as a side channel, allowing an attacker to
    iteratively guess the correct result.
    This patch adds a new method crypto_memneq ("memory not equal to each
    other") to the crypto API that compares memory areas of the same length
    in roughly "constant time" (cache misses could change the timing, but
    since they don't reveal information about the content of the strings
    being compared, they are effectively benign). Iow, best and worst case
    behaviour take the same amount of time to complete (in contrast to
    Note that crypto_memneq (unlike memcmp) can only be used to test for
    equality or inequality, NOT for lexicographical order. This, however,
    is not an issue for its use-cases within the crypto API.
    We tried to locate all of the places in the crypto API where memcmp was
    being used for authentication or integrity checking, and convert them
    over to crypto_memneq.
    crypto_memneq is declared noinline, placed in its own source file,
    and compiled with optimizations that might increase code size disabled
    ("Os") because a smart compiler (or LTO) might notice that the return
    value is always compared against zero/nonzero, and might then
    reintroduce the same early-return optimization that we are trying to
    Using #pragma or __attribute__ optimization annotations of the code
    for disabling optimization was avoided as it seems to be considered
    broken or unmaintained for long time in GCC [1]. Therefore, we work
    around that by specifying the compile flag for memneq.o directly in
    the Makefile. We found that this seems to be most appropriate.
    As we use ("Os"), this patch also provides a loop-free "fast-path" for
    frequently used 16 byte digests. Similarly to kernel library string
    functions, leave an option for future even further optimized architecture
    specific assembler implementations.
    This was a joint work of James Yonan and Daniel Borkmann. Also thanks
    for feedback from Florian Weimer on this and earlier proposals [2].
      [1] http://gcc.gnu.org/ml/gcc/2012-07/msg00211.html
      [2] https://lkml.org/lkml/2013/2/10/131Signed-off-by: default avatarJames Yonan <james@openvpn.net>
    Signed-off-by: default avatarDaniel Borkmann <dborkman@redhat.com>
    Cc: Florian Weimer <fw@deneb.enyo.de>
    Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
algapi.h 10.6 KB