• Cesar Eduardo Barros's avatar
    crypto: more robust crypto_memneq · fe8c8a12
    Cesar Eduardo Barros authored
    Disabling compiler optimizations can be fragile, since a new
    optimization could be added to -O0 or -Os that breaks the assumptions
    the code is making.
    
    Instead of disabling compiler optimizations, use a dummy inline assembly
    (based on RELOC_HIDE) to block the problematic kinds of optimization,
    while still allowing other optimizations to be applied to the code.
    
    The dummy inline assembly is added after every OR, and has the
    accumulator variable as its input and output. The compiler is forced to
    assume that the dummy inline assembly could both depend on the
    accumulator variable and change the accumulator variable, so it is
    forced to compute the value correctly before the inline assembly, and
    cannot assume anything about its value after the inline assembly.
    
    This change should be enough to make crypto_memneq work correctly (with
    data-independent timing) even if it is inlined at its call sites. That
    can be done later in a followup patch.
    
    Compile-tested on x86_64.
    Signed-off-by: default avatarCesar Eduardo Barros <cesarb@cesarb.eti.br>
    Acked-by: default avatarDaniel Borkmann <dborkman@redhat.com>
    Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
    fe8c8a12
memneq.c 6.15 KB