Re: [PATCH v3] crypto: more robust crypto_memneq

[Daniel Borkmann <dborkman@redhat.com>]

On 11/26/2013 01:00 AM, Cesar Eduardo Barros wrote:
> 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.

Actually with yet another version, I hoped that the "compile-tested"-only
statement would eventually disappear, ohh well. ;)

> Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.eti.br>

Resolving the OPTIMIZER_HIDE_VAR() macro for others than GCC jnto a
barrier() seems a bit suboptimal, but assuming 99% of people will use
GCC anyway, then for the minority of the remaining, they will worst case
have a clever compiler and eventually mimic memcmp() in some situations,
or have a not-so-clever compiler and execute the full code as is.

Anyway, I think still better than the rather ugly Makefile workaround
imho, so I'm generally fine with this.