Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage

From: Josh Poimboeuf
Date: Tue Dec 19 2017 - 17:37:14 EST

On Mon, Dec 18, 2017 at 04:40:26PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> Using %rbp as a temporary register breaks frame pointer convention and
> breaks stack traces when unwinding from an interrupt in the crypto code.
> In twofish-3way, we can't simply replace %rbp with another register
> because there are none available. Instead, we use the stack to hold the
> values that %rbp, %r11, and %r12 were holding previously. Each of these
> values represents the half of the output from the previous Feistel round
> that is being passed on unchanged to the following round. They are only
> used once per round, when they are exchanged with %rax, %rbx, and %rcx.
> As a result, we free up 3 registers (one per block) and can reassign
> them so that %rbp is not used, and additionally %r14 and %r15 are not
> used so they do not need to be saved/restored.
> There may be a small overhead caused by replacing 'xchg REG, REG' with
> the needed sequence 'mov MEM, REG; mov REG, MEM; mov REG, REG' once per
> round. But, counterintuitively, when I tested "ctr-twofish-3way" on a
> Haswell processor, the new version was actually about 2% faster.
> (Perhaps 'xchg' is not as well optimized as plain moves.)
> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Thanks a lot for fixing this!

Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>