linux-kernel - Re: [PATCH] crypto: x86/twofish-3way

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Message-ID: <20171219223704.x2mrgejbkzkuuqxr@treble>
Date:   Tue, 19 Dec 2017 16:37:04 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Jussi Kivilinna <jussi.kivilinna@....fi>, x86@...nel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage

On Mon, Dec 18, 2017 at 04:40:26PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> 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@...glegroups.com>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Thanks a lot for fixing this!

Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh