linux-kernel - Re: [PATCH] crypto: x86/aes - Don't use %rbp as temporary register

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Message-ID: <20170519015632.byuezk5ky22rqnkt@treble>
Date:   Thu, 18 May 2017 20:56:32 -0500
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>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] crypto: x86/aes - Don't use %rbp as temporary register

On Wed, May 17, 2017 at 03:21:41PM -0700, Eric Biggers wrote:
> On Wed, May 17, 2017 at 03:44:27PM -0500, Josh Poimboeuf wrote:
> > On Tue, May 16, 2017 at 09:03:08PM -0700, Eric Biggers wrote:
> > > From: Eric Biggers <ebiggers@...gle.com>
> > > 
> > > When using the "aes-asm" implementation of AES (*not* the AES-NI
> > > implementation) on an x86_64, v4.12-rc1 kernel with lockdep enabled, the
> > > following warning was reported, along with a long unwinder dump:
> > > 
> > > 	WARNING: kernel stack regs at ffffc90000643558 in kworker/u4:2:155 has bad 'bp' value 000000000000001c
> > > 
> > > The problem is that aes_enc_block() and aes_dec_block() use %rbp as a
> > > temporary register, which breaks stack traces if an interrupt occurs.
> > > 
> > > Fix this by replacing %rbp with %r9, which was being used to hold the
> > > saved value of %rbp.  This required rearranging the AES round macro
> > > slightly since %r9d cannot be used as the target of a move from %ah-%dh.
> > > 
> > > Performance is essentially unchanged --- actually about 0.2% faster than
> > > before.  Interestingly, I also measured aes-generic as being nearly 7%
> > > faster than aes-asm, so perhaps aes-asm has outlived its usefulness...
> > > 
> > > Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> > 
> > Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > 
> 
> Hmm, it looks like a number of other algorithms in arch/x86/crypto/ use %rbp (or
> %ebp), e.g. blowfish, camellia, cast5, and aes-i586.  Presumably they have the
> same problem.  I'm a little confused: do these all need to be fixed, and
> when/why did this start being considered broken?

This warning was only recently added, with the goal of flushing out
these types of issues with hand-coded asm to make frame pointer based
stack traces more reliable.  I can take a look at fixing the rest of
them if you want.

-- 
Josh