linux-kernel - Re: [PATCH] crypto: skcipher - set walk.iv for zero-length inputs

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Message-ID: <20171211114513.GD12014@gondor.apana.org.au>
Date:   Mon, 11 Dec 2017 22:45:13 +1100
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-crypto@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] crypto: skcipher - set walk.iv for zero-length inputs

On Wed, Nov 29, 2017 at 01:18:57AM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> All the ChaCha20 algorithms as well as the ARM bit-sliced AES-XTS
> algorithms call skcipher_walk_virt(), then access the IV (walk.iv)
> before checking whether any bytes need to be processed (walk.nbytes).
> 
> But if the input is empty, then skcipher_walk_virt() doesn't set the IV,
> and the algorithms crash trying to use the uninitialized IV pointer.
> 
> Fix it by setting the IV earlier in skcipher_walk_virt().  Also fix it
> for the AEAD walk functions.
> 
> This isn't a perfect solution because we can't actually align the IV to
> ->cra_alignmask unless there are bytes to process, for one because the
> temporary buffer for the aligned IV is freed by skcipher_walk_done(),
> which is only called when there are bytes to process.  Thus, algorithms
> that require aligned IVs will still need to avoid accessing the IV when
> walk.nbytes == 0.  Still, many algorithms/architectures are fine with
> IVs having any alignment, and even for those that aren't, a misaligned
> pointer bug is much less severe than an uninitialized pointer bug.
> 
> This change also matches the behavior of the older blkcipher_walk API.
> 
> Fixes: 0cabf2af6f5a ("crypto: skcipher - Fix crash on zero-length input")
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Cc: <stable@...r.kernel.org> # v4.14+
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt