Re: [PATCH] crypto: arm64/neonbs - fix out-of-bounds access on short input

Ard Biesheuvel
2024-02-22T08:19:11Z

On Thu, 22 Feb 2024 at 07:34, Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Sat, Feb 17, 2024 at 05:11:52PM +0100, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > The bit-sliced implementation of AES-CTR operates on blocks of 128
> > bytes, and will fall back to the plain NEON version for tail blocks or
> > inputs that are shorter than 128 bytes to begin with.
> >
> > It will call straight into the plain NEON asm helper, which performs all
> > memory accesses in granules of 16 bytes (the size of a NEON register).
> > For this reason, the associated plain NEON glue code will copy inputs
> > shorter than 16 bytes into a temporary buffer, given that this is a rare
> > occurrence and it is not worth the effort to work around this in the asm
> > code.
> >
> > The fallback from the bit-sliced NEON version fails to take this into
> > account, potentially resulting in out-of-bounds accesses. So clone the
> > same workaround, and use a temp buffer for short in/outputs.
> >
> > Cc: <stable@vger.kernel.org>
> > Reported-by: syzbot+f1ceaa1a09ab891e1934@syzkaller.appspotmail.com
> > Tested-by: syzbot+f1ceaa1a09ab891e1934@syzkaller.appspotmail.com
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>
> Looks like this could use:
>
> Fixes: fc074e130051 ("crypto: arm64/aes-neonbs-ctr - fallback to plain NEON for final chunk")
>

Indeed.

> > + if (unlikely(nbytes < AES_BLOCK_SIZE))
> > + src = dst = memcpy(buf + sizeof(buf) - nbytes,
> > + src, nbytes);
> > +
> > neon_aes_ctr_encrypt(dst, src, ctx->enc, ctx->key.rounds,
> > nbytes, walk.iv);
> > +
> > + if (unlikely(nbytes < AES_BLOCK_SIZE))
> > + memcpy(d, buf + sizeof(buf) - nbytes, nbytes);
>
> The second one could use 'dst' instead of 'buf + sizeof(buf) - nbytes', right?
>

Correct.

> Otherwise this looks good.
>
> Reviewed-by: Eric Biggers <ebiggers@google.com>
>

I'll respin with these changes. Thanks.