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

Eric Biggers
2024-02-22T06:34:35Z

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")

> + 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?

Otherwise this looks good.

Reviewed-by: Eric Biggers <ebiggers@google.com>

- Eric