[dm-devel] [PATCH v8 7/9] crypto: qat: Remove VLA usage

[Arnd Bergmann]

On Tue, Aug 7, 2018 at 11:18 PM Kees Cook <keescook at chromium.org> wrote:
>
> In the quest to remove all stack VLA usage from the kernel[1], this uses
> the new upper bound for the stack buffer. Also adds a sanity check.
>
> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>
> Signed-off-by: Kees Cook <keescook at chromium.org>

After rebasing to linux-next, I now get a warning about this file:

drivers/crypto/qat/qat_common/qat_algs.c: In function 'qat_alg_do_precomputes':
drivers/crypto/qat/qat_common/qat_algs.c:257:1: error: the frame size
of 1112 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

I assume it was already possible to get into that state with the VLA,
but it seems bad enough that I think we need to do something
about it.

The large stack variables add up to 1084 bytes, which fully explains
how we got here:

        SHASH_DESC_ON_STACK(shash, ctx->hash_tfm); /* 360 */
        struct sha1_state sha1;  /* 92 */
        struct sha256_state sha256; /* 104 */
        struct sha512_state sha512; /* 208 */
        char ipad[MAX_ALGAPI_BLOCKSIZE]; /* 160 */
        char opad[MAX_ALGAPI_BLOCKSIZE]; /* 160 */

The question is what we can do about it. One simple step I've tried
is to move the sha1/sha256/sha512 into a union, which saves around
200 bytes and should bring us (slightly) below the warning
limit, but I suspect we can do better than that. Any ideas?

        Arnd