linux-kernel - Re: [PATCH v8 7/9] crypto: qat: Remove VLA usage

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Message-ID: <CAK8P3a3kiH-a+JG8FqUw7nc_hk+9z-FdsGFT57SxR83W32zKzQ@mail.gmail.com>
Date:   Tue, 25 Sep 2018 18:12:10 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Eric Biggers <ebiggers@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Will Deacon <will.deacon@....com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Matthew Wilcox <willy@...radead.org>,
        David Miller <davem@...emloft.net>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, dm-devel@...hat.com,
        qat-linux@...el.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 7/9] crypto: qat: Remove VLA usage

On Tue, Aug 7, 2018 at 11:18 PM Kees Cook <keescook@...omium.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@...omium.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