linux-kernel - Re: [PATCH v3 4/9] dm integrity: Remove VLA usage

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Date:   Fri, 29 Jun 2018 22:43:34 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Lars Persson <larper@...s.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Rabin Vincent <rabinv@...s.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, qat-linux@...el.com,
        dm-devel@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 4/9] dm integrity: Remove VLA usage

On Fri, Jun 29, 2018 at 2:28 AM, Kees Cook <keescook@...omium.org> wrote:

> diff --git a/drivers/md/dm-integrity.c b/drivers/md/dm-integrity.c
> index 86438b2f10dd..85e8ce1625a2 100644
> --- a/drivers/md/dm-integrity.c
> +++ b/drivers/md/dm-integrity.c
> @@ -521,7 +521,12 @@ static void section_mac(struct dm_integrity_c *ic, unsigned section, __u8 result
>                 }
>                 memset(result + size, 0, JOURNAL_MAC_SIZE - size);
>         } else {
> -               __u8 digest[size];
> +               __u8 digest[SHASH_MAX_DIGESTSIZE];
> +
> +               if (WARN_ON(size > sizeof(digest))) {
> +                       dm_integrity_io_error(ic, "digest_size", -EINVAL);
> +                       goto err;
> +               }

I'm still slightly worried that some patches like this one could make
things worse
and lead to an actual stack overflow. You define SHASH_MAX_DIGESTSIZE
as '512', which is still quite a lot to put on the kernel stack. The
function also
uses SHASH_DESC_ON_STACK(), so now you have two copies. Then you
could call shash_final_unaligned(), which seems to put a third copy on
the stack,
so replacing each one with a fixed-size buffer adds quite a bit of bloat.

Is there actually a digest that can be used in dm-integrity with more than 64
byte output (matching JOURNAL_MAC_SIZE) here?

       Arnd