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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 20 Jun 2018 16:44:58 -0700
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Subject: Re: [dm-devel] [PATCH 04/11] dm verity fec: Remove VLA usage
To:     Eric Biggers <ebiggers3@gmail.com>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        Giovanni Cabiddu <giovanni.cabiddu@intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Biggers <ebiggers@google.com>,
        Mike Snitzer <snitzer@redhat.com>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        qat-linux@intel.com, LKML <linux-kernel@vger.kernel.org>,
        dm-devel@redhat.com, linux-crypto <linux-crypto@vger.kernel.org>,
        Lars Persson <larper@axis.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alasdair Kergon <agk@redhat.com>,
        Rabin Vincent <rabinv@axis.com>
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On Wed, Jun 20, 2018 at 4:33 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> On Wed, Jun 20, 2018 at 12:04:01PM -0700, Kees Cook wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> uses the newly defined max digest size macro. Also adds a sanity-check
>> at use-time.
>>
>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>  drivers/md/dm-verity-fec.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
>> index 684af08d0747..0dfcc52835bc 100644
>> --- a/drivers/md/dm-verity-fec.c
>> +++ b/drivers/md/dm-verity-fec.c
>> @@ -212,12 +212,15 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
>>       struct dm_verity_fec_io *fio = fec_io(io);
>>       u64 block, ileaved;
>>       u8 *bbuf, *rs_block;
>> -     u8 want_digest[v->digest_size];
>> +     u8 want_digest[AHASH_MAX_DIGESTSIZE];
>>       unsigned n, k;
>>
>>       if (neras)
>>               *neras = 0;
>>
>> +     if (WARN_ON(v->digest_size < sizeof(want_digest)))
>> +             return -EINVAL;
>> +
>
> This is backwards; it should be 'v->digest_size > sizeof(want_digest)'.

Yikes. Thank you for catching that! I will fix in v2.

-Kees

-- 
Kees Cook
Pixel Security