Re: [dm-devel] [PATCH 04/11] dm verity fec: Remove VLA usage

[Kees Cook]

On Wed, Jun 20, 2018 at 4:33 PM, Eric Biggers <ebiggers3@xxxxxxxxx> 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@xxxxxxxxxxxxxx
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  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