Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

On Fri, Jul 13, 2018 at 3:14 AM, Arnd Bergmann [off-list ref] wrote:

On Fri, Jul 13, 2018 at 8:00 AM, Kees Cook [off-list ref] wrote:

quoted

On Thu, Jul 12, 2018 at 10:20 PM, Herbert Xu
[off-list ref] wrote:

quoted

On Thu, Jul 12, 2018 at 10:17:29PM -0700, Kees Cook wrote:

quoted

What is the correct value to use for AHASH_REQUEST_ON_STACK?

As I said to arrive at a fixed value you should examine all sync
ahash algorithms (e.g., all shash ones plus ahash ones marked as
sync if there are any).

The "value" for the ahash I understand: it has a request size
(tfm->reqsize, as set by crypto_ahash_set_reqsize()) what should be
used to measure the shash value? (And how does this relate to the
value returned by crypto_ahash_reqsize()?) The closest clue I can find
is this:

crypto_init_shash_ops_async() does:
        crt->reqsize = sizeof(struct shash_desc) + crypto_shash_descsize(shash);

and that gets called from crypto_ahash_init_tfm(), so if it starts
with the above reqsize and adds to it with a call to
crypto_ahash_set_reqsize() later, we'll have that maximum?

So, do I want to calculate this answer as:

sizeof(struct shash_desc) + max(shash descsize) + max(ahash-sync reqsize) ?
16 + 360 + 0

I arrived at the same number, looking at all the sizes in shash,
The largest I found are sha3_state (360 bytes) and s390_sha_ctx
(336 bytes), everything else is way smaller.

Excellent. Thanks for double-checking this. :)

quoted

It's 0 above because if I look at all the callers of
crypto_ahash_set_reqsize() that do wrapping, all are ASYNC.

So, should this really just be 376? Where is best to validate this
size, as it seems checking in crypto_ahash_set_reqsize() is
inappropriate?

How about crypto_init_shash_ops_async()?

Ah yes, that looks good. Nice find!

After my ahash to shash conversions, only ccm is left as an ahash
user, since it actually uses sg. But with the hard-coded value reduced
to 376, this doesn't trip the frame warnings any more. :)

I'll send an updated series soon.

-Kees

-- 
Kees Cook
Pixel Security