Re: [PATCH v6 12/18] drbd: Convert from ahash to shash

[Kees Cook]

On Fri, Aug 3, 2018 at 6:44 AM, Lars Ellenberg
<lars.ellenberg@xxxxxxxxxx> wrote:
> On Thu, Aug 02, 2018 at 04:43:19PM -0700, Kees Cook wrote:
>> On Tue, Jul 24, 2018 at 9:49 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> > In preparing to remove all stack VLA usage from the kernel[1], this
>> > removes the discouraged use of AHASH_REQUEST_ON_STACK in favor of
>> > the smaller SHASH_DESC_ON_STACK by converting from ahash-wrapped-shash
>> > to direct shash. By removing a layer of indirection this both improves
>> > performance and reduces stack usage. The stack allocation will be made
>> > a fixed size in a later patch to the crypto subsystem.
>>
>> Philipp or Lars, what do you think of this? Can this go via your tree
>> or maybe via Jens?
>>
>> I'd love an Ack or Review.
>
> I'm sorry, summer time and all, limited online time ;-)
>
> I'm happy for this to go in via Jens, or any other way.
>
> Being not that fluent in the crypto stuff,
> I will just "believe" most of it.
>
> I still think I found something, comments below:
>
>> > diff --git a/drivers/block/drbd/drbd_worker.c b/drivers/block/drbd/drbd_worker.c
>> > index 1476cb3439f4..62dd3700dd84 100644
>> > --- a/drivers/block/drbd/drbd_worker.c
>> > +++ b/drivers/block/drbd/drbd_worker.c
>> > @@ -295,60 +295,54 @@ void drbd_request_endio(struct bio *bio)
>> >                 complete_master_bio(device, &m);
>> >  }
>> >
>> > -void drbd_csum_ee(struct crypto_ahash *tfm, struct drbd_peer_request *peer_req, void *digest)
>> > +void drbd_csum_ee(struct crypto_shash *tfm, struct drbd_peer_request *peer_req, void *digest)
>> >  {
>> > -       AHASH_REQUEST_ON_STACK(req, tfm);
>> > -       struct scatterlist sg;
>> > +       SHASH_DESC_ON_STACK(desc, tfm);
>> >         struct page *page = peer_req->pages;
>> >         struct page *tmp;
>> >         unsigned len;
>> >
>> > -       ahash_request_set_tfm(req, tfm);
>> > -       ahash_request_set_callback(req, 0, NULL, NULL);
>> > +       desc->tfm = tfm;
>> > +       desc->flags = 0;
>> >
>> > -       sg_init_table(&sg, 1);
>> > -       crypto_ahash_init(req);
>> > +       crypto_shash_init(desc);
>> >
>> >         while ((tmp = page_chain_next(page))) {
>> >                 /* all but the last page will be fully used */
>> > -               sg_set_page(&sg, page, PAGE_SIZE, 0);
>> > -               ahash_request_set_crypt(req, &sg, NULL, sg.length);
>> > -               crypto_ahash_update(req);
>> > +               crypto_shash_update(desc, page_to_virt(page), PAGE_SIZE);
>
> These pages may be "highmem", so page_to_virt() seem not appropriate.
> I think the crypto_ahash_update() thingy did an implicit kmap() for us
> in the crypto_hash_walk()?
> Maybe it is good enough to add a kmap() here,
> and a kunmap() on the next line?

Oooh, excellent point. Thanks, I'll see what I can do to sort this out.

-Kees

-- 
Kees Cook
Pixel Security