Re: [dm-devel] [PATCH v5 05/11] crypto: ahash: Remove VLA usage

On Tue, Jul 17, 2018 at 9:39 AM, Eric Biggers [off-list ref] wrote:

On Mon, Jul 16, 2018 at 09:21:44PM -0700, Kees Cook wrote:

quoted

In the quest to remove all stack VLA usage from the kernel[1], this
introduces max size macros for ahash, as already done for shash, and
adjust the crypto user to max state size.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <redacted>
---
 crypto/ahash.c        | 4 ++--
 crypto/algif_hash.c   | 2 +-
 include/crypto/hash.h | 3 +++
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/crypto/ahash.c b/crypto/ahash.c
index a64c143165b1..6435bdbe42fd 100644
--- a/crypto/ahash.c
+++ b/crypto/ahash.c

@@ -550,8 +550,8 @@ static int ahash_prepare_alg(struct ahash_alg *alg)
 {
      struct crypto_alg *base = &alg->halg.base;

-     if (alg->halg.digestsize > PAGE_SIZE / 8 ||
-         alg->halg.statesize > PAGE_SIZE / 8 ||
+     if (alg->halg.digestsize > AHASH_MAX_DIGESTSIZE ||
+         alg->halg.statesize > AHASH_MAX_STATESIZE ||
          alg->halg.statesize == 0)
              return -EINVAL;

diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
index bfcf595fd8f9..8974ee8ebead 100644
--- a/crypto/algif_hash.c
+++ b/crypto/algif_hash.c

@@ -239,7 +239,7 @@ static int hash_accept(struct socket *sock, struct socket *newsock, int flags,
      struct alg_sock *ask = alg_sk(sk);
      struct hash_ctx *ctx = ask->private;
      struct ahash_request *req = &ctx->req;
-     char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req)) ? : 1];
+     char state[AHASH_MAX_STATESIZE];
      struct sock *sk2;
      struct alg_sock *ask2;
      struct hash_ctx *ctx2;

diff --git a/include/crypto/hash.h b/include/crypto/hash.h
index ae14cc0e0cdb..4fcd0e2368cd 100644
--- a/include/crypto/hash.h
+++ b/include/crypto/hash.h

@@ -64,6 +64,9 @@ struct ahash_request {
      void *__ctx[] CRYPTO_MINALIGN_ATTR;
 };

+#define AHASH_MAX_DIGESTSIZE 512
+#define AHASH_MAX_STATESIZE  512
+

Why is AHASH_MAX_DIGESTSIZE (512) so much larger than SHASH_MAX_DIGESTSIZE (64)?
I would have expected them to be the same.

This was a direct replacement of the PAGE_SIZE / 8 original limit.
This this didn't trip any frame size warnings, it seemed like we
didn't need to do the more narrow limitations done elsewhere.

I am, of course, happy to do a manual review and find a lower
alternative if that's desired.

-Kees

-- 
Kees Cook
Pixel Security