Re: [PATCH v7 2/9] crypto: cbc: Remove VLA usage

On Tue, Aug 7, 2018 at 2:47 AM, Herbert Xu [off-list ref] wrote:

On Thu, Aug 02, 2018 at 03:51:45PM -0700, Kees Cook wrote:

quoted

In the quest to remove all stack VLA usage from the kernel[1], this
uses the upper bounds on blocksize. Since this is always a cipher
blocksize, use the existing cipher max blocksize.

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

Signed-off-by: Kees Cook <redacted>
---
 include/crypto/cbc.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/crypto/cbc.h b/include/crypto/cbc.h
index f5b8bfc22e6d..47db0aac2ab9 100644
--- a/include/crypto/cbc.h
+++ b/include/crypto/cbc.h

@@ -113,7 +113,9 @@ static inline int crypto_cbc_decrypt_inplace(
      unsigned int bsize = crypto_skcipher_blocksize(tfm);
      unsigned int nbytes = walk->nbytes;
      u8 *src = walk->src.virt.addr;
-     u8 last_iv[bsize];
+     u8 last_iv[MAX_CIPHER_BLOCKSIZE];
+
+     BUG_ON(bsize > sizeof(last_iv));

Ugh, please don't add these BUG_ONs.  Add them to the places where
the algorithm is created (if they aren't checking that already).

It's already being checked (cra_blocksize vs MAX_CIPHER_BLOCKSIZE) so
I was just adding the BUG_ON to catch "impossible" behavior. I'll
leave it out in the next revision.

-Kees

-- 
Kees Cook
Pixel Security