Re: [PATCH] mac80211: aes-cmac: remove VLA usage

On 03/21/2018 08:48 AM, Johannes Berg wrote:

On Wed, 2018-03-21 at 08:42 -0500, Gustavo A. R. Silva wrote:

quoted

In preparation to enabling -Wvla, remove VLAs and replace them
with dynamic memory allocation instead.

The use of stack Variable Length Arrays needs to be avoided, as they
can be a vector for stack exhaustion, which can be both a runtime bug
or a security flaw. Also, in general, as code evolves it is easy to
lose track of how big a VLA can get. Thus, we can end up having runtime
failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Gustavo A. R. Silva <redacted>
---
  net/mac80211/aes_cmac.c | 36 ++++++++++++++++++++++++------------
  1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/net/mac80211/aes_cmac.c b/net/mac80211/aes_cmac.c
index 2fb6558..c9444bf 100644
--- a/net/mac80211/aes_cmac.c
+++ b/net/mac80211/aes_cmac.c

@@ -27,30 +27,42 @@ static const u8 zero[CMAC_TLEN_256];
  void ieee80211_aes_cmac(struct crypto_shash *tfm, const u8 *aad,
  			const u8 *data, size_t data_len, u8 *mic)
  {
-	SHASH_DESC_ON_STACK(desc, tfm);
+	struct shash_desc *shash;
  	u8 out[AES_BLOCK_SIZE];
  
-	desc->tfm = tfm;
+	shash = kmalloc(sizeof(*shash) + crypto_shash_descsize(tfm),
+			GFP_KERNEL);
+	if (!shash)
+		return;

Honestly, this seems like a really bad idea - you're now hitting
kmalloc for every TX/RX frame here.

SHA_DESC_ON_STACK() should just be fixed to not need a VLA, but take
some sort of maximum, I guess?

SHA_DESC_ON_STACK is currently being used in multiple places. But, yeah, I think we can define multiple macros of the same kind and adjust to the characteristics of each the component.

How big do you think tfm can get?

Thanks
--
Gustavo