Re: [PATCH] Bluetooth: Remove VLA usage in aes_cmac

[Gustavo A. R. Silva]

Hi,

I've just discovered an issue in this patch. Please, drop it. I'll send 
v2 shortly.

Thanks
--
Gustavo

On 03/20/2018 06:34 PM, Gustavo A. R. Silva wrote:
In preparation to enabling -Wvla, remove VLA and replace it
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 <garsi...@embeddedor.com>
---
  net/bluetooth/smp.c | 16 +++++++++++-----
  1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index a2ddae2..23c694d 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -173,7 +173,7 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 
k[16], const u8 *m,
                    size_t len, u8 mac[16])
  {
        uint8_t tmp[16], mac_msb[16], msg_msb[CMAC_MSG_MAX];
-       SHASH_DESC_ON_STACK(desc, tfm);
+       struct shash_desc *shash;
        int err;
  
  	if (len > CMAC_MSG_MAX)
@@ -184,8 +184,13 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 
k[16], const u8 *m,
                return -EINVAL;
        }
  
-	desc->tfm = tfm;
-       desc->flags = 0;
+       shash = kzalloc(sizeof(*shash) + crypto_shash_descsize(tfm),
+                       GFP_KERNEL);
+       if (!shash)
+               return -ENOMEM;
+
+       shash->tfm = tfm;
+       shash->flags = 0;
  
  	/* Swap key and message from LSB to MSB */
        swap_buf(k, tmp, 16);
@@ -200,8 +205,9 @@ static int aes_cmac(struct crypto_shash *tfm, const u8 
k[16], const u8 *m,
                return err;
        }
  
-	err = crypto_shash_digest(desc, msg_msb, len, mac_msb);
-       shash_desc_zero(desc);
+       err = crypto_shash_digest(shash, msg_msb, len, mac_msb);
+       shash_desc_zero(shash);
+       kfree(shash);
        if (err) {
                BT_ERR("Hash computation error %d", err);
                return err;