Bluetooth: Remove VLA usage in aes_cmac

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 <[email protected]>

---
  net/bluetooth/smp.c | 16 +++++++++++-----
  1 file changed, 11 insertions(+), 5 deletions(-)

net/bluetooth/smp.c +11 -5

index a2ddae2..23c694d 100644
@@ -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;