bna: integer overflow bug in debugfs

commit 13e2d518 upstream.

We could allocate less memory than intended because we do:

	bnad->regdata = kzalloc(len << 2, GFP_KERNEL);

The shift can overflow leading to a crash.  This is debugfs code so the
impact is very small.

Fixes: 7afc5dbd ("bna: Add debugfs interface.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rasesh Mody <rasesh.mody@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/ethernet/brocade/bna/bnad_debugfs.c

@@ -331,7 +331,7 @@ bnad_debugfs_write_regrd(struct file *file, const char __user *buf,
 	}
 
 	rc = sscanf(kern_buf, "%x:%x", &addr, &len);
-	if (rc < 2) {
+	if (rc < 2 || len > UINT_MAX >> 2) {
 		pr_warn("bna %s: Failed to read user buffer\n",
 			pci_name(bnad->pcidev));
 		kfree(kern_buf);