mac80211: fix failure to check kmalloc return value in key_key_read

I noticed two small issues in mac80211/debugfs_key.c::key_key_read while
reading through the code. Patch below.

The key_key_read() function returns ssize_t and the value that's actually
returned is the return value of simple_read_from_buffer() which also
returns ssize_t, so let's hold the return value in a ssize_t local
variable rather than a int one.

Also, memory is allocated dynamically with kmalloc() which can fail, but
the return value of kmalloc() is not checked, so we may end up operating
on a null pointer further on. So check for a NULL return and bail out with
-ENOMEM in that case.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/debugfs_key.c

@@ -203,9 +203,13 @@ static ssize_t key_key_read(struct file *file, char __user *userbuf,
 			    size_t count, loff_t *ppos)
 {
 	struct ieee80211_key *key = file->private_data;
-	int i, res, bufsize = 2 * key->conf.keylen + 2;
+	int i, bufsize = 2 * key->conf.keylen + 2;
 	char *buf = kmalloc(bufsize, GFP_KERNEL);
 	char *p = buf;
+	ssize_t res;
+
+	if (!buf)
+		return -ENOMEM;
 
 	for (i = 0; i < key->conf.keylen; i++)
 		p += scnprintf(p, bufsize + buf - p, "%02x", key->conf.key[i]);