irda: prevent integer underflow in IRLMP_ENUMDEVICES

If the user-provided len is less than the expected offset, the
IRLMP_ENUMDEVICES getsockopt will do a copy_to_user() with a very large
size value.  While this isn't be a security issue on x86 because it will
get caught by the access_ok() check, it may leak large amounts of kernel
heap on other architectures.  In any event, this patch fixes it.
Signed-off-by: NDan Rosenberg <drosenberg@vsecurity.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/irda/af_irda.c

@@ -2281,6 +2281,16 @@ static int irda_getsockopt(struct socket *sock, int level, int optname,
 
 	switch (optname) {
 	case IRLMP_ENUMDEVICES:
+
+		/* Offset to first device entry */
+		offset = sizeof(struct irda_device_list) -
+			sizeof(struct irda_device_info);
+
+		if (len < offset) {
+			err = -EINVAL;
+			goto out;
+		}
+
 		/* Ask lmp for the current discovery log */
 		discoveries = irlmp_get_discoveries(&list.len, self->mask.word,
 						    self->nslots);
@@ -2291,15 +2301,9 @@ static int irda_getsockopt(struct socket *sock, int level, int optname,
 		}
 
 		/* Write total list length back to client */
-		if (copy_to_user(optval, &list,
-				 sizeof(struct irda_device_list) -
-				 sizeof(struct irda_device_info)))
+		if (copy_to_user(optval, &list, offset))
 			err = -EFAULT;
 
-		/* Offset to first device entry */
-		offset = sizeof(struct irda_device_list) -
-			sizeof(struct irda_device_info);
-
 		/* Copy the list itself - watch for overflow */
 		if (list.len > 2048) {
 			err = -EINVAL;