[PATCH net 5/6] ovpn: fix use after free in unlock_ovpn()

[Antonio Quartulli <antonio@openvpn.net>]

From: Marco Baffo <marco@mandelbit.com>

unlock_ovpn() iterates over the release_list using llist_for_each_entry()
and drops the peer reference inside the loop body via ovpn_peer_put().

If this drops the last reference, the peer is eventually freed. However,
llist_for_each_entry() reads peer->release_entry.next in the loop advance
expression, which runs after the body. By that time the peer may have
already been freed, resulting in a use after free when advancing to the
next list entry.

Fix this by using llist_for_each_entry_safe(), which caches the next
pointer before executing the loop body.

Fixes: 80747caef33d ("ovpn: introduce the ovpn_peer object")
Signed-off-by: Marco Baffo <marco@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
---
 drivers/net/ovpn/peer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ovpn/peer.c b/drivers/net/ovpn/peer.c
index 2b6096d8b1cc..8fdbb5050690 100644
--- a/drivers/net/ovpn/peer.c
+++ b/drivers/net/ovpn/peer.c
@@ -26,11 +26,12 @@ static void unlock_ovpn(struct ovpn_priv *ovpn,
 			 struct llist_head *release_list)
 	__releases(&ovpn->lock)
 {
-	struct ovpn_peer *peer;
+	struct ovpn_peer *peer, *next;
 
 	spin_unlock_bh(&ovpn->lock);
 
-	llist_for_each_entry(peer, release_list->first, release_entry) {
+	llist_for_each_entry_safe(peer, next, release_list->first,
+				  release_entry) {
 		ovpn_socket_release(peer);
 		ovpn_peer_put(peer);
 	}
-- 
2.54.0