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

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.

Signed-off-by: Marco Baffo <[email protected]>
---
 drivers/net/ovpn/peer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ovpn/peer.c b/drivers/net/ovpn/peer.c
index c02dfab51a6e..ff7c6ce9fcad 100644
--- a/drivers/net/ovpn/peer.c
+++ b/drivers/net/ovpn/peer.c
@@ -26,11 +26,11 @@ 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);
 	}
-- 
2.43.0