[PATCH ovpn net v2 1/9] ovpn: skip rehash for peers already removed from by_id

From: Antonio Quartulli <[email protected]>

ovpn_nl_peer_set_doit() resolves the target peer via
ovpn_peer_get_by_id() before taking ovpn->lock. In the window between
the lookup (which only takes a refcount) and the subsequent
spin_lock_bh(&ovpn->lock), a concurrent OVPN_CMD_PEER_DEL, keepalive
expiry, or socket teardown can take ovpn->lock first, run
ovpn_peer_remove() to unhash the peer from all four tables (by_id,
by_vpn_addr4/6, by_transp_addr) and release the lock. set_doit then
acquires ovpn->lock and calls ovpn_peer_hash_vpn_ip(), which
re-inserts the now-removed peer back into the rehashing tables.

The same race affects the float path: ovpn_peer_endpoints_update()
holds only a refcount and acquires ovpn->lock very late (after async
AEAD decrypt and a netlink notification), then rehashes the peer
in the by_transp_addr table.

The resurrected peer becomes reachable again from the RX lookup
(ovpn_peer_get_by_transp_addr) and the TX VPN-IP lookup, even though
userspace believes it is gone. Once the data-path refcount drops the
peer is freed via call_rcu while the hash entries embedded in it
remain linked, opening a UAF window.

Bail out of the rehash when hash_entry_id is unhashed, mirroring
the sentinel already used by ovpn_peer_remove() to detect the
already-removed state. The check is safe under ovpn->lock, which
serializes every mutation of hash_entry_id, and is a no-op for the
add path because ovpn_peer_add_mp() inserts hash_entry_id before
calling ovpn_peer_hash_vpn_ip().

Fixes: 1d36a36f6d53 ("ovpn: implement peer add/get/dump/delete via netlink")
Signed-off-by: Antonio Quartulli <[email protected]>
---
Changes since v1:
* simplified flow in ovpn_peer_endpoints_update() and introduced new
  unlock2 label
---
 drivers/net/ovpn/peer.c | 73 ++++++++++++++++++++++++-----------------
 1 file changed, 43 insertions(+), 30 deletions(-)

diff --git a/drivers/net/ovpn/peer.c b/drivers/net/ovpn/peer.c
index a09d61296425..c855435edc46 100644
--- a/drivers/net/ovpn/peer.c
+++ b/drivers/net/ovpn/peer.c
@@ -296,40 +296,46 @@ void ovpn_peer_endpoints_update(struct ovpn_peer *peer, struct sk_buff *skb)
 	/* rehashing is required only in MP mode as P2P has one peer
 	 * only and thus there is no hashtable
 	 */
-	if (peer->ovpn->mode == OVPN_MODE_MP) {
-		spin_lock_bh(&peer->ovpn->lock);
-		spin_lock_bh(&peer->lock);
-		bind = rcu_dereference_protected(peer->bind,
-						 lockdep_is_held(&peer->lock));
-		if (unlikely(!bind)) {
-			spin_unlock_bh(&peer->lock);
-			spin_unlock_bh(&peer->ovpn->lock);
-			return;
-		}
+	if (peer->ovpn->mode != OVPN_MODE_MP)
+		return;
 
-		/* This function may be invoked concurrently, therefore another
-		 * float may have happened in parallel: perform rehashing
-		 * using the peer->bind->remote directly as key
-		 */
+	spin_lock_bh(&peer->ovpn->lock);
+	spin_lock_bh(&peer->lock);
+	bind = rcu_dereference_protected(peer->bind,
+					 lockdep_is_held(&peer->lock));
+	if (unlikely(!bind))
+		goto unlock2;
 
-		switch (bind->remote.in4.sin_family) {
-		case AF_INET:
-			salen = sizeof(*sa);
-			break;
-		case AF_INET6:
-			salen = sizeof(*sa6);
-			break;
-		}
+	/* peer may have been concurrently removed between the caller's
+	 * initial lookup and our acquisition of ovpn->lock; skip the
+	 * rehash so we don't re-insert a removed peer
+	 */
+	if (unlikely(hlist_unhashed(&peer->hash_entry_id)))
+		goto unlock2;
 
-		/* remove old hashing */
-		hlist_nulls_del_init_rcu(&peer->hash_entry_transp_addr);
-		/* re-add with new transport address */
-		nhead = ovpn_get_hash_head(peer->ovpn->peers->by_transp_addr,
-					   &bind->remote, salen);
-		hlist_nulls_add_head_rcu(&peer->hash_entry_transp_addr, nhead);
-		spin_unlock_bh(&peer->lock);
-		spin_unlock_bh(&peer->ovpn->lock);
+	/* This function may be invoked concurrently, therefore another
+	 * float may have happened in parallel: perform rehashing
+	 * using the peer->bind->remote directly as key
+	 */
+
+	switch (bind->remote.in4.sin_family) {
+	case AF_INET:
+		salen = sizeof(*sa);
+		break;
+	case AF_INET6:
+		salen = sizeof(*sa6);
+		break;
 	}
+
+	/* remove old hashing */
+	hlist_nulls_del_init_rcu(&peer->hash_entry_transp_addr);
+	/* re-add with new transport address */
+	nhead = ovpn_get_hash_head(peer->ovpn->peers->by_transp_addr,
+				   &bind->remote, salen);
+	hlist_nulls_add_head_rcu(&peer->hash_entry_transp_addr, nhead);
+unlock2:
+	spin_unlock_bh(&peer->lock);
+	spin_unlock_bh(&peer->ovpn->lock);
 	return;
 unlock:
 	spin_unlock_bh(&peer->lock);
@@ -905,6 +911,13 @@ void ovpn_peer_hash_vpn_ip(struct ovpn_peer *peer)
 	if (peer->ovpn->mode != OVPN_MODE_MP)
 		return;
 
+	/* peer may have been concurrently removed between the caller's
+	 * initial lookup and our acquisition of ovpn->lock; skip the
+	 * rehash so we don't re-insert a removed peer
+	 */
+	if (hlist_unhashed(&peer->hash_entry_id))
+		return;
+
 	if (peer->vpn_addrs.ipv4.s_addr != htonl(INADDR_ANY)) {
 		/* remove potential old hashing */
 		hlist_nulls_del_init_rcu(&peer->hash_entry_addr4);
-- 
2.53.0