tjh.dev / kernel
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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ovpn: tcp - don't deref NULL sk_socket member after tcp_close()

When deleting a peer in case of keepalive expiration, the peer is
removed from the OpenVPN hashtable and is temporary inserted in a
"release list" for further processing.

This happens in:
ovpn_peer_keepalive_work()
unlock_ovpn(release_list)

This processing includes detaching from the socket being used to
talk to this peer, by restoring its original proto and socket
ops/callbacks.

In case of TCP it may happen that, while the peer is sitting in
the release list, userspace decides to close the socket.
This will result in a concurrent execution of:

tcp_close(sk)
__tcp_close(sk)
sock_orphan(sk)
sk_set_socket(sk, NULL)

The last function call will set sk->sk_socket to NULL.

When the releasing routine is resumed, ovpn_tcp_socket_detach()
will attempt to dereference sk->sk_socket to restore its original
ops member. This operation will crash due to sk->sk_socket being NULL.

Fix this race condition by testing-and-accessing
sk->sk_socket atomically under sk->sk_callback_lock.

Link: https://lore.kernel.org/netdev/176996279620.3109699.15382994681575380467@eldamar.lan/
Link: https://github.com/OpenVPN/ovpn-net-next/issues/29
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Fixes: 11851cbd60ea ("ovpn: implement TCP transport")
Link: https://patch.msgid.link/20260212213130.11497-1-antonio@openvpn.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

authored by

Antonio Quartulli and committed by
Paolo Abeni 94560267 e74ff27f

+13 -1
+13 -1
drivers/net/ovpn/tcp.c
··· 199 199 sk->sk_data_ready = peer->tcp.sk_cb.sk_data_ready; 200 200 sk->sk_write_space = peer->tcp.sk_cb.sk_write_space; 201 201 sk->sk_prot = peer->tcp.sk_cb.prot; 202 - sk->sk_socket->ops = peer->tcp.sk_cb.ops; 202 + 203 + /* tcp_close() may race this function and could set 204 + * sk->sk_socket to NULL. It does so by invoking 205 + * sock_orphan(), which holds sk_callback_lock before 206 + * doing the assignment. 207 + * 208 + * For this reason we acquire the same lock to avoid 209 + * sk_socket to disappear under our feet 210 + */ 211 + write_lock_bh(&sk->sk_callback_lock); 212 + if (sk->sk_socket) 213 + sk->sk_socket->ops = peer->tcp.sk_cb.ops; 214 + write_unlock_bh(&sk->sk_callback_lock); 203 215 204 216 rcu_assign_sk_user_data(sk, NULL); 205 217 }