[PATCH net] mptcp: pm: Fix uaf in __timer_delete_sync

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

#syz test

Fixes: 00cfd77b9063 ("mptcp: retransmit ADD_ADDR when timeout")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/pm_netlink.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index f891bc714668..792b5f09a915 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -332,17 +332,21 @@ struct mptcp_pm_add_entry *
 mptcp_pm_del_add_timer(struct mptcp_sock *msk,
 		       const struct mptcp_addr_info *addr, bool check_id)
 {
-	struct mptcp_pm_add_entry *entry;
 	struct sock *sk = (struct sock *)msk;
+	struct timer_list *add_timer = NULL;
+	struct mptcp_pm_add_entry *entry;
 
 	spin_lock_bh(&msk->pm.lock);
 	entry = mptcp_lookup_anno_list_by_saddr(msk, addr);
-	if (entry && (!check_id || entry->addr.id == addr->id))
+	if (entry && (!check_id || entry->addr.id == addr->id)) {
 		entry->retrans_times = ADD_ADDR_RETRANS_MAX;
+		add_timer = &entry->add_timer;
+	}
 	spin_unlock_bh(&msk->pm.lock);
 
-	if (entry && (!check_id || entry->addr.id == addr->id))
-		sk_stop_timer_sync(sk, &entry->add_timer);
+	/* no lock, because sk_stop_timer_sync() is calling del_timer_sync() */
+	if (add_timer)
+		sk_stop_timer_sync(sk, add_timer);
 
 	return entry;
 }
-- 
2.45.2