[PATCH net v2] net/x25: fix use-after-free of the socket by its timers

[PATCH net v2] net/x25: fix use-after-free of the socket by its timers

[Baul Lee]

The x25 timers are armed with mod_timer() and cancelled with
timer_delete(), so a pending timer holds no reference on the socket and a
cancel does not wait for a callback already running on another CPU.

x25_heartbeat_expiry() also rearms unconditionally, so it can reinstall
sk->sk_timer after __x25_destroy_socket() has passed its cancel point.
The following __sock_put() frees the socket while the timer is still
queued, and the next expiry uses freed memory.  KASAN reports a
slab-use-after-free on the kmalloc-2k object freed by close().

timer_delete_sync() cannot be used here: x25_heartbeat_expiry() and
x25_timer_expiry() both reach the cancels from inside the timer they
would wait on, through __x25_destroy_socket() and x25_disconnect().

Arm the timers with sk_reset_timer() and cancel them with sk_stop_timer()
so that an armed timer owns a reference, and release it in both expiry
handlers.  Rearm the heartbeat only while sk_hashed(sk) is still true,
since __x25_destroy_socket() unlinks the socket before dropping it.  Arm
the deferred destroy timer the same way and drop its reference in
x25_destroy_timer().

Reproduced on net with KASAN, with the heartbeat period shortened so the
window recurs.  With this patch the reproducer no longer triggers a
report and /proc/net/x25 drains.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
v2:
 - shorten the commit message (Andrew Lunn)
 - drop the Reported-by tags; there is no public report to credit
   (Andrew Lunn)
 - retest on net instead of v7.2-rc4; the bug is still present there as of
   53658c6f3682 (Andrew Lunn)

Link to v1: https://lore.kernel.org/netdev/20260726071808.47781-1-baul.lee@xbow.com/

 net/x25/af_x25.c    |  4 ++--
 net/x25/x25_timer.c | 25 ++++++++++++++++---------
 2 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c
index 8aae9273b..033e7d059 100644
--- a/net/x25/af_x25.c
+++ b/net/x25/af_x25.c
@@ -363,6 +363,7 @@ static void x25_destroy_timer(struct timer_list *t)
 	struct sock *sk = timer_container_of(sk, t, sk_timer);
 
 	x25_destroy_socket_from_timer(sk);
+	sock_put(sk);
 }
 
 /*
@@ -398,9 +399,8 @@ static void __x25_destroy_socket(struct sock *sk)
 
 	if (sk_has_allocations(sk)) {
 		/* Defer: outstanding buffers */
-		sk->sk_timer.expires  = jiffies + 10 * HZ;
 		sk->sk_timer.function = x25_destroy_timer;
-		add_timer(&sk->sk_timer);
+		sk_reset_timer(sk, &sk->sk_timer, jiffies + 10 * HZ);
 	} else {
 		/* drop last reference so sock_put will free */
 		__sock_put(sk);
diff --git a/net/x25/x25_timer.c b/net/x25/x25_timer.c
index 2ec63a1f4..7896cd43f 100644
--- a/net/x25/x25_timer.c
+++ b/net/x25/x25_timer.c
@@ -36,45 +36,45 @@ void x25_init_timers(struct sock *sk)
 
 void x25_start_heartbeat(struct sock *sk)
 {
-	mod_timer(&sk->sk_timer, jiffies + 5 * HZ);
+	sk_reset_timer(sk, &sk->sk_timer, jiffies + 5 * HZ);
 }
 
 void x25_stop_heartbeat(struct sock *sk)
 {
-	timer_delete(&sk->sk_timer);
+	sk_stop_timer(sk, &sk->sk_timer);
 }
 
 void x25_start_t2timer(struct sock *sk)
 {
 	struct x25_sock *x25 = x25_sk(sk);
 
-	mod_timer(&x25->timer, jiffies + x25->t2);
+	sk_reset_timer(sk, &x25->timer, jiffies + x25->t2);
 }
 
 void x25_start_t21timer(struct sock *sk)
 {
 	struct x25_sock *x25 = x25_sk(sk);
 
-	mod_timer(&x25->timer, jiffies + x25->t21);
+	sk_reset_timer(sk, &x25->timer, jiffies + x25->t21);
 }
 
 void x25_start_t22timer(struct sock *sk)
 {
 	struct x25_sock *x25 = x25_sk(sk);
 
-	mod_timer(&x25->timer, jiffies + x25->t22);
+	sk_reset_timer(sk, &x25->timer, jiffies + x25->t22);
 }
 
 void x25_start_t23timer(struct sock *sk)
 {
 	struct x25_sock *x25 = x25_sk(sk);
 
-	mod_timer(&x25->timer, jiffies + x25->t23);
+	sk_reset_timer(sk, &x25->timer, jiffies + x25->t23);
 }
 
 void x25_stop_timer(struct sock *sk)
 {
-	timer_delete(&x25_sk(sk)->timer);
+	sk_stop_timer(sk, &x25_sk(sk)->timer);
 }
 
 unsigned long x25_display_timer(struct sock *sk)
@@ -108,7 +108,7 @@ static void x25_heartbeat_expiry(struct timer_list *t)
 			     sock_flag(sk, SOCK_DEAD))) {
 				bh_unlock_sock(sk);
 				x25_destroy_socket_from_timer(sk);
-				return;
+				goto out;
 			}
 			break;
 
@@ -120,8 +120,14 @@ static void x25_heartbeat_expiry(struct timer_list *t)
 			break;
 	}
 restart_heartbeat:
-	x25_start_heartbeat(sk);
+	/* Do not rearm once __x25_destroy_socket() has unlinked the socket:
+	 * it is past its cancel point and owns the teardown from there on.
+	 */
+	if (sk_hashed(sk))
+		x25_start_heartbeat(sk);
 	bh_unlock_sock(sk);
+out:
+	sock_put(sk);
 }
 
 /*
@@ -166,4 +172,5 @@ static void x25_timer_expiry(struct timer_list *t)
 	} else
 		x25_do_timer_expiry(sk);
 	bh_unlock_sock(sk);
+	sock_put(sk);
 }
-- 
2.53.0