[PATCH net v2] tipc: purge cong_links under the socket lock in tipc_release()

From: Jun Yang <redacted>

tipc_release() frees the elements of tsk->cong_links after release_sock(),
i.e. with no lock held:

	tipc_sk_remove(tsk);
	sock_orphan(sk);
	release_sock(sk);
	tipc_dest_list_purge(&tsk->cong_links);	/* no lock */
	tsk->cong_link_cnt = 0;

Every other accessor of that list runs under the socket lock, including
the SOCK_WAKEUP handler in tipc_sk_proto_rcv(), which does
tipc_dest_del(&tsk->cong_links, ...) while holding only sk->sk_lock.slock
via tipc_sk_rcv()'s spin_trylock_bh().  Because the purge never acquires
that spinlock, it provides no mutual exclusion against the wakeup path.

A SOCK_WAKEUP delivered for this port can therefore run concurrently with
the purge: tipc_sk_rcv() looks the socket up and takes a reference before
tipc_sk_remove() unhashes it, is then delayed past release_sock() so
sock_owned_by_user() is false, its spin_trylock_bh() succeeds, and it
list_del()s and kfree()s a struct tipc_dest that the closing task is
walking at the same time.  Both paths free entries of the same list.

__tipc_shutdown() does not close this window: it waits on
!tsk->cong_link_cnt but ignores the return value of tipc_wait_for_cond(),
which returns early on timeout, on a pending signal, or on sk_err, so the
close can proceed with cong_links still populated.

  BUG: KASAN: slab-use-after-free in
   __list_del_entry_valid_or_report (lib/list_debug.c:49)
  Read of size 8 at addr ffff888028b4b6c8 by task poc_cong_race/9425
   __list_del_entry_valid_or_report (lib/list_debug.c:49)
   tipc_dest_list_purge (net/tipc/name_table.c:1225)
   tipc_release (net/tipc/socket.c:653)
   __sock_release (net/socket.c:735)
   sock_close (net/socket.c:1526)
   __x64_sys_close (fs/open.c:1560)
  Allocated by task 9425:
   tipc_dest_push (net/tipc/name_table.c:1183)
   __tipc_sendmsg (net/tipc/socket.c:1518)
  Freed by task 9418:
   kfree (mm/slub.c:6792)
   tipc_dest_del (net/tipc/name_table.c:1216)
   tipc_sk_filter_rcv (net/tipc/socket.c:2164)
   tipc_sk_rcv (net/tipc/socket.c:2450)
   tipc_rcv (net/tipc/node.c:2210)
   tipc_udp_recv (net/tipc/udp_media.c:389)
  kmalloc-32, freed 32-byte region [ffff888028b4b6c0, ffff888028b4b6e0)
  Oops: general protection fault, probably for non-canonical address
   0xe0347c4420000499
  Kernel panic - not syncing: Fatal exception

Move the purge above release_sock() so it runs under the socket lock, the
same discipline commit 844cf763fba6 ("tipc: make macro tipc_wait_for_cond()
smp safe") established for the wait condition.  A concurrent tipc_sk_rcv()
then either backlogs the wakeup because the socket is owned, or processes
it against an already empty list, and no new delivery can arrive because
tipc_sk_remove() has already unhashed the socket.  tsk->cong_link_cnt is
left where it is, so a wakeup taken from the backlog by release_sock()
still decrements the real count.

Fixes: 365ad353c256 ("tipc: reduce risk of user starvation during link congestion")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <redacted>
Assisted-by: tencentos-corvus-ai:hy4
Signed-off-by: Jun Yang <redacted>
---
A KASAN reproducer for this issue is available if requested.

v2:
 - Move only the purge.  v1 moved "tsk->cong_link_cnt = 0" above
   release_sock() as well, which makes a SOCK_WAKEUP taken from the backlog
   by release_sock() decrement an already-zeroed u16 down to 65535.  The
   value is dead by then, but the race does not need that hunk.

v1: https://lore.kernel.org/netdev/20260731101926.31514-1-juny24602@gmail.com/ (local)

 net/tipc/socket.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index d5d70eb230b5..cf7ee31638ab 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c

@@ -647,10 +647,14 @@ static int tipc_release(struct socket *sock)
 	sk_stop_timer(sk, &sk->sk_timer);
 	tipc_sk_remove(tsk);
 
+	/* Purge under the socket lock: a SOCK_WAKEUP looked up before
+	 * tipc_sk_remove() can still reach tipc_dest_del() on this list.
+	 */
+	tipc_dest_list_purge(&tsk->cong_links);
+
 	sock_orphan(sk);
 	/* Reject any messages that accumulated in backlog queue */
 	release_sock(sk);
-	tipc_dest_list_purge(&tsk->cong_links);
 	tsk->cong_link_cnt = 0;
 	call_rcu(&tsk->rcu, tipc_sk_callback);
 	sock->sk = NULL;

-- 
2.55.0