Subject: [PATCH net] tipc: clear sock->sk on the failed-insert path in
tipc_sk_create()

When tipc_sk_create() fails to insert the new socket (tipc_sk_insert() returns
non-zero), its error path frees the sk with sk_free() but leaves
sock->sk pointing at the freed object:

if (tipc_sk_insert(tsk)) {
	sk_free(sk);
	pr_warn("Socket create failed; port number exhausted\n");
	return -EINVAL;
}

This is harmless for plain socket(): the syscall layer clears sock->ops before
releasing, so tipc_release() is never called. It is not harmless on the accept()
path. tipc_accept() creates the pre-allocated child socket with
tipc_sk_create(net, new_sock, 0, kern); on failure it leaves new_sock->sk
dangling and new_sock->ops non-NULL, and do_accept() then fput()s the new
file, so __sock_release() -> tipc_release() runs
lock_sock(new_sock->sk) on the freed sk -- a use-after-free write of the sk_lock
spinlock.

tipc_release() already guards this exact "failed accept() releases a pre-allocated
child" case with "if (sk == NULL) return 0;", but the guard is bypassed because
tipc_sk_create() left sock->sk non-NULL
(dangling) rather than NULL.

Clear sock->sk on the failed-insert path so the existing tipc_release() NULL
check fires and the use-after-free is avoided.

The tipc_sk_insert() failure is reached when the per-netns socket rhashtable
hits its max_size (tsk_rht_params.max_size = 1048576, ~2M
elements) -- i.e. once a netns holds ~2M TIPC sockets every insert returns -
E2BIG.

 BUG: KASAN: slab-use-after-free in lock_sock_nested+0x98/0x150
 Write of size 8 at addr ffff8880047cdc38 by task init/1
  lock_sock_nested+0x98/0x150
  tipc_release+0xa4/0x7a0
  __sock_release+0x61/0x120
  sock_close+0x10/0x20
  __fput+0x1d6/0x490
 Allocated by task 1:
  sk_alloc+0x2b/0x380
  tipc_sk_create+0x82/0xb90
  tipc_accept+0x14c/0x650
 Freed by task 1:
  __sk_destruct+0x22d/0x2d0
  tipc_sk_create+0x7b8/0xb90
  tipc_accept+0x14c/0x650
  do_accept+0x1d2/0x2a0

Fixes: 07f6c4bc048a ("tipc: convert tipc reference table to use generic
rhashtable")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <redacted>
---
This was reported to security@kernel.org (Cc: the TIPC maintainer) with no
response; posting the fix directly to netdev as it is a straightforward one-line
fix. Full C reproducer available on request.

Yes, please send me your C reproducer.