tipc: clear sock->sk on the failed-insert path in tipc_sk_create()

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 (net/core/sock.c:3839)
  Write of size 8 at addr ffff8880047cdc38 by task init/1
   lock_sock_nested (net/core/sock.c:3839)
   tipc_release (net/tipc/socket.c:638)
   __sock_release (net/socket.c:710)
   sock_close (net/socket.c:1501)
   __fput (fs/file_table.c:512)
  Allocated by task 1:
   sk_alloc (net/core/sock.c:2308)
   tipc_sk_create (net/tipc/socket.c:487)
   tipc_accept (net/tipc/socket.c:2744)
   do_accept (net/socket.c:2034)
  Freed by task 1:
   __sk_destruct (net/core/sock.c:2391)
   tipc_sk_create (net/tipc/socket.c:504)
   tipc_accept (net/tipc/socket.c:2744)
   do_accept (net/socket.c:2034)

Fixes: 00aff3590fc0 ("net: tipc: fix possible refcount leak in tipc_sk_create()")
Cc: stable@vger.kernel.org
Reviewed-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260714131939.1255974-1-4ncienth@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>