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

[Daehyeon Ko <4ncienth@gmail.com>]

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 <4ncienth@gmail.com>
---
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.

 net/tipc/socket.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index e564341e0216..55e695748332 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -502,6 +502,7 @@ static int tipc_sk_create(struct net *net, struct socket *sock,
 	tipc_set_sk_state(sk, TIPC_OPEN);
 	if (tipc_sk_insert(tsk)) {
 		sk_free(sk);
+		sock->sk = NULL;
 		pr_warn("Socket create failed; port number exhausted\n");
 		return -EINVAL;
 	}
-- 
2.54.0