tcp: prevent concurrent execution of tcp_sk_exit_batch

Its possible that two threads call tcp_sk_exit_batch() concurrently,
once from the cleanup_net workqueue, once from a task that failed to clone
a new netns.  In the latter case, error unwinding calls the exit handlers
in reverse order for the 'failed' netns.

tcp_sk_exit_batch() calls tcp_twsk_purge().
Problem is that since commit b099ce2 ("net: Batch inet_twsk_purge"),
this function picks up twsk in any dying netns, not just the one passed
in via exit_batch list.

This means that the error unwind of setup_net() can "steal" and destroy
timewait sockets belonging to the exiting netns.

This allows the netns exit worker to proceed to call

WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));

without the expected 1 -> 0 transition, which then splats.

At same time, error unwind path that is also running inet_twsk_purge()
will splat as well:

WARNING: .. at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210
...
 refcount_dec include/linux/refcount.h:351 [inline]
 inet_twsk_kill+0x758/0x9c0 net/ipv4/inet_timewait_sock.c:70
 inet_twsk_deschedule_put net/ipv4/inet_timewait_sock.c:221
 inet_twsk_purge+0x725/0x890 net/ipv4/inet_timewait_sock.c:304
 tcp_sk_exit_batch+0x1c/0x170 net/ipv4/tcp_ipv4.c:3522
 ops_exit_list+0x128/0x180 net/core/net_namespace.c:178
 setup_net+0x714/0xb40 net/core/net_namespace.c:375
 copy_net_ns+0x2f0/0x670 net/core/net_namespace.c:508
 create_new_namespaces+0x3ea/0xb10 kernel/nsproxy.c:110

... because refcount_dec() of tw_refcount unexpectedly dropped to 0.

This doesn't seem like an actual bug (no tw sockets got lost and I don't
see a use-after-free) but as erroneous trigger of debug check.

Add a mutex to force strict ordering: the task that calls tcp_twsk_purge()
blocks other task from doing final _dec_and_test before mutex-owner has
removed all tw sockets of dying netns.

Fixes: e9bd0cc ("tcp: Don't allocate tcp_death_row outside of struct netns_ipv4.")
Reported-by: [email protected]
Closes: https://lore.kernel.org/netdev/[email protected]/
Link: https://lore.kernel.org/netdev/[email protected]/
Signed-off-by: Florian Westphal <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Jason Xing <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Florian Westphal

net/ipv4/tcp_ipv4.c

@@ -97,6 +97,8 @@ static DEFINE_PER_CPU(struct sock_bh_locked, ipv4_tcp_sk) = {
 	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
 };
 
+static DEFINE_MUTEX(tcp_exit_batch_mutex);
+
 static u32 tcp_v4_init_seq(const struct sk_buff *skb)
 {
 	return secure_tcp_seq(ip_hdr(skb)->daddr,
@@ -3514,13 +3516,25 @@ static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
 {
 	struct net *net;
 
+	/* make sure concurrent calls to tcp_sk_exit_batch from net_cleanup_work
+	 * and failed setup_net error unwinding path are serialized.
+	 *
+	 * tcp_twsk_purge() handles twsk in any dead netns, not just those in
+	 * net_exit_list, the thread that dismantles a particular twsk must
+	 * do so without other thread progressing to refcount_dec_and_test() of
+	 * tcp_death_row.tw_refcount.
+	 */
+	mutex_lock(&tcp_exit_batch_mutex);
+
 	tcp_twsk_purge(net_exit_list);
 
 	list_for_each_entry(net, net_exit_list, exit_list) {
 		inet_pernet_hashinfo_free(net->ipv4.tcp_death_row.hashinfo);
 		WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
 		tcp_fastopen_ctx_destroy(net);
 	}
+
+	mutex_unlock(&tcp_exit_batch_mutex);
 }
 
 static struct pernet_operations __net_initdata tcp_sk_ops = {