[PATCH net] tcp: wait for TIME_WAIT teardown before freeing per-netns ehash

[Eric Dumazet <edumazet@kernel.org>]

From: Kyle Zeng <kylebot@openai.com>

inet_twsk_kill() removes a TIME_WAIT socket from the established hash
before it finishes using the ehash lock, hashinfo and bind tables, and
before it drops the death-row reference. A running timer can therefore
unhash its socket before inet_twsk_purge() reaches it. The purge cannot
synchronize with that timer, and tcp_sk_exit_batch() can free the
private ehash and hashinfo while inet_twsk_kill() is still using them.

An unprivileged reproducer using user and network namespaces, a private
TCP ehash and a FIN_WAIT2 timer triggers a KASAN slab-use-after-free in
inet_twsk_kill(). The allocation and free stacks point to
inet_pernet_hashinfo_alloc() and inet_pernet_hashinfo_free().

Wait for an RCU grace period after purging the whole batch.
TIME_WAIT removal runs with bottom halves disabled, so this also waits
for removers that were already invisible to the purge. Keep the wait
under tcp_exit_batch_mutex, before freeing any private hashinfo.

Fixes: d1e5e6408b30 ("tcp: Introduce optional per-netns ehash.")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
[edumazet: only call synchronize_net() if one netns in the batch uses a private ehash]
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/ipv4/tcp_ipv4.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index ebe5aa70374a3fd9e3305377efebec3004989945..3f8563a36db1e9ea853699ecb547af7ee9491b11 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -3561,6 +3561,17 @@ static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
 
 	tcp_twsk_purge(net_exit_list);
 
+	/* inet_twsk_kill() can unhash a socket before the purge sees it.
+	 * Wait for such BH-disabled callers to finish before freeing a
+	 * private ehash. This is not needed for the shared tcp_hashinfo.
+	 */
+	list_for_each_entry(net, net_exit_list, exit_list) {
+		if (net->ipv4.tcp_death_row.hashinfo->pernet) {
+			synchronize_net();
+			break;
+		}
+	}
+
 	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));
-- 
2.53.0