bpf, sockmap: Fix psock incorrectly pointing to sk

We observed an issue from the latest selftest: sockmap_redir where
sk_psock(psock->sk) != psock in the backlog. The root cause is the special
behavior in sockmap_redir - it frequently performs map_update() and
map_delete() on the same socket. During map_update(), we create a new
psock and during map_delete(), we eventually free the psock via rcu_work
in sk_psock_drop(). However, pending workqueues might still exist and not
be processed yet. If users immediately perform another map_update(), a new
psock will be allocated for the same sk, resulting in two psocks pointing
to the same sk.

When the pending workqueue is later triggered, it uses the old psock to
access sk for I/O operations, which is incorrect.

Timing Diagram:

cpu0 cpu1

map_update(sk):
sk->psock = psock1
psock1->sk = sk
map_delete(sk):
rcu_work_free(psock1)

map_update(sk):
sk->psock = psock2
psock2->sk = sk
workqueue:
wakeup with psock1, but the sk of psock1
doesn't belong to psock1
rcu_handler:
clean psock1
free(psock1)

Previously, we used reference counting to address the concurrency issue
between backlog and sock_map_close(). This logic remains necessary as it
prevents the sk from being freed while processing the backlog. But this
patch prevents pending backlogs from using a psock after it has been
stopped.

Note: We cannot call cancel_delayed_work_sync() in map_delete() since this
might be invoked in BPF context by BPF helper, and the function may sleep.

Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: Jiayuan Chen <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/core/skmsg.c

···
 	bool ingress;
 	int ret;
 
+	/* If sk is quickly removed from the map and then added back, the old
+	 * psock should not be scheduled, because there are now two psocks
+	 * pointing to the same sk.
+	 */
+	if (!sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED))
+		return;
+
 	/* Increment the psock refcnt to synchronize with close(fd) path in
 	 * sock_map_close(), ensuring we wait for backlog thread completion
 	 * before sk_socket freed. If refcnt increment fails, it indicates