Re: [RFC net] net: stream: don't purge sk_error_queue without holding its lock

On 9/14/21 7:18 AM, Jakub Kicinski wrote:

On Mon, 13 Sep 2021 22:14:00 -0700 Eric Dumazet wrote:

quoted

On 9/13/21 3:38 PM, Jakub Kicinski wrote:

quoted

sk_stream_kill_queues() can be called when there are still
outstanding skbs to transmit. Those skbs may try to queue
notifications to the error queue (e.g. timestamps).
If sk_stream_kill_queues() purges the queue without taking
its lock the queue may get corrupted.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
Sending as an RFC for review, compile-tested only.

Seems far more likely that I'm missing something than that
this has been broken forever and nobody noticed :S
---
 net/core/stream.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/stream.c b/net/core/stream.c
index 4f1d4aa5fb38..7c585088f394 100644
--- a/net/core/stream.c
+++ b/net/core/stream.c

@@ -196,7 +196,7 @@ void sk_stream_kill_queues(struct sock *sk)
 	__skb_queue_purge(&sk->sk_receive_queue);
 
 	/* Next, the error queue. */
-	__skb_queue_purge(&sk->sk_error_queue);
+	skb_queue_purge(&sk->sk_error_queue);
 
 	/* Next, the write queue. */
 	WARN_ON(!skb_queue_empty(&sk->sk_write_queue));

This should not be needed.

By definition, sk_stream_kill_queues() is only called when there is no
more references on the sockets.

So all outstanding packets must have been orphaned or freed.

I don't see the wait anywhere, would you mind spelling it out?
My (likely flawed) understanding is that inet_sock_destruct() gets
called when refs are gone (via sk->sk_destruct).

But tcp_disconnect() + tcp_close() seem to happily call
inet_csk_destroy_sock() -> sk_stream_kill_queues() with outstanding
sk_wmem_alloc refs.

tcp_disconnect() should probably leave the error queue as is.

For some reason I thought your report was about inet_sock_destruct()

tcp_disconnect() has always been full of bugs, it is surprising real applications
(not fuzzers) are still trying to use it.

quoted

Anyway, Linux-2.6.12-rc2 had no timestamps yet.

I see, thanks, if some form of the patch stands perhaps:

Fixes: cb9eff097831 ("net: new user space API for time stamping of incoming and outgoing packets")

Except that this patch wont prevent a packet being added to sk_error_queue
right after skb_queue_purge(&sk->sk_error_queue).

If you think there is a bug, it must be fixed in another way.

IMO, preventing err packets from a prior session being queued after a tcp_disconnect()
is rather hard. We should not even try (packets could be stuck for hours in a qdisc)