Re: [Question]: A non NULL req->sk in tcp_rtx_synack. Not a fastopen connection.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Martin KaFai Lau <martin.lau@linux.dev>
Date: Thu, 3 Oct 2024 21:00:20 -0700
> On 10/3/24 7:02 PM, Kuniyuki Iwashima wrote:
> > From: Martin KaFai Lau <martin.lau@linux.dev>
> > Date: Thu, 3 Oct 2024 18:14:09 -0700
> >> Hi,
> >>
> >> We are seeing a use-after-free from a bpf prog attached to
> >> trace_tcp_retransmit_synack. The program passes the req->sk to the
> >> bpf_sk_storage_get_tracing kernel helper which does check for null before using it.
> >>
> >> fastopen is not used.
> >>
> >> We got a kfence report on use-after-free (pasted at the end). It is running with
> >> an older 6.4 kernel and we hardly hit this in production.
> >>
> >>   From the upstream code, del_timer_sync() should have been done by
> >> inet_csk_reqsk_queue_drop() before "req->sk = child;" is assigned in
> >> inet_csk_reqsk_queue_add(). My understanding is the req->rsk_timer should have
> >> been stopped before the "req->sk = child;" assignment.
> > 
> > There seems to be a small race window in reqsk_queue_unlink().
> > 
> > expire_timers() first calls detach_timer(, true), which marks the timer
> > as not pending, and then calls reqsk_timer_handler().
> > 
> > If reqsk_queue_unlink() calls timer_pending() just before expire_timers()
> > calls reqsk_timer_handler(), reqsk_queue_unlink() could miss
> > del_timer_sync() ?
> 
> This seems to explain it. :)
> 
> Does it mean there is a chance that the reqsk_timer_handler() may rearm the 
> timer again and I guess only a few more synack will be sent in this case and 
> should be no harm?

Ah, it seems possible.  I was wondering how the timer can be delayed
until sk is freed.  In such a case, the timer will just let the peer
generate some challenge ACKs.


> 
> > 
> > ---8<---
> > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > index 2c5632d4fddb..4ba47ee6c9da 100644
> > --- a/net/ipv4/inet_connection_sock.c
> > +++ b/net/ipv4/inet_connection_sock.c
> > @@ -1045,7 +1045,7 @@ static bool reqsk_queue_unlink(struct request_sock *req)
> >   		found = __sk_nulls_del_node_init_rcu(sk);
> >   		spin_unlock(lock);
> >   	}
> > -	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
> > +	if (del_timer_sync(&req->rsk_timer))
> 
> It seems the reqsk_timer_handler() will also call reqsk_queue_unlink() through 
> inet_csk_reqsk_queue_drop_and_put(). Not sure if the reqsk_timer_handler() can 
> del_timer_sync() itself.

Exactly, it seems illegal to call it from the timer.
Then, we need a variant of inet_csk_reqsk_queue_drop() to see if
the caller is tiemr or not. (compile-test only)

---8<---
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 2c5632d4fddb..2623964d8817 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1045,21 +1045,31 @@ static bool reqsk_queue_unlink(struct request_sock *req)
 		found = __sk_nulls_del_node_init_rcu(sk);
 		spin_unlock(lock);
 	}
-	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
-		reqsk_put(req);
+
 	return found;
 }
 
-bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req)
+static bool __inet_csk_reqsk_queue_drop(struct sock *sk,
+					struct request_sock *req,
+					bool from_timer)
 {
 	bool unlinked = reqsk_queue_unlink(req);
 
+	if (!from_timer && del_timer_sync(&req->rsk_timer))
+		reqsk_put(req);
+
 	if (unlinked) {
 		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
 		reqsk_put(req);
 	}
+
 	return unlinked;
 }
+
+bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req)
+{
+	return __inet_csk_reqsk_queue_drop(sk, req, false);
+}
 EXPORT_SYMBOL(inet_csk_reqsk_queue_drop);
 
 void inet_csk_reqsk_queue_drop_and_put(struct sock *sk, struct request_sock *req)
@@ -1152,7 +1162,7 @@ static void reqsk_timer_handler(struct timer_list *t)
 
 		if (!inet_ehash_insert(req_to_sk(nreq), req_to_sk(oreq), NULL)) {
 			/* delete timer */
-			inet_csk_reqsk_queue_drop(sk_listener, nreq);
+			__inet_csk_reqsk_queue_drop(sk_listener, nreq, true);
 			goto no_ownership;
 		}
 
@@ -1178,7 +1188,8 @@ static void reqsk_timer_handler(struct timer_list *t)
 	}
 
 drop:
-	inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
+	__inet_csk_reqsk_queue_drop(sk_listener, nreq, true);
+	reqsk_put(req);
 }
 
 static bool reqsk_queue_hash_req(struct request_sock *req,
---8<---