[PATCH v1 net 1/2] tcp: Fix potential UAF in reqsk_timer_handler().

[Kuniyuki Iwashima <kuniyu@google.com>]

When TCP socket migration fails at inet_ehash_insert() in
reqsk_timer_handler(), we jump to the no_ownership: label
and free the new reqsk immediately with __reqsk_free().

Thus, we must stop the new reqsk's timer before jumping to the
label, but the timer might be missed since the cited commit,
resulting in UAF.

As we are in the original reqsk's timer context, we can safely
call timer_delete_sync() for the new reqsk.

Let's pass false to __inet_csk_reqsk_queue_drop() to stop
the new reqsk's timer.

Fixes: 83fccfc3940c ("inet: fix potential deadlock in reqsk_queue_unlink()")
Reported-by: Damiano Melotti <melotti@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
In case Sashiko asks

  "What happens if TFO reqsk is migrated in reqsk_timer_handler() ?"

, the answer is

  "TFO does not use reqsk_timer_handler()."
---
 net/ipv4/inet_connection_sock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 928654c34156..971f9db2c586 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1108,7 +1108,7 @@ 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, true);
+			__inet_csk_reqsk_queue_drop(sk_listener, nreq, false);
 			goto no_ownership;
 		}
 
-- 
2.54.0.545.g6539524ca2-goog