dccp related oops in inet_csk_listen_start

[Dave Jones <davej@codemonkey.org.uk>]

While experimenting with some dccp fuzzing, I hit this..

Oops: 0010 [#1] PREEMPT SMP DEBUG_PAGEALLOC 
CPU: 3 PID: 19269 Comm: trinity-c22 Not tainted 4.2.0-rc2-think+ #2
task: ffff88006f3954c0 ti: ffff8802b89b0000 task.ti: ffff8802b89b0000
RIP: 0010:[<0000000000000000>]  [<          (null)>]           (null)
RSP: 0018:ffff8802b89b3e30  EFLAGS: 00010246
RAX: ffffffffc063b200 RBX: 000000000000908c RCX: 000000000000908c
RDX: 0000000000000001 RSI: ffff8800cb94ef00 RDI: ffff880501ad8c40
RBP: ffff8802b89b3eb8 R08: ffff8800cb94ef00 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000005 R12: 000000000000908c
R13: ffffffffc0631940 R14: ffffffffafeefd40 R15: ffff8800cc97e850
FS:  00007fc948b0b740(0000) GS:ffff880507e00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000024f2d2000 CR4: 00000000001407e0
DR0: 00007fe46baf0000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Stack:
 ffffffffaf725ae5 ffff8802b89b3e48 000003e800000005 ffff8802b89b3e68
 ffff880501ad8c40 ffff8800cb94ef00 ffff880033cc8000 ffffffffb89b3e88
 ffffffff00000000 ffff880501ad9200 ffff880501ad9200 ffff8802b89b3eb8
Call Trace:
 [<ffffffffaf725ae5>] ? inet_csk_get_port+0x3a5/0x4f0
 [<ffffffffaf7260a9>] inet_csk_listen_start+0x79/0xe0
 [<ffffffffc06260bb>] inet_dccp_listen+0x8b/0xc0 [dccp]
 [<ffffffffaf6b509e>] SyS_listen+0x4e/0x80
 [<ffffffffaf80063c>] tracesys_phase2+0x84/0x89


inet_csk_listen_start has an insightful comment about a potential race.
I added this for debugging..

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 60021d0d9326..d53cba9c1dfd 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -814,11 +814,15 @@ int inet_csk_listen_start(struct sock *sk, const int nr_table_entries)
 		inet->inet_sport = htons(inet->inet_num);
 
 		sk_dst_reset(sk);
+		if (sk->sk_prot->hash == NULL) {
+			printk("sk->sk_prot->hash WAS NULL!\n");
+			goto out;
+		}
 		sk->sk_prot->hash(sk);
 
 		return 0;
 	}
-
+out:
 	sk->sk_state = TCP_CLOSE;
 	__reqsk_queue_destroy(&icsk->icsk_accept_queue);
 	return -EADDRINUSE;

But haven't been able to provoke it into happening again.

Is a null check sufficient here, or should this be solved
elsewhere ?

	Dave