af_rxrpc: Avoid setting up double-free on checksum error

skb_kill_datagram() does not dequeue the skb when MSG_PEEK is unset.
This leaves a free'd skb on the queue, resulting a double-free later.

Without this, the following oops can occur:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
IP: [<ffffffff8154fcf7>] skb_dequeue+0x47/0x70
PGD 0
Oops: 0002 [#1] SMP
Modules linked in: af_rxrpc ...
CPU: 0 PID: 1191 Comm: listen Not tainted 3.12.0+ #4
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
task: ffff8801183536b0 ti: ffff880035c92000 task.ti: ffff880035c92000
RIP: 0010:[<ffffffff8154fcf7>] skb_dequeue+0x47/0x70
RSP: 0018:ffff880035c93db8  EFLAGS: 00010097
RAX: 0000000000000246 RBX: ffff8800d2754b00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000202 RDI: ffff8800d254c084
RBP: ffff880035c93dd0 R08: ffff880035c93cf0 R09: ffff8800d968f270
R10: 0000000000000000 R11: 0000000000000293 R12: ffff8800d254c070
R13: ffff8800d254c084 R14: ffff8800cd861240 R15: ffff880119b39720
FS:  00007f37a969d740(0000) GS:ffff88011fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000008 CR3: 00000000d4413000 CR4: 00000000000006f0
Stack:
 ffff8800d254c000 ffff8800d254c070 ffff8800d254c2c0 ffff880035c93df8
 ffffffffa041a5b8 ffff8800cd844c80 ffffffffa04385a0 ffff8800cd844cb0
 ffff880035c93e18 ffffffff81546cef ffff8800d45fea00 0000000000000008
Call Trace:
 [<ffffffffa041a5b8>] rxrpc_release+0x128/0x2e0 [af_rxrpc]
 [<ffffffff81546cef>] sock_release+0x1f/0x80
 [<ffffffff81546d62>] sock_close+0x12/0x20
 [<ffffffff811aaba1>] __fput+0xe1/0x230
 [<ffffffff811aad3e>] ____fput+0xe/0x10
 [<ffffffff810862cc>] task_work_run+0xbc/0xe0
 [<ffffffff8106a3be>] do_exit+0x2be/0xa10
 [<ffffffff8116dc47>] ? do_munmap+0x297/0x3b0
 [<ffffffff8106ab8f>] do_group_exit+0x3f/0xa0
 [<ffffffff8106ac04>] SyS_exit_group+0x14/0x20
 [<ffffffff8166b069>] system_call_fastpath+0x16/0x1b
Signed-off-by: Tim Smith <tim@electronghost.co.uk>
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/ar-recvmsg.c

@@ -353,6 +353,10 @@ int rxrpc_recvmsg(struct kiocb *iocb, struct socket *sock,
 	if (continue_call)
 		rxrpc_put_call(continue_call);
 	rxrpc_kill_skb(skb);
+	if (!(flags & MSG_PEEK)) {
+		if (skb_dequeue(&rx->sk.sk_receive_queue) != skb)
+			BUG();
+	}
 	skb_kill_datagram(&rx->sk, skb, flags);
 	rxrpc_put_call(call);
 	return -EAGAIN;