rxrpc: Fix a use-after-push in data_ready handler

Fix a use of a packet after it has been enqueued onto the packet processing
queue in the data_ready handler.  Once on a call's Rx queue, we mustn't
touch it any more as it may be dequeued and freed by the call processor
running on a work queue.

Save the values we need before enqueuing.

Without this, we can get an oops like the following:

BUG: unable to handle kernel NULL pointer dereference at 000000000000009c
IP: [<ffffffffa01854e8>] rxrpc_fast_process_packet+0x724/0xa11 [af_rxrpc]
PGD 0 
Oops: 0000 [#1] SMP
Modules linked in: kafs(E) af_rxrpc(E) [last unloaded: af_rxrpc]
CPU: 2 PID: 0 Comm: swapper/2 Tainted: G            E   4.7.0-fsdevel+ #1336
Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014
task: ffff88040d6863c0 task.stack: ffff88040d68c000
RIP: 0010:[<ffffffffa01854e8>]  [<ffffffffa01854e8>] rxrpc_fast_process_packet+0x724/0xa11 [af_rxrpc]
RSP: 0018:ffff88041fb03a78  EFLAGS: 00010246
RAX: ffffffffffffffff RBX: ffff8803ff195b00 RCX: 0000000000000001
RDX: ffffffffa01854d1 RSI: 0000000000000008 RDI: ffff8803ff195b00
RBP: ffff88041fb03ab0 R08: 0000000000000000 R09: 0000000000000001
R10: ffff88041fb038c8 R11: 0000000000000000 R12: ffff880406874800
R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff88041fb00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000000009c CR3: 0000000001c14000 CR4: 00000000001406e0
Stack:
 ffff8803ff195ea0 ffff880408348800 ffff880406874800 ffff8803ff195b00
 ffff880408348800 ffff8803ff195ed8 0000000000000000 ffff88041fb03af0
 ffffffffa0186072 0000000000000000 ffff8804054da000 0000000000000000
Call Trace:
 <IRQ> 
 [<ffffffffa0186072>] rxrpc_data_ready+0x89d/0xbae [af_rxrpc]
 [<ffffffff814c94d7>] __sock_queue_rcv_skb+0x24c/0x2b2
 [<ffffffff8155c59a>] __udp_queue_rcv_skb+0x4b/0x1bd
 [<ffffffff8155e048>] udp_queue_rcv_skb+0x281/0x4db
 [<ffffffff8155ea8f>] __udp4_lib_rcv+0x7ed/0x963
 [<ffffffff8155ef9a>] udp_rcv+0x15/0x17
 [<ffffffff81531d86>] ip_local_deliver_finish+0x1c3/0x318
 [<ffffffff81532544>] ip_local_deliver+0xbb/0xc4
 [<ffffffff81531bc3>] ? inet_del_offload+0x40/0x40
 [<ffffffff815322a9>] ip_rcv_finish+0x3ce/0x42c
 [<ffffffff81532851>] ip_rcv+0x304/0x33d
 [<ffffffff81531edb>] ? ip_local_deliver_finish+0x318/0x318
 [<ffffffff814dff9d>] __netif_receive_skb_core+0x601/0x6e8
 [<ffffffff814e072e>] __netif_receive_skb+0x13/0x54
 [<ffffffff814e082a>] netif_receive_skb_internal+0xbb/0x17c
 [<ffffffff814e1838>] napi_gro_receive+0xf9/0x1bd
 [<ffffffff8144eb9f>] rtl8169_poll+0x32b/0x4a8
 [<ffffffff814e1c7b>] net_rx_action+0xe8/0x357
 [<ffffffff81051074>] __do_softirq+0x1aa/0x414
 [<ffffffff810514ab>] irq_exit+0x3d/0xb0
 [<ffffffff810184a2>] do_IRQ+0xe4/0xfc
 [<ffffffff81612053>] common_interrupt+0x93/0x93
 <EOI> 
 [<ffffffff814af837>] ? cpuidle_enter_state+0x1ad/0x2be
 [<ffffffff814af832>] ? cpuidle_enter_state+0x1a8/0x2be
 [<ffffffff814af96a>] cpuidle_enter+0x12/0x14
 [<ffffffff8108956f>] call_cpuidle+0x39/0x3b
 [<ffffffff81089855>] cpu_startup_entry+0x230/0x35d
 [<ffffffff810312ea>] start_secondary+0xf4/0xf7

Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/input.c

@@ -124,11 +124,15 @@ static int rxrpc_fast_process_data(struct rxrpc_call *call,
 	struct rxrpc_skb_priv *sp;
 	bool terminal;
 	int ret, ackbit, ack;
+	u32 serial;
+	u8 flags;
 
 	_enter("{%u,%u},,{%u}", call->rx_data_post, call->rx_first_oos, seq);
 
 	sp = rxrpc_skb(skb);
 	ASSERTCMP(sp->call, ==, NULL);
+	flags = sp->hdr.flags;
+	serial = sp->hdr.serial;
 
 	spin_lock(&call->lock);
 
@@ -192,8 +196,8 @@ static int rxrpc_fast_process_data(struct rxrpc_call *call,
 	sp->call = call;
 	rxrpc_get_call(call);
 	atomic_inc(&call->skb_count);
-	terminal = ((sp->hdr.flags & RXRPC_LAST_PACKET) &&
-		    !(sp->hdr.flags & RXRPC_CLIENT_INITIATED));
+	terminal = ((flags & RXRPC_LAST_PACKET) &&
+		    !(flags & RXRPC_CLIENT_INITIATED));
 	ret = rxrpc_queue_rcv_skb(call, skb, false, terminal);
 	if (ret < 0) {
 		if (ret == -ENOMEM || ret == -ENOBUFS) {
@@ -205,12 +209,13 @@ static int rxrpc_fast_process_data(struct rxrpc_call *call,
 	}
 
 	skb = NULL;
+	sp = NULL;
 
 	_debug("post #%u", seq);
 	ASSERTCMP(call->rx_data_post, ==, seq);
 	call->rx_data_post++;
 
-	if (sp->hdr.flags & RXRPC_LAST_PACKET)
+	if (flags & RXRPC_LAST_PACKET)
 		set_bit(RXRPC_CALL_RCVD_LAST, &call->flags);
 
 	/* if we've reached an out of sequence packet then we need to drain
@@ -226,7 +231,7 @@ static int rxrpc_fast_process_data(struct rxrpc_call *call,
 
 	spin_unlock(&call->lock);
 	atomic_inc(&call->ackr_not_idle);
-	rxrpc_propose_ACK(call, RXRPC_ACK_DELAY, sp->hdr.serial, false);
+	rxrpc_propose_ACK(call, RXRPC_ACK_DELAY, serial, false);
 	_leave(" = 0 [posted]");
 	return 0;
 
@@ -239,7 +244,7 @@ out:
 
 discard_and_ack:
 	_debug("discard and ACK packet %p", skb);
-	__rxrpc_propose_ACK(call, ack, sp->hdr.serial, true);
+	__rxrpc_propose_ACK(call, ack, serial, true);
 discard:
 	spin_unlock(&call->lock);
 	rxrpc_free_skb(skb);
@@ -247,7 +252,7 @@ discard:
 	return 0;
 
 enqueue_and_ack:
-	__rxrpc_propose_ACK(call, ack, sp->hdr.serial, true);
+	__rxrpc_propose_ACK(call, ack, serial, true);
 enqueue_packet:
 	_net("defer skb %p", skb);
 	spin_unlock(&call->lock);