tipc: correctly unlink packets from deferred packet queue

When we pull a received packet from a link's 'deferred packets' queue
for processing, its 'next' pointer is not cleared, and still refers to
the next packet in that queue, if any. This is incorrect, but caused
no harm before commit 40ba3cdf ("tipc:
message reassembly using fragment chain") was introduced. After that
commit, it may sometimes lead to the following oops:

general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
Modules linked in: tipc
CPU: 4 PID: 0 Comm: swapper/4 Tainted: G        W 3.13.0-rc2+ #6
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
task: ffff880017af4880 ti: ffff880017aee000 task.ti: ffff880017aee000
RIP: 0010:[<ffffffff81710694>]  [<ffffffff81710694>] skb_try_coalesce+0x44/0x3d0
RSP: 0018:ffff880016603a78  EFLAGS: 00010212
RAX: 6b6b6b6bd6d6d6d6 RBX: ffff880013106ac0 RCX: ffff880016603ad0
RDX: ffff880016603ad7 RSI: ffff88001223ed00 RDI: ffff880013106ac0
RBP: ffff880016603ab8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff88001223ed00
R13: ffff880016603ad0 R14: 000000000000058c R15: ffff880012297650
FS:  0000000000000000(0000) GS:ffff880016600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 000000000805b000 CR3: 0000000011f5d000 CR4: 00000000000006e0
Stack:
 ffff880016603a88 ffffffff810a38ed ffff880016603aa8 ffff88001223ed00
 0000000000000001 ffff880012297648 ffff880016603b68 ffff880012297650
 ffff880016603b08 ffffffffa0006c51 ffff880016603b08 00ffffffa00005fc
Call Trace:
 <IRQ>
 [<ffffffff810a38ed>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffffa0006c51>] tipc_link_recv_fragment+0xd1/0x1b0 [tipc]
 [<ffffffffa0007214>] tipc_recv_msg+0x4e4/0x920 [tipc]
 [<ffffffffa00016f0>] ? tipc_l2_rcv_msg+0x40/0x250 [tipc]
 [<ffffffffa000177c>] tipc_l2_rcv_msg+0xcc/0x250 [tipc]
 [<ffffffffa00016f0>] ? tipc_l2_rcv_msg+0x40/0x250 [tipc]
 [<ffffffff8171e65b>] __netif_receive_skb_core+0x80b/0xd00
 [<ffffffff8171df94>] ? __netif_receive_skb_core+0x144/0xd00
 [<ffffffff8171eb76>] __netif_receive_skb+0x26/0x70
 [<ffffffff8171ed6d>] netif_receive_skb+0x2d/0x200
 [<ffffffff8171fe70>] napi_gro_receive+0xb0/0x130
 [<ffffffff815647c2>] e1000_clean_rx_irq+0x2c2/0x530
 [<ffffffff81565986>] e1000_clean+0x266/0x9c0
 [<ffffffff81985f7b>] ? notifier_call_chain+0x2b/0x160
 [<ffffffff8171f971>] net_rx_action+0x141/0x310
 [<ffffffff81051c1b>] __do_softirq+0xeb/0x480
 [<ffffffff819817bb>] ? _raw_spin_unlock+0x2b/0x40
 [<ffffffff810b8c42>] ? handle_fasteoi_irq+0x72/0x100
 [<ffffffff81052346>] irq_exit+0x96/0xc0
 [<ffffffff8198cbc3>] do_IRQ+0x63/0xe0
 [<ffffffff81981def>] common_interrupt+0x6f/0x6f
 <EOI>

This happens when the last fragment of a message has passed through the
the receiving link's 'deferred packets' queue, and at least one other
packet was added to that queue while it was there. After the fragment
chain with the complete message has been successfully delivered to the
receiving socket, it is released. Since 'next' pointer of the last
fragment in the released chain now is non-NULL, we get the crash shown
above.

We fix this by clearing the 'next' pointer of all received packets,
including those being pulled from the 'deferred' queue, before they
undergo any further processing.

Fixes: 40ba3cdf ("tipc: message reassembly using fragment chain")
Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Reported-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/link.c

@@ -1498,6 +1498,7 @@ void tipc_recv_msg(struct sk_buff *head, struct tipc_bearer *b_ptr)
 		int type;
 
 		head = head->next;
+		buf->next = NULL;
 
 		/* Ensure bearer is still enabled */
 		if (unlikely(!b_ptr->active))