sctp: Fix race between OOTB responce and route removal

commit 29c4afc4 upstream.

There is NULL pointer dereference possible during statistics update if the route
used for OOTB responce is removed at unfortunate time. If the route exists when
we receive OOTB packet and we finally jump into sctp_packet_transmit() to send
ABORT, but in the meantime route is removed under our feet, we take "no_route"
path and try to update stats with IP_INC_STATS(sock_net(asoc->base.sk), ...).

But sctp_ootb_pkt_new() used to prepare responce packet doesn't call
sctp_transport_set_owner() and therefore there is no asoc associated with this
packet. Probably temporary asoc just for OOTB responces is overkill, so just
introduce a check like in all other places in sctp_packet_transmit(), where
"asoc" is dereferenced.

To reproduce this, one needs to
0. ensure that sctp module is loaded (otherwise ABORT is not generated)
1. remove default route on the machine
2. while true; do
     ip route del [interface-specific route]
     ip route add [interface-specific route]
   done
3. send enough OOTB packets (i.e. HB REQs) from another host to trigger ABORT
   responce

On x86_64 the crash looks like this:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
IP: [<ffffffffa05ec9ac>] sctp_packet_transmit+0x63c/0x730 [sctp]
PGD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: ...
CPU: 0 PID: 0 Comm: swapper/0 Tainted: G           O    4.0.5-1-ARCH #1
Hardware name: ...
task: ffffffff818124c0 ti: ffffffff81800000 task.ti: ffffffff81800000
RIP: 0010:[<ffffffffa05ec9ac>]  [<ffffffffa05ec9ac>] sctp_packet_transmit+0x63c/0x730 [sctp]
RSP: 0018:ffff880127c037b8  EFLAGS: 00010296
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00000015ff66b480
RDX: 00000015ff66b400 RSI: ffff880127c17200 RDI: ffff880123403700
RBP: ffff880127c03888 R08: 0000000000017200 R09: ffffffff814625af
R10: ffffea00047e4680 R11: 00000000ffffff80 R12: ffff8800b0d38a28
R13: ffff8800b0d38a28 R14: ffff8800b3e88000 R15: ffffffffa05f24e0
FS:  0000000000000000(0000) GS:ffff880127c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000020 CR3: 00000000c855b000 CR4: 00000000000007f0
Stack:
 ffff880127c03910 ffff8800b0d38a28 ffffffff8189d240 ffff88011f91b400
 ffff880127c03828 ffffffffa05c94c5 0000000000000000 ffff8800baa1c520
 0000000000000000 0000000000000001 0000000000000000 0000000000000000
Call Trace:
 <IRQ>
 [<ffffffffa05c94c5>] ? sctp_sf_tabort_8_4_8.isra.20+0x85/0x140 [sctp]
 [<ffffffffa05d6b42>] ? sctp_transport_put+0x52/0x80 [sctp]
 [<ffffffffa05d0bfc>] sctp_do_sm+0xb8c/0x19a0 [sctp]
 [<ffffffff810b0e00>] ? trigger_load_balance+0x90/0x210
 [<ffffffff810e0329>] ? update_process_times+0x59/0x60
 [<ffffffff812c7a40>] ? timerqueue_add+0x60/0xb0
 [<ffffffff810e0549>] ? enqueue_hrtimer+0x29/0xa0
 [<ffffffff8101f599>] ? read_tsc+0x9/0x10
 [<ffffffff8116d4b5>] ? put_page+0x55/0x60
 [<ffffffff810ee1ad>] ? clockevents_program_event+0x6d/0x100
 [<ffffffff81462b68>] ? skb_free_head+0x58/0x80
 [<ffffffffa029a10b>] ? chksum_update+0x1b/0x27 [crc32c_generic]
 [<ffffffff81283f3e>] ? crypto_shash_update+0xce/0xf0
 [<ffffffffa05d3993>] sctp_endpoint_bh_rcv+0x113/0x280 [sctp]
 [<ffffffffa05dd4e6>] sctp_inq_push+0x46/0x60 [sctp]
 [<ffffffffa05ed7a0>] sctp_rcv+0x880/0x910 [sctp]
 [<ffffffffa05ecb50>] ? sctp_packet_transmit_chunk+0xb0/0xb0 [sctp]
 [<ffffffffa05ecb70>] ? sctp_csum_update+0x20/0x20 [sctp]
 [<ffffffff814b05a5>] ? ip_route_input_noref+0x235/0xd30
 [<ffffffff81051d6b>] ? ack_ioapic_level+0x7b/0x150
 [<ffffffff814b27be>] ip_local_deliver_finish+0xae/0x210
 [<ffffffff814b2e15>] ip_local_deliver+0x35/0x90
 [<ffffffff814b2a15>] ip_rcv_finish+0xf5/0x370
 [<ffffffff814b3128>] ip_rcv+0x2b8/0x3a0
 [<ffffffff81474193>] __netif_receive_skb_core+0x763/0xa50
 [<ffffffff81476c28>] __netif_receive_skb+0x18/0x60
 [<ffffffff81476cb0>] netif_receive_skb_internal+0x40/0xd0
 [<ffffffff814776c8>] napi_gro_receive+0xe8/0x120
 [<ffffffffa03946aa>] rtl8169_poll+0x2da/0x660 [r8169]
 [<ffffffff8147896a>] net_rx_action+0x21a/0x360
 [<ffffffff81078dc1>] __do_softirq+0xe1/0x2d0
 [<ffffffff8107912d>] irq_exit+0xad/0xb0
 [<ffffffff8157d158>] do_IRQ+0x58/0xf0
 [<ffffffff8157b06d>] common_interrupt+0x6d/0x6d
 <EOI>
 [<ffffffff810e1218>] ? hrtimer_start+0x18/0x20
 [<ffffffffa05d65f9>] ? sctp_transport_destroy_rcu+0x29/0x30 [sctp]
 [<ffffffff81020c50>] ? mwait_idle+0x60/0xa0
 [<ffffffff810216ef>] arch_cpu_idle+0xf/0x20
 [<ffffffff810b731c>] cpu_startup_entry+0x3ec/0x480
 [<ffffffff8156b365>] rest_init+0x85/0x90
 [<ffffffff818eb035>] start_kernel+0x48b/0x4ac
 [<ffffffff818ea120>] ? early_idt_handlers+0x120/0x120
 [<ffffffff818ea339>] x86_64_start_reservations+0x2a/0x2c
 [<ffffffff818ea49c>] x86_64_start_kernel+0x161/0x184
Code: 90 48 8b 80 b8 00 00 00 48 89 85 70 ff ff ff 48 83 bd 70 ff ff ff 00 0f 85 cd fa ff ff 48 89 df 31 db e8 18 63 e7 e0 48 8b 45 80 <48> 8b 40 20 48 8b 40 30 48 8b 80 68 01 00 00 65 48 ff 40 78 e9
RIP  [<ffffffffa05ec9ac>] sctp_packet_transmit+0x63c/0x730 [sctp]
 RSP <ffff880127c037b8>
CR2: 0000000000000020
---[ end trace 5aec7fd2dc983574 ]---
Kernel panic - not syncing: Fatal exception in interrupt
Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)
drm_kms_helper: panic occurred, switching back to text console
---[ end Kernel panic - not syncing: Fatal exception in interrupt
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/sctp/output.c

@@ -599,7 +599,9 @@ int sctp_packet_transmit(struct sctp_packet *packet)
 	return err;
 no_route:
 	kfree_skb(nskb);
-	IP_INC_STATS(sock_net(asoc->base.sk), IPSTATS_MIB_OUTNOROUTES);
+
+	if (asoc)
+		IP_INC_STATS(sock_net(asoc->base.sk), IPSTATS_MIB_OUTNOROUTES);
 
 	/* FIXME: Returning the 'err' will effect all the associations
 	 * associated with a socket, although only one of the paths of the