net/dccp: use-after-free in dccp_invalid_packet

Hi!

I've got the following error report while running the syzkaller fuzzer.

On commit d8e435f3ab6fea2ea324dce72b51dd7761747523 (Nov 26).

dh->dccph_doff is being accessed (line 731) right after skb was freed
(line 732) in net/dccp/ipv4.c.

A reproducer is attached.

==================================================================
BUG: KASAN: use-after-free in dccp_invalid_packet+0x788/0x800
Read of size 1 at addr ffff880066f0e7c8 by task a.out/3895

page:ffffea00019bc380 count:1 mapcount:0 mapping:          (null)
index:0x0 compound_mapcount: 0
flags: 0x100000000004080(slab|head)
page dumped because: kasan: bad access detected

CPU: 1 PID: 3895 Comm: a.out Not tainted 4.9.0-rc6+ #457
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffff88006cd07758 ffffffff81c73b14 ffff88006cd077e8 ffff880066f0e7c8
 00000000000000fa 00000000000000fb ffff88006cd077d8 ffffffff81637962
 ffff88006cd07810 ffffffff82cc9c90 ffffffff82cabf7c 0000000000000296
Call Trace:
 <IRQ> [   27.672034]  [<ffffffff81c73b14>] dump_stack+0xb3/0x10f
 [<     inline     >] describe_address mm/kasan/report.c:259
 [<ffffffff81637962>] kasan_report_error+0x122/0x560 mm/kasan/report.c:365
 [<     inline     >] kasan_report mm/kasan/report.c:387
 [<ffffffff81637dde>] __asan_report_load1_noabort+0x3e/0x40
mm/kasan/report.c:405
 [<ffffffff839e8158>] dccp_invalid_packet+0x788/0x800 net/dccp/ipv4.c:732
 [<ffffffff839f4fc1>] dccp_v6_rcv+0x21/0x1720 net/dccp/ipv6.c:658
 [<ffffffff83418f13>] ip6_input_finish+0x423/0x15f0 net/ipv6/ip6_input.c:279
 [<     inline     >] NF_HOOK_THRESH ./include/linux/netfilter.h:232
 [<     inline     >] NF_HOOK ./include/linux/netfilter.h:255
 [<ffffffff8341a1ae>] ip6_input+0xce/0x340 net/ipv6/ip6_input.c:322
 [<     inline     >] dst_input ./include/net/dst.h:507
 [<ffffffff834185ae>] ip6_rcv_finish+0x23e/0x780 net/ipv6/ip6_input.c:69
 [<     inline     >] NF_HOOK_THRESH ./include/linux/netfilter.h:232
 [<     inline     >] NF_HOOK ./include/linux/netfilter.h:255
 [<ffffffff8341b4bd>] ipv6_rcv+0x109d/0x1dc0 net/ipv6/ip6_input.c:203
 [<ffffffff82d0805b>] __netif_receive_skb_core+0x187b/0x2a10 net/core/dev.c:4208
 [<ffffffff82d0921a>] __netif_receive_skb+0x2a/0x170 net/core/dev.c:4246
 [<ffffffff82d0bbed>] process_backlog+0xed/0x6e0 net/core/dev.c:4855
 [<     inline     >] napi_poll net/core/dev.c:5156
 [<ffffffff82d0b4cd>] net_rx_action+0x76d/0xda0 net/core/dev.c:5221
 [<ffffffff840f59ef>] __do_softirq+0x23f/0x8e5 kernel/softirq.c:284
 [<ffffffff840f3c1c>] do_softirq_own_stack+0x1c/0x30
arch/x86/entry/entry_64.S:904
 <EOI> [   27.672034]  [<ffffffff81251370>] do_softirq.part.17+0x60/0xa0
 [<     inline     >] do_softirq kernel/softirq.c:176
 [<ffffffff81251466>] __local_bh_enable_ip+0xb6/0xc0 kernel/softirq.c:181
 [<     inline     >] local_bh_enable ./include/linux/bottom_half.h:31
 [<     inline     >] rcu_read_unlock_bh ./include/linux/rcupdate.h:967
 [<ffffffff8340b6c0>] ip6_finish_output2+0xb70/0x1f30 net/ipv6/ip6_output.c:122
 [<ffffffff834152b9>] ip6_finish_output+0x3c9/0x7e0 net/ipv6/ip6_output.c:139
 [<     inline     >] NF_HOOK_COND ./include/linux/netfilter.h:246
 [<ffffffff8341588d>] ip6_output+0x1bd/0x6b0 net/ipv6/ip6_output.c:153
 [<     inline     >] dst_output ./include/net/dst.h:501
 [<ffffffff8357a116>] ip6_local_out+0x96/0x170 net/ipv6/output_core.c:170
 [<ffffffff83417b63>] ip6_send_skb+0xa3/0x340 net/ipv6/ip6_output.c:1712
 [<ffffffff83417eb5>] ip6_push_pending_frames+0xb5/0xe0
net/ipv6/ip6_output.c:1732
 [<     inline     >] rawv6_push_pending_frames net/ipv6/raw.c:607
 [<ffffffff83489a9e>] rawv6_sendmsg+0x1c4e/0x2c20 net/ipv6/raw.c:920
 [<ffffffff832a1057>] inet_sendmsg+0x317/0x4e0 net/ipv4/af_inet.c:734
 [<     inline     >] sock_sendmsg_nosec net/socket.c:621
 [<ffffffff82c9d76c>] sock_sendmsg+0xcc/0x110 net/socket.c:631
 [<ffffffff82c9f1b7>] ___sys_sendmsg+0x2d7/0x8b0 net/socket.c:1954
 [<ffffffff82ca1888>] __sys_sendmmsg+0x158/0x390 net/socket.c:2044
 [<     inline     >] SYSC_sendmmsg net/socket.c:2075
 [<ffffffff82ca1af5>] SyS_sendmmsg+0x35/0x60 net/socket.c:2070
 [<ffffffff840f2c41>] entry_SYSCALL_64_fastpath+0x1f/0xc2
arch/x86/entry/entry_64.S:209

The buggy address belongs to the object at ffff880066f0e780
 which belongs to the cache kmalloc-512 of size 512
The buggy address ffff880066f0e7c8 is located 72 bytes inside
 of 512-byte region [ffff880066f0e780, ffff880066f0e980)

Freed by task 3895:
 [<ffffffff811aa1b6>] save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
 [<ffffffff81636a76>] save_stack+0x46/0xd0 mm/kasan/kasan.c:495
 [<     inline     >] set_track mm/kasan/kasan.c:507
 [<ffffffff816372d3>] kasan_slab_free+0x73/0xc0 mm/kasan/kasan.c:571
 [<     inline     >] slab_free_hook mm/slub.c:1352
 [<     inline     >] slab_free_freelist_hook mm/slub.c:1374
 [<     inline     >] slab_free mm/slub.c:2951
 [<ffffffff816337b8>] kfree+0xe8/0x2b0 mm/slub.c:3871
 [<ffffffff82cb6748>] skb_free_head+0x78/0xb0 net/core/skbuff.c:580
 [<ffffffff82cc50fb>] pskb_expand_head+0x28b/0x8f0 net/core/skbuff.c:1244
 [<ffffffff82cc954c>] __pskb_pull_tail+0xcc/0x1190 net/core/skbuff.c:1613
 [<     inline     >] pskb_may_pull ./include/linux/skbuff.h:1966
 [<ffffffff839e80bb>] dccp_invalid_packet+0x6eb/0x800 net/dccp/ipv4.c:731
 [<ffffffff839f4fc1>] dccp_v6_rcv+0x21/0x1720 net/dccp/ipv6.c:658
 [<ffffffff83418f13>] ip6_input_finish+0x423/0x15f0 net/ipv6/ip6_input.c:279
 [<     inline     >] NF_HOOK_THRESH ./include/linux/netfilter.h:232
 [<     inline     >] NF_HOOK ./include/linux/netfilter.h:255
 [<ffffffff8341a1ae>] ip6_input+0xce/0x340 net/ipv6/ip6_input.c:322
 [<     inline     >] dst_input ./include/net/dst.h:507
 [<ffffffff834185ae>] ip6_rcv_finish+0x23e/0x780 net/ipv6/ip6_input.c:69
 [<     inline     >] NF_HOOK_THRESH ./include/linux/netfilter.h:232
 [<     inline     >] NF_HOOK ./include/linux/netfilter.h:255
 [<ffffffff8341b4bd>] ipv6_rcv+0x109d/0x1dc0 net/ipv6/ip6_input.c:203
 [<ffffffff82d0805b>] __netif_receive_skb_core+0x187b/0x2a10 net/core/dev.c:4208
 [<ffffffff82d0921a>] __netif_receive_skb+0x2a/0x170 net/core/dev.c:4246
 [<ffffffff82d0bbed>] process_backlog+0xed/0x6e0 net/core/dev.c:4855
 [<     inline     >] napi_poll net/core/dev.c:5156
 [<ffffffff82d0b4cd>] net_rx_action+0x76d/0xda0 net/core/dev.c:5221
 [<ffffffff840f59ef>] __do_softirq+0x23f/0x8e5 kernel/softirq.c:284

Allocated by task 3895:
 [<ffffffff811aa1b6>] save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
 [<ffffffff81636a76>] save_stack+0x46/0xd0 mm/kasan/kasan.c:495
 [<     inline     >] set_track mm/kasan/kasan.c:507
 [<ffffffff81636ceb>] kasan_kmalloc+0xab/0xe0 mm/kasan/kasan.c:598
 [<ffffffff81637252>] kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:537
 [<     inline     >] slab_post_alloc_hook mm/slab.h:417
 [<     inline     >] slab_alloc_node mm/slub.c:2708
 [<ffffffff81635fab>] __kmalloc_node_track_caller+0xcb/0x390 mm/slub.c:4270
 [<ffffffff82cb7671>] __kmalloc_reserve.isra.35+0x41/0xe0 net/core/skbuff.c:138
 [<ffffffff82cc4f8c>] pskb_expand_head+0x11c/0x8f0 net/core/skbuff.c:1212
 [<ffffffff82cc954c>] __pskb_pull_tail+0xcc/0x1190 net/core/skbuff.c:1613
 [<     inline     >] pskb_may_pull ./include/linux/skbuff.h:1966
 [<ffffffff839e804b>] dccp_invalid_packet+0x67b/0x800 net/dccp/ipv4.c:708
 [<ffffffff839f4fc1>] dccp_v6_rcv+0x21/0x1720 net/dccp/ipv6.c:658
 [<ffffffff83418f13>] ip6_input_finish+0x423/0x15f0 net/ipv6/ip6_input.c:279
 [<     inline     >] NF_HOOK_THRESH ./include/linux/netfilter.h:232
 [<     inline     >] NF_HOOK ./include/linux/netfilter.h:255
 [<ffffffff8341a1ae>] ip6_input+0xce/0x340 net/ipv6/ip6_input.c:322
 [<     inline     >] dst_input ./include/net/dst.h:507
 [<ffffffff834185ae>] ip6_rcv_finish+0x23e/0x780 net/ipv6/ip6_input.c:69
 [<     inline     >] NF_HOOK_THRESH ./include/linux/netfilter.h:232
 [<     inline     >] NF_HOOK ./include/linux/netfilter.h:255
 [<ffffffff8341b4bd>] ipv6_rcv+0x109d/0x1dc0 net/ipv6/ip6_input.c:203
 [<ffffffff82d0805b>] __netif_receive_skb_core+0x187b/0x2a10 net/core/dev.c:4208
 [<ffffffff82d0921a>] __netif_receive_skb+0x2a/0x170 net/core/dev.c:4246
 [<ffffffff82d0bbed>] process_backlog+0xed/0x6e0 net/core/dev.c:4855
 [<     inline     >] napi_poll net/core/dev.c:5156
 [<ffffffff82d0b4cd>] net_rx_action+0x76d/0xda0 net/core/dev.c:5221
 [<ffffffff840f59ef>] __do_softirq+0x23f/0x8e5 kernel/softirq.c:284

Memory state around the buggy address:
 ffff880066f0e680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff880066f0e700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

ffff880066f0e780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

                                              ^
 ffff880066f0e800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff880066f0e880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================