Re: KASAN: use-after-free Write in skb_release_data (2)

[Dmitry Vyukov]

On Mon, Oct 15, 2018 at 8:30 AM, syzbot
<syzbot+580be3953ed99133804f@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    e40a826a6cbc qed: Add support for virtual link.
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=168954a5400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=7e7e2279c0020d5f
> dashboard link: https://syzkaller.appspot.com/bug?extid=580be3953ed99133804f
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+580be3953ed99133804f@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ==================================================================
> BUG: KASAN: use-after-free in atomic_sub_return
> include/asm-generic/atomic-instrumented.h:305 [inline]
> BUG: KASAN: use-after-free in skb_release_data+0x1a3/0x880
> net/core/skbuff.c:559
> Write of size 4 at addr ffff8801d8cb4120 by task syz-executor0/8395
>
> CPU: 0 PID: 8395 Comm: syz-executor0 Not tainted 4.19.0-rc6+ #254
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>  print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>  kasan_report_error mm/kasan/report.c:354 [inline]
>  kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>  check_memory_region_inline mm/kasan/kasan.c:260 [inline]
>  check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
>  kasan_check_write+0x14/0x20 mm/kasan/kasan.c:278
>  atomic_sub_return include/asm-generic/atomic-instrumented.h:305 [inline]
>  skb_release_data+0x1a3/0x880 net/core/skbuff.c:559
>  skb_release_all+0x4a/0x60 net/core/skbuff.c:627
>  __kfree_skb net/core/skbuff.c:641 [inline]
>  kfree_skb+0x1bb/0x580 net/core/skbuff.c:659
>  sit_tunnel_xmit+0x173/0x30d0 net/ipv6/sit.c:1044
>  __netdev_start_xmit include/linux/netdevice.h:4328 [inline]
>  netdev_start_xmit include/linux/netdevice.h:4337 [inline]
>  xmit_one net/core/dev.c:3219 [inline]
>  dev_hard_start_xmit+0x295/0xc90 net/core/dev.c:3235
>  __dev_queue_xmit+0x2f0d/0x3950 net/core/dev.c:3805
>  dev_queue_xmit+0x17/0x20 net/core/dev.c:3838
>  packet_sendmsg_spkt+0xf44/0x16f0 net/packet/af_packet.c:1975
>  sock_sendmsg_nosec net/socket.c:621 [inline]
>  sock_sendmsg+0xd5/0x120 net/socket.c:631
>  ___sys_sendmsg+0x7fd/0x930 net/socket.c:2116
>  __sys_sendmsg+0x11d/0x280 net/socket.c:2154
>  __do_sys_sendmsg net/socket.c:2163 [inline]
>  __se_sys_sendmsg net/socket.c:2161 [inline]
>  __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
>  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x457519
> Code: 1d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
> 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff
> 0f 83 eb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f2a52d30c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457519
> RDX: 0000000000000000 RSI: 00000000200003c0 RDI: 0000000000000003
> RBP: 000000000072bf00 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2a52d316d4
> R13: 00000000004c34d5 R14: 00000000004d52c8 R15: 00000000ffffffff
>
> Allocated by task 8395:
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
>  set_track mm/kasan/kasan.c:460 [inline]
>  kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:553
>  __do_kmalloc_node mm/slab.c:3682 [inline]
>  __kmalloc_node_track_caller+0x47/0x70 mm/slab.c:3696
>  __kmalloc_reserve.isra.39+0x41/0xe0 net/core/skbuff.c:137
>  __alloc_skb+0x155/0x770 net/core/skbuff.c:205
>  alloc_skb include/linux/skbuff.h:997 [inline]
>  sock_wmalloc+0x16d/0x1f0 net/core/sock.c:1934
>  packet_sendmsg_spkt+0x48a/0x16f0 net/packet/af_packet.c:1922
>  sock_sendmsg_nosec net/socket.c:621 [inline]
>  sock_sendmsg+0xd5/0x120 net/socket.c:631
>  ___sys_sendmsg+0x7fd/0x930 net/socket.c:2116
>  __sys_sendmsg+0x11d/0x280 net/socket.c:2154
>  __do_sys_sendmsg net/socket.c:2163 [inline]
>  __se_sys_sendmsg net/socket.c:2161 [inline]
>  __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
>  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Freed by task 8395:
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
>  set_track mm/kasan/kasan.c:460 [inline]
>  __kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
>  kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
>  __cache_free mm/slab.c:3498 [inline]
>  kfree+0xcf/0x230 mm/slab.c:3813
>  skb_free_head+0x99/0xc0 net/core/skbuff.c:550
>  skb_release_data+0x6a4/0x880 net/core/skbuff.c:570
>  skb_release_all+0x4a/0x60 net/core/skbuff.c:627
>  __kfree_skb net/core/skbuff.c:641 [inline]
>  consume_skb+0x1ae/0x570 net/core/skbuff.c:701
>  packet_rcv+0x172/0x1820 net/packet/af_packet.c:2139
>  dev_queue_xmit_nit+0x8ae/0xb30 net/core/dev.c:2020
>  xmit_one net/core/dev.c:3215 [inline]
>  dev_hard_start_xmit+0x186/0xc90 net/core/dev.c:3235
>  __dev_queue_xmit+0x2f0d/0x3950 net/core/dev.c:3805
>  dev_queue_xmit+0x17/0x20 net/core/dev.c:3838
>  packet_sendmsg_spkt+0xf44/0x16f0 net/packet/af_packet.c:1975
>  sock_sendmsg_nosec net/socket.c:621 [inline]
>  sock_sendmsg+0xd5/0x120 net/socket.c:631
>  ___sys_sendmsg+0x7fd/0x930 net/socket.c:2116
>  __sys_sendmsg+0x11d/0x280 net/socket.c:2154
>  __do_sys_sendmsg net/socket.c:2163 [inline]
>  __se_sys_sendmsg net/socket.c:2161 [inline]
>  __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
>  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> The buggy address belongs to the object at ffff8801d8cb4040
>  which belongs to the cache kmalloc-512 of size 512
> The buggy address is located 224 bytes inside of
>  512-byte region [ffff8801d8cb4040, ffff8801d8cb4240)
> The buggy address belongs to the page:
> page:ffffea0007632d00 count:1 mapcount:0 mapping:ffff8801da800940
> index:0xffff8801d8cb42c0
> flags: 0x2fffc0000000100(slab)
> raw: 02fffc0000000100 ffffea00070baf08 ffffea000736a788 ffff8801da800940
> raw: ffff8801d8cb42c0 ffff8801d8cb4040 0000000100000005 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
>  ffff8801d8cb4000: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
>  ffff8801d8cb4080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>
>> ffff8801d8cb4100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>
>                                ^
>  ffff8801d8cb4180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff8801d8cb4200: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
> ==================================================================

FWIW from the log this was triggered on this program:

05:22:33 executing program 0:
r0 = socket(0x11, 0xa, 0x0)
sendmsg(r0, &(0x7f00000003c0)={&(0x7f00000000c0)=@pptp={0x18, 0x2,
{0x0, @dev}}, 0x80, &(0x7f0000000380), 0x0, &(0x7f0000001880)}, 0x0)

Looks like a race with a very short inconsistency window since it was
triggered only once.

> ---
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