Re: KASAN: use-after-free Read in handle_userfault (2)

[Dmitry Vyukov]

On Wed, Dec 12, 2018 at 10:45 AM syzbot
<syzbot+cbc64b24b2b2d54c07a9@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    14cf8c1d5b90 Add linux-next specific files for 20181210
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=133296db400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c9133d0a4284c012
> dashboard link: https://syzkaller.appspot.com/bug?extid=cbc64b24b2b2d54c07a9
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

This is a corrupted/intermixed kernel output, the bug actually
happened somewhere in net/core/neighbour.c.
syzkaller has a bunch of code to try to deal to corrupted kernel
output, but it's not always possible as a parallel thread printing
something can inject an unrelated frame just in the right place, and
then it's indistinguishable from a crash happened there.

The question is: what's the significance of that
"FAULT_FLAG_ALLOW_RETRY missing 70"?
Is it something to fix in kernel? Can the check be removed? Should it
be moved somewhere earlier when flags are passed to kernel and cause
EINVAL? Can it be at least converted to a single-line pr_warn?
+Sasha go as far as suggesting that any "Call Trace:" in kernel output
means a kernel bugs. This is one of few places in kernel that dumps
stacks left and right without a kernel bug (?).



> 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+cbc64b24b2b2d54c07a9@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> RDX: 00000000000003ff RSI: 0000000020012fe0 RDI: 00007f5dbe489850
> RBP: 000000000072bf00 R08: 00000000000003ff R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f5dbe48a6d4
> R13: 00000000004c578a R14: 00000000004d9d90 R15: 00000000ffffffff
> ==================================================================
> BUG: KASAN: use-after-free in __list_del_entry_valid+0xf1/0x100
> lib/list_debug.c:51
> CPU: 1 PID: 20306 Comm: syz-executor2 Not tainted 4.20.0-rc6-next-20181210+
> #164
> Read of size 8 at addr ffff8881c5e72bb0 by task kworker/0:1/12
> 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+0x244/0x39d lib/dump_stack.c:113
>   handle_userfault.cold.30+0x47/0x62 fs/userfaultfd.c:431
>   do_anonymous_page mm/memory.c:2938 [inline]
>   handle_pte_fault mm/memory.c:3780 [inline]
>   __handle_mm_fault+0x4d26/0x5b70 mm/memory.c:3906
>   handle_mm_fault+0x54f/0xc70 mm/memory.c:3943
>   do_user_addr_fault arch/x86/mm/fault.c:1475 [inline]
>   __do_page_fault+0x5f6/0xd70 arch/x86/mm/fault.c:1541
>   do_page_fault+0xf2/0x7e0 arch/x86/mm/fault.c:1572
>   page_fault+0x1e/0x30 arch/x86/entry/entry_64.S:1143
> RIP: 0033:0x4510a0
> Code: 0f 84 c4 0f 00 00 48 89 f1 48 89 f8 48 83 e1 3f 48 83 f9 20 0f 86 7b
> 02 00 00 48 83 e6 f0 48 83 e1 0f 66 0f ef c0 66 0f ef c9 <66> 0f 74 0e 66
> 0f d7 d1 48 d3 ea 49 c7 c2 11 00 00 00 49 29 ca 4d
> RSP: 002b:00007fab1fbba7a8 EFLAGS: 00010202
> RAX: 00007fab1fbba850 RBX: 0000000000000003 RCX: 000000000000000e
> RDX: 00000000000003ff RSI: 0000000020012fe0 RDI: 00007fab1fbba850
> RBP: 000000000072bf00 R08: 00000000000003ff R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fab1fbbb6d4
> R13: 00000000004c578a R14: 00000000004d9d90 R15: 00000000ffffffff
> CPU: 0 PID: 12 Comm: kworker/0:1 Not tainted 4.20.0-rc6-next-20181210+ #164
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Workqueue: events_power_efficient neigh_periodic_work
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x244/0x39d lib/dump_stack.c:113
>   print_address_description.cold.4+0x9/0x1ff mm/kasan/report.c:187
>   kasan_report.cold.5+0x1b/0x39 mm/kasan/report.c:317
>   __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
>   __list_del_entry_valid+0xf1/0x100 lib/list_debug.c:51
>   __list_del_entry include/linux/list.h:117 [inline]
>   list_del_init include/linux/list.h:159 [inline]
>   neigh_mark_dead+0x13b/0x410 net/core/neighbour.c:125
>   neigh_periodic_work+0x89a/0xc30 net/core/neighbour.c:905
>   process_one_work+0xc90/0x1c40 kernel/workqueue.c:2153
>   worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
>   kthread+0x35a/0x440 kernel/kthread.c:246
>   ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
>
> Allocated by task 8166:
>   save_stack+0x43/0xd0 mm/kasan/common.c:73
>   set_track mm/kasan/common.c:85 [inline]
>   kasan_kmalloc+0xcb/0xd0 mm/kasan/common.c:482
>   __do_kmalloc_node mm/slab.c:3671 [inline]
>   __kmalloc_node_track_caller+0x4d/0x70 mm/slab.c:3685
>   __kmalloc_reserve.isra.38+0x41/0xe0 net/core/skbuff.c:137
>   __alloc_skb+0x155/0x770 net/core/skbuff.c:205
>   alloc_skb include/linux/skbuff.h:1008 [inline]
>   netlink_alloc_large_skb net/netlink/af_netlink.c:1182 [inline]
>   netlink_sendmsg+0xb29/0xfc0 net/netlink/af_netlink.c:1892
>   sock_sendmsg_nosec net/socket.c:622 [inline]
>   sock_sendmsg+0xd5/0x120 net/socket.c:632
>   ___sys_sendmsg+0x7fd/0x930 net/socket.c:2117
>   __sys_sendmsg+0x11d/0x280 net/socket.c:2155
>   __do_sys_sendmsg net/socket.c:2164 [inline]
>   __se_sys_sendmsg net/socket.c:2162 [inline]
>   __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2162
>   do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Freed by task 8166:
>   save_stack+0x43/0xd0 mm/kasan/common.c:73
>   set_track mm/kasan/common.c:85 [inline]
>   __kasan_slab_free+0x102/0x150 mm/kasan/common.c:444
>   kasan_slab_free+0xe/0x10 mm/kasan/common.c:452
>   __cache_free mm/slab.c:3485 [inline]
>   kfree+0xcf/0x230 mm/slab.c:3804
>   skb_free_head+0x99/0xc0 net/core/skbuff.c:550
>   skb_release_data+0x70c/0x9a0 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
>   netlink_unicast_kernel net/netlink/af_netlink.c:1311 [inline]
>   netlink_unicast+0x5ad/0x760 net/netlink/af_netlink.c:1336
>   netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1917
>   sock_sendmsg_nosec net/socket.c:622 [inline]
>   sock_sendmsg+0xd5/0x120 net/socket.c:632
>   ___sys_sendmsg+0x7fd/0x930 net/socket.c:2117
>   __sys_sendmsg+0x11d/0x280 net/socket.c:2155
>   __do_sys_sendmsg net/socket.c:2164 [inline]
>   __se_sys_sendmsg net/socket.c:2162 [inline]
>   __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2162
>   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 ffff8881c5e72940
>   which belongs to the cache kmalloc-1k of size 1024
> The buggy address is located 624 bytes inside of
>   1024-byte region [ffff8881c5e72940, ffff8881c5e72d40)
> The buggy address belongs to the page:
> page:ffffea0007179c80 count:1 mapcount:0 mapping:ffff8881da800ac0
> index:0xffff8881c5e72dc0 compound_mapcount: 0
> flags: 0x2fffc0000010200(slab|head)
> raw: 02fffc0000010200 ffffea00072cf188 ffffea00075d0c08 ffff8881da800ac0
> raw: ffff8881c5e72dc0 ffff8881c5e72040 0000000100000005 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
>   ffff8881c5e72a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>   ffff8881c5e72b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > ffff8881c5e72b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                                       ^
>   ffff8881c5e72c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>   ffff8881c5e72c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> kobject: 'loop5' (00000000a3f9d928): kobject_uevent_env
>
>
> ---
> This bug is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx
>
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