netdev - Re: [vlan_device_event] BUG: unable to handle kernel paging request at 6b6b6ccf

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Date:   Tue, 7 Nov 2017 08:25:03 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Fengguang Wu <fengguang.wu@...el.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [vlan_device_event] BUG: unable to handle kernel paging request
 at 6b6b6ccf

On Tue, Nov 7, 2017 at 2:21 AM, Fengguang Wu <fengguang.wu@...el.com> wrote:
>
> FYI this happens in v4.14-rc8 -- it's not necessarily a new bug.

Probably not.

Looks like a use-after-free bug in vlan_device_event() judging by the
base pointer:

    ECX: 6b6b6b6b

this is one of those circumstances where having the faddr2line output
for that EIP would make it much easier to see exactly which access it
is that causes problems. There's lots of inlining going on, so without
that it's a pain to figure out.

The code is

   0: 31 c0                xor    %eax,%eax
   2: 8d 76 00              lea    0x0(%esi),%esi
   5: 89 c2                mov    %eax,%edx
   7: 89 c3                mov    %eax,%ebx
   9: 81 e2 ff 0f 00 00    and    $0xfff,%edx
   f: 89 d1                mov    %edx,%ecx
  11: c1 fb 0c              sar    $0xc,%ebx
  14: c1 e9 09              shr    $0x9,%ecx
  17: 8d 0c d9              lea    (%ecx,%ebx,8),%ecx
  1a: 8b 4c 8e 10          mov    0x10(%esi,%ecx,4),%ecx
  1e: 85 c9                test   %ecx,%ecx
  20: 74 34                je     0x56
  22: 81 e2 ff 01 00 00    and    $0x1ff,%edx
  28:* 8b 14 91              mov    (%ecx,%edx,4),%edx <-- trapping instruction
  2b: 85 d2                test   %edx,%edx
  2d: 74 27                je     0x56
  2f: f6 82 30 01 00 00 01 testb  $0x1,0x130(%edx)
  36: 74 1e                je     0x56

and just by going by the constants in question (0xfff and 0x1ff), I
can see that it's one of

    vlan_group_for_each_dev(..) {
        ...
    }

things, but that's pretty much all I can tell.

Apparently we'll get that faddr2line output soon. In the meantime, I
think this is a real bug report but I don't see enough information to
really go on.

Of course, if it's bisectable, that would be great too.

                    Linus