Re: kernel BUG at mm/memory.c:LINE!

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Jul 9, 2018 at 12:48 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Mon, Jul 9, 2018 at 12:15 PM, Kirill A. Shutemov
> <kirill@shutemov.name> wrote:
>> On Sun, Jul 08, 2018 at 10:51:03PM -0700, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following crash on:
>>>
>>> HEAD commit:    b2d44d145d2a Merge tag '4.18-rc3-smb3fixes' of git://git.s..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=11d07748400000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=2ca6c7a31d407f86
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=3f84280d52be9b7083cc
>>> 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+3f84280d52be9b7083cc@syzkaller.appspotmail.com
>>>
>>> next ffff8801ce5e7040 prev ffff8801d20eca50 mm ffff88019c1e13c0
>>> prot 27 anon_vma ffff88019680cdd8 vm_ops 0000000000000000
>>> pgoff 0 file ffff8801b2ec2d00 private_data 0000000000000000
>>> flags: 0xff(read|write|exec|shared|mayread|maywrite|mayexec|mayshare)
>>> ------------[ cut here ]------------
>>> kernel BUG at mm/memory.c:1422!
>>
>> Looks like vma_is_anonymous() false-positive.
>>
>> Any clues what file is it? I would guess some kind of socket, but it's not
>> clear from log which exactly.
>
>
> From the log it looks like it was this program (number 3 matches Comm:
> syz-executor3):
>
> 08:39:32 executing program 3:
> r0 = socket$nl_route(0x10, 0x3, 0x0)
> bind$netlink(r0, &(0x7f00000002c0)={0x10, 0x0, 0x0, 0x100000}, 0xc)
> getsockname(r0, &(0x7f0000000000)=@pppol2tpv3in6={0x0, 0x0, {0x0,
> <r1=>0xffffffffffffffff, 0x0, 0x0, 0x0, 0x0, {0x0, 0x0, 0x0,
> @loopback}}}, &(0x7f00000000c0)=0x3a)
> mmap(&(0x7f0000e00000/0x200000)=nil, 0x200000, 0x7fdff, 0x11, r1, 0x0)
> ioctl$FIBMAP(r0, 0x1, &(0x7f0000000100)=0x9)
> r2 = socket$inet6(0xa, 0x1000000000002, 0x0)
> ioctl(r2, 0x8912, &(0x7f00000001c0)="796d05ad441e829115ac7fd77200")
> r3 = syz_open_dev$vcsa(&(0x7f0000000140)='/dev/vcsa#\x00', 0x3, 0x2)
> ioctl$VHOST_SET_VRING_ENDIAN(r3, 0x4008af13, &(0x7f0000000180)={0x0, 0x8})
> sendto$inet(0xffffffffffffffff, &(0x7f0000a88f88), 0xffffffffffffff31,
> 0x0, &(0x7f0000e68000)={0x2, 0x0, @multicast2=0xe0000002},
> 0xfffffffffffffeb3)
> ftruncate(r1, 0x6)
> mmap(&(0x7f0000e00000/0x200000)=nil, 0x200000, 0x0, 0x11, r0, 0x0)
> setsockopt$SO_TIMESTAMPING(r1, 0x1, 0x25, &(0x7f0000000080)=0x804, 0x4)
>
> But take what happens here with a grain of salt, it can pretend that
> it's doing one thing, but actually do something different.
> So that r1 passed to ftruncate is something that getsockname returned
> somewhere in the middle of address. And since the socket is not
> actually ppp, it can be just some bytes in the middle of netlink
> address, that than happened to be small and match some existing fd...


This also happened only once so far:
https://syzkaller.appspot.com/bug?extid=3f84280d52be9b7083cc
and I can't reproduce it rerunning this program. So it's either a very
subtle race, or fd in the middle of netlink address magically matched
some fd once, or something else...