linux-kernel - Re: WARNING: refcount bug in kvm_vm

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Date:   Thu, 11 Oct 2018 20:39:45 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     syzbot <syzbot+6df4d24a0a50fd5d1a08@...kaller.appspotmail.com>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Janosch Frank <frankja@...ux.vnet.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: WARNING: refcount bug in kvm_vm_ioctl

On Thu, Oct 11, 2018 at 8:23 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Thu, Oct 11, 2018 at 4:17 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>> On 10/10/2018 09:58, syzbot wrote:
>>>  do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
>>>  invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:993
>>> RIP: 0010:refcount_inc_checked+0x5d/0x70 lib/refcount.c:153
>>>  kvm_get_kvm arch/x86/kvm/../../../virt/kvm/kvm_main.c:766 [inline]
>>>  kvm_ioctl_create_device arch/x86/kvm/../../../virt/kvm/kvm_main.c:2924
>>>  kvm_vm_ioctl+0xed7/0x1d40 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3114
>>>  vfs_ioctl fs/ioctl.c:46 [inline]
>>>  file_ioctl fs/ioctl.c:501 [inline]
>>>  do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685
>>>  ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702
>>>  __do_sys_ioctl fs/ioctl.c:709 [inline]
>>>  __se_sys_ioctl fs/ioctl.c:707 [inline]
>>>  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707
>>>  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> The trace here is fairly simple, but I don't understand how this could
>> happen.
>>
>> The kvm_get_kvm is done within kvm_ioctl_create_device, which is called
>> from ioctl; the last reference cannot disappear inside a ioctl, because:
>>
>> 1) kvm_ioctl is called from vfs_ioctl, which does fdget and holds the fd
>> reference until after kvm_vm_ioctl returns
>>
>> 2) the file descriptor holds one reference to the struct kvm*, and this
>> reference is not released until kvm_vm_release is called by the last
>> fput (which could be fdput's call to fput if the process has exited in
>> the meanwhile)
>>
>> 3) for completeness, in case anon_inode_getfd fails, put_unused_fd will
>> not invoke the file descriptor's ->release callback (in this case
>> kvm_device_release).
>>
>> CCing some random people to get their opinion...
>>
>> Paolo
>>
>>> # See https://goo.gl/kgGztJ for information about syzkaller reproducers.
>>> #{"threaded":true,"collide":true,"repeat":true,"procs":6,"sandbox":"none","fault_call":-1,"tun":true,"tmpdir":true,"cgroups":true,"netdev":true,"resetnet":true,"segv":true}
>>> r0 = openat$kvm(0xffffffffffffff9c, &(0x7f0000000380)='/dev/kvm\x00', 0x0, 0x0)
>>> r1 = syz_open_dev$dspn(&(0x7f0000000100)='/dev/dsp#\x00', 0x3fe, 0x400)
>>> r2 = ioctl$KVM_CREATE_VM(r0, 0xae01, 0x0)
>>> perf_event_open(&(0x7f0000000040)={0x1, 0x70, 0x0, 0x0, 0x0, 0x0, 0x0, 0x50d}, 0x0, 0xffffffffffffffff, 0xffffffffffffffff, 0x0)
>>> mincore(&(0x7f0000ffc000/0x1000)=nil, 0x1000, &(0x7f00000003c0)=""/4096)
>>> setrlimit(0x0, &(0x7f0000000000))
>>> readahead(r1, 0x3, 0x9a6)
>>> ioctl$KVM_CREATE_DEVICE(r2, 0xc00caee0, &(0x7f00000002c0)={0x4})
>>> setsockopt$inet_sctp6_SCTP_FRAGMENT_INTERLEAVE(r1, 0x84, 0x12, &(0x7f00000001c0)=0x9, 0x4)
>
>
> Looking at crash rate and dates here:
> https://syzkaller.appspot.com/bug?id=2b9fab00a235b50a34adbc35adc236f7dbe490fd
> https://syzkaller.appspot.com/bug?id=d6e4dd59a9b708895738b9cc59e6bdcb3a43ff14
>
> It looks like something that reached upstream tree yesterday.
> However, there is one outliner: 2018/09/30 08:58. So either that one
> crash was something else, or syzkaller learned how to reproduce it
> more often...
> But maybe if you know some recent suspect patch that touched that
> area, that may be the easiest way to localize the bug.

I can't reproduce it locally qemu on top of 4.17.17 host kernel, at
least easily. What are the chances this is provoked by L0 kernel
(syzbot also runs in VMs)?