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

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Message-ID: <CACT4Y+Ys3E7Au6-wZ7DFi+yxu0sJ_Gi_6jgBHWGTQ9YNDnjE=g@mail.gmail.com>
Date:   Fri, 15 Feb 2019 16:39:52 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, Jann Horn <jannh@...gle.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>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        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 4:18 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


Jann, is it what you fixed in "kvm: fix kvm_ioctl_create_device()
reference counting (CVE-2019-6974)"?
If so, we need to close the syzbot bug.


> > # 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)