Re: WARNING: refcount bug in kvm_vm_ioctl

[Paolo Bonzini <pbonzini@redhat.com>]

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)