[PATCH v3 0/2] KVM: x86/xen: Prevent Xen timer init when running

[PATCH v3 0/2] KVM: x86/xen: Prevent Xen timer init when running

[Coleman Dietsch]

The following issue was discovered by syzbot:
WARNING: ODEBUG bug in kvm_xen_vcpu_set_attr.

When running the syzbot reproducer code, the following crash dump occurs:

ODEBUG: init active (active state 0)
object type: hrtimer hint: xen_timer_callbac0
RIP: 0010:debug_print_object+0x16e/0x250 lib/debugobjects.c:502
Call Trace:
__debug_object_init
debug_hrtimer_init
debug_init
hrtimer_init
kvm_xen_init_timer
kvm_xen_vcpu_set_attr
kvm_arch_vcpu_ioctl
kvm_vcpu_ioctl
vfs_ioctl

The ODEBUG bug crash appears to be happening when vcpu->arch.xen.timer is
already set and kvm_xen_init_timer() is called, which appears to be the
result of two separate issues.

The first issue is that kvm_xen_init_timer() is run "every"
KVM_XEN_VCPU_ATTR_TYPE_TIMER. This is addressed in patch 1.

The second issue is that the stop xen timer code should be run before
changing the IRQ vector. This is addressed in patch 2 with some cleanup.

version 3 changes
-removed ambiguous comment
-updated another comment to be more descriptive
-continue cleaning up changelogs

version 2 changes (mostly feedback from Sean Christopherson)
-split patch into 2 patches
-fix changelogs to be more descriptive
-fix formatting issues
-add check for existing xen timer before trying to initialize another one
-removed conditional for kvm_xen_stop_timer() so that it always runs
-ensure that xen timer is stopped before changing IRQ vector
-streamlined switch case KVM_XEN_VCPU_ATTR_TYPE_TIMER a bit

Coleman Dietsch (2):
  KVM: x86/xen: Initialize Xen timer only once
  KVM: x86/xen: Stop Xen timer before changing IRQ

 arch/x86/kvm/xen.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

-- 
2.34.1

Re: [PATCH v3 0/2] KVM: x86/xen: Prevent Xen timer init when running

[Sean Christopherson]

+David

On Mon, Aug 08, 2022, Coleman Dietsch wrote:
> Coleman Dietsch (2):
>   KVM: x86/xen: Initialize Xen timer only once
>   KVM: x86/xen: Stop Xen timer before changing IRQ

Paolo, can you grab these for the current cycle?  FWIW, I believe David's ack still
stands too.

Thanks!

[*] https://lore.kernel.org/all/9bad724858b6a06c25ead865b2b3d9dfc216d01c.camel@infradead.org