Re: [PATCH v3 07/26] KVM: VMX: Move preemption timer <=> hrtimer dance to common x86

Paolo Bonzini
2023-03-29T13:48:34Z

On 3/29/23 14:34, Tudor Ambarus wrote:
> This patch fixes the bug reported at:
> LINK:
> https://syzkaller.appspot.com/bug?id=489beb3d76ef14cc6cd18125782dc6f86051a605
>
> One may find the strace at:
> LINK:https://syzkaller.appspot.com/text?tag=CrashLog&x=1798b54ec80000
> and the c reproducer at:
> LINK:https://syzkaller.appspot.com/text?tag=ReproC&x=10365781c80000
>
> Since I've no experience with kvm, it would be helpful if one of you can
> provide some guidance. Do you think it is worth to backport this patch
> to stable (together with its prerequisite patches), or shall I try to
> get familiar with the code and try to provide a less invasive fix?

I think it is enough to fix the conflicts in vmx_pre_block and
vmx_post_block, there are no prerequisites:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0718658268fe..895069038856 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7577,17 +7577,11 @@ static int vmx_pre_block(struct kvm_vcpu *vcpu)
if (pi_pre_block(vcpu))
return 1;

- if (kvm_lapic_hv_timer_in_use(vcpu))
- kvm_lapic_switch_to_sw_timer(vcpu);
-
return 0;
}

static void vmx_post_block(struct kvm_vcpu *vcpu)
{
- if (kvm_x86_ops.set_hv_timer)
- kvm_lapic_switch_to_hv_timer(vcpu);
-
pi_post_block(vcpu);
}

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fcfa3fedf84f..4eca3ec38afd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10022,12 +10022,28 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)

static inline int vcpu_block(struct kvm *kvm, struct kvm_vcpu *vcpu)
{
+ bool hv_timer;
+
if (!kvm_arch_vcpu_runnable(vcpu) &&
(!kvm_x86_ops.pre_block || static_call(kvm_x86_pre_block)(vcpu) == 0)) {
+ /*
+ * Switch to the software timer before halt-polling/blocking as
+ * the guest's timer may be a break event for the vCPU, and the
+ * hypervisor timer runs only when the CPU is in guest mode.
+ * Switch before halt-polling so that KVM recognizes an expired
+ * timer before blocking.
+ */
+ hv_timer = kvm_lapic_hv_timer_in_use(vcpu);
+ if (hv_timer)
+ kvm_lapic_switch_to_sw_timer(vcpu);
+
srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
kvm_vcpu_block(vcpu);
vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);

+ if (hv_timer)
+ kvm_lapic_switch_to_hv_timer(vcpu);
+
if (kvm_x86_ops.post_block)
static_call(kvm_x86_post_block)(vcpu);

@@ -10266,6 +10282,11 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
r = -EINTR;
goto out;
}
+ /*
+ * It should be impossible for the hypervisor timer to be in
+ * use before KVM has ever run the vCPU.
+ */
+ WARN_ON_ONCE(kvm_lapic_hv_timer_in_use(vcpu));
kvm_vcpu_block(vcpu);
if (kvm_apic_accept_events(vcpu) < 0) {
r = 0;

The fix is due to the second "if" changing from
kvm_x86_ops.set_hv_timer to hv_timer.

Paolo