Re: [PATCH v3 07/26] KVM: VMX: Move preemption timer <=> hrtimer dance to common x86
From: Tudor Ambarus
Date: Wed Mar 29 2023 - 11:22:44 EST
On 3/29/23 14:47, Paolo Bonzini wrote:
Hi, Paolo!
> 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.
>
Thanks for the prompt answer! I fixed the conflicts as per your
suggestion and tested the patch with the reproducer on top of
stable/linux-5.15.y and I confirm the reproducer is silenced. Sent the
patch proposal (with you in To:) at:
https://lore.kernel.org/all/20230329151747.2938509-1-tudor.ambarus@xxxxxxxxxx/T/#u
Feel free to ACK/NACK it.
Cheers,
ta