Re: [PATCH v3] KVM: hyperv: Fix Direct Synthetic timers assert an interrupt w/o lapic_in_kernel

[Vitaly Kuznetsov]

Wanpeng Li <kernellwp@xxxxxxxxx> writes:

> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> Reported by syzkaller:
>
> 	kasan: GPF could be caused by NULL-ptr deref or user memory access
> 	general protection fault: 0000 [#1] PREEMPT SMP KASAN
> 	RIP: 0010:__apic_accept_irq+0x46/0x740 arch/x86/kvm/lapic.c:1029
> 	Call Trace:
> 	kvm_apic_set_irq+0xb4/0x140 arch/x86/kvm/lapic.c:558
> 	stimer_notify_direct arch/x86/kvm/hyperv.c:648 [inline]
> 	stimer_expiration arch/x86/kvm/hyperv.c:659 [inline]
> 	kvm_hv_process_stimers+0x594/0x1650 arch/x86/kvm/hyperv.c:686
> 	vcpu_enter_guest+0x2b2a/0x54b0 arch/x86/kvm/x86.c:7896
> 	vcpu_run+0x393/0xd40 arch/x86/kvm/x86.c:8152
> 	kvm_arch_vcpu_ioctl_run+0x636/0x900 arch/x86/kvm/x86.c:8360
> 	kvm_vcpu_ioctl+0x6cf/0xaf0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2765
>
> The testcase programs HV_X64_MSR_STIMERn_CONFIG/HV_X64_MSR_STIMERn_COUNT,
> in addition, there is no lapic in the kernel, the counters value are small
> enough in order that kvm_hv_process_stimers() inject this already-expired
> timer interrupt into the guest through lapic in the kernel which triggers
> the NULL deferencing. This patch fixes it by don't advertise direct mode 
> synthetic timers and discarding the inject when lapic is not in kernel.
>
> syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=1752fe0a600000
>
> Reported-by: syzbot+dff25ee91f0c7d5c1695@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> v2 -> v3:
>  * add the link of syzkaller source
> v1 -> v2:
>  * don't advertise direct mode synthetic timers when lapic is not in kernel
>
>  arch/x86/kvm/hyperv.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index c10a8b1..069e655 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -645,7 +645,9 @@ static int stimer_notify_direct(struct kvm_vcpu_hv_stimer *stimer)
>  		.vector = stimer->config.apic_vector
>  	};
>  
> -	return !kvm_apic_set_irq(vcpu, &irq, NULL);
> +	if (lapic_in_kernel(vcpu))
> +		return !kvm_apic_set_irq(vcpu, &irq, NULL);
> +	return 0;

We can go even further and forbid to enable direct mode by adding
lapic_in_kernel() check to stimer_set_config() but the guest (or
userspace setting CPUIDs) is already misbehaving and we can't magically
fix things in KVM.

>  }
>  
>  static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)
> @@ -1849,7 +1851,13 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
>  
>  			ent->edx |= HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
>  			ent->edx |= HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;
> -			ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
> +
> +			/*
> +			 * Direct Synthetic timers only make sense with in-kernel
> +			 * LAPIC
> +			 */
> +			if (lapic_in_kernel(vcpu))
> +				ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
>  
>  			break;

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly