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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Subject: Re: [PATCH] KVM: LAPIC: Fix SynIC Timers inject timer interrupt w/o LAPIC present
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References: <1567680270-14022-1-git-send-email-wanpengli@tencent.com>
Date:   Thu, 05 Sep 2019 15:16:29 +0200
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Wanpeng Li <kernellwp@gmail.com> writes:

> From: Wanpeng Li <wanpengli@tencent.com>
>
> 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 checking lapic_in_kernel, 
> discarding the inject if it is 0.
>
> Reported-by: syzbot+dff25ee91f0c7d5c1695@syzkaller.appspotmail.com
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/hyperv.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index c10a8b1..461fcc5 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;
>  }
>  
>  static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)

Hm, but this basically means direct mode synthetic timers won't work
when LAPIC is not in kernel but the feature will still be advertised to
the guest, not good. Shall we stop advertizing it? Something like
(completely untested):

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 3f5ad84853fb..1dfa594eaab6 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1856,7 +1856,13 @@ 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;


-- 
Vitaly