linux-kernel - Re: [PATCH v2] KVM: nVMX: Fix exception injection

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Date:   Mon, 5 Jun 2017 14:26:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2] KVM: nVMX: Fix exception injection



On 05/06/2017 14:19, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
>  WARNING: CPU: 3 PID: 2840 at arch/x86/kvm/vmx.c:10966 nested_vmx_vmexit+0xdcd/0xde0 [kvm_intel]
>  CPU: 3 PID: 2840 Comm: qemu-system-x86 Tainted: G           OE   4.12.0-rc3+ #23
>  RIP: 0010:nested_vmx_vmexit+0xdcd/0xde0 [kvm_intel]
>  Call Trace:
>   ? kvm_check_async_pf_completion+0xef/0x120 [kvm]
>   ? rcu_read_lock_sched_held+0x79/0x80
>   vmx_queue_exception+0x104/0x160 [kvm_intel]
>   ? vmx_queue_exception+0x104/0x160 [kvm_intel]
>   kvm_arch_vcpu_ioctl_run+0x1171/0x1ce0 [kvm]
>   ? kvm_arch_vcpu_load+0x47/0x240 [kvm]
>   ? kvm_arch_vcpu_load+0x62/0x240 [kvm]
>   kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
>   ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
>   ? __fget+0xf3/0x210
>   do_vfs_ioctl+0xa4/0x700
>   ? __fget+0x114/0x210
>   SyS_ioctl+0x79/0x90
>   do_syscall_64+0x81/0x220
>   entry_SYSCALL64_slow_path+0x25/0x25
> 
> This is triggered occasionally by running both win7 and win2016 in L2, in 
> addition, EPT is disabled on both L1 and L2. It can't be reproduced easily.
> 
> Commit 0b6ac343fc (KVM: nVMX: Correct handling of exception injection) mentioned 
> that "KVM wants to inject page-faults which it got to the guest. This function 
> assumes it is called with the exit reason in vmcs02 being a #PF exception". 
> Commit e011c663 (KVM: nVMX: Check all exceptions for intercept during delivery to 
> L2) allows to check all exceptions for intercept during delivery to L2. However, 
> there is no guarantee the exit reason is exception currently, when there is an 
> external interrupt occurred on host, maybe a time interrupt for host which should 
> not be injected to guest, and somewhere queues an exception, then the function 
> nested_vmx_check_exception() will be called and the vmexit emulation codes will 
> try to emulate the "Acknowledge interrupt on exit" behavior, the warning is 
> triggered.
> 
> This patch fixes it by confirming to inject exception to the guest when the exit 
> reason in vmcs02 is exception. 

I think the final part of the commit message needs to be reworded, like:

Reusing the exit reason from the L2->L0 vmexit is wrong in this case,
the reason must always be EXCEPTION_NMI when injecting an exception into
L1 as a nested vmexit.

Radim can fix it when applying.

Paolo

> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v1 -> v2:
>  * pass EXIT_REASON_EXCEPTION_NMI instead of reusing to_vmx(vcpu)->exit_reason
> 
>  arch/x86/kvm/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9b4b5d6..ca5d2b9 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2425,7 +2425,7 @@ static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned nr)
>  	if (!(vmcs12->exception_bitmap & (1u << nr)))
>  		return 0;
>  
> -	nested_vmx_vmexit(vcpu, to_vmx(vcpu)->exit_reason,
> +	nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
>  			  vmcs_read32(VM_EXIT_INTR_INFO),
>  			  vmcs_readl(EXIT_QUALIFICATION));
>  	return 1;
>