linux-kernel - Re: [PATCH v2] KVM: nVMX: do not use dangling shadow VMCS after guest reset

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Date:   Sat, 20 Jul 2019 00:21:11 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] KVM: nVMX: do not use dangling shadow VMCS after guest
 reset

On 20/07/19 00:06, Liran Alon wrote:
> 
> 
>> On 20 Jul 2019, at 0:39, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> If a KVM guest is reset while running a nested guest, free_nested will
>> disable the shadow VMCS execution control in the vmcs01.  However,
>> on the next KVM_RUN vmx_vcpu_run would nevertheless try to sync
>> the VMCS12 to the shadow VMCS which has since been freed.
>>
>> This causes a vmptrld of a NULL pointer on my machime, but Jan reports
>> the host to hang altogether.  Let's see how much this trivial patch fixes.
>>
>> Reported-by: Jan Kiszka <jan.kiszka@...mens.com>
>> Cc: Liran Alon <liran.alon@...cle.com>
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> 1) Are we sure we prefer WARN_ON() instead of WARN_ON_ONCE()?

I don't think you can get it to be called in a loop, the calls are
generally guarded by ifs.

> 2) Should we also check for WARN_ON(!vmcs12)? As free_nested() also kfree(vmx->nested.cached_vmcs12).

Well, it doesn't NULL it but it does NULL shadow_vmcs so the extra
warning wouldn't add much.

> In fact, because free_nested() don’t put NULL in cached_vmcs12 after kfree() it, I wonder if we shouldn’t create a separate patch that does:
> (a) Modify free_nested() to put NULL in cached_vmcs12 after kfree().
> (b) Put BUG_ON(!cached_vmcs12) in get_vmcs12() before returning value.

This is useful but a separate improvement (and not a bugfix, I want this
patch to be small so it applies to older trees).

Paolo

> -Liran
> 
>> ---
>> arch/x86/kvm/vmx/nested.c | 8 +++++++-
>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index 4f23e34f628b..0f1378789bd0 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -194,6 +194,7 @@ static void vmx_disable_shadow_vmcs(struct vcpu_vmx *vmx)
>> {
>> 	secondary_exec_controls_clearbit(vmx, SECONDARY_EXEC_SHADOW_VMCS);
>> 	vmcs_write64(VMCS_LINK_POINTER, -1ull);
>> +	vmx->nested.need_vmcs12_to_shadow_sync = false;
>> }
>>
>> static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
>> @@ -1341,6 +1342,9 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>> 	unsigned long val;
>> 	int i;
>>
>> +	if (WARN_ON(!shadow_vmcs))
>> +		return;
>> +
>> 	preempt_disable();
>>
>> 	vmcs_load(shadow_vmcs);
>> @@ -1373,6 +1377,9 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>> 	unsigned long val;
>> 	int i, q;
>>
>> +	if (WARN_ON(!shadow_vmcs))
>> +		return;
>> +
>> 	vmcs_load(shadow_vmcs);
>>
>> 	for (q = 0; q < ARRAY_SIZE(fields); q++) {
>> @@ -4436,7 +4443,6 @@ static inline void nested_release_vmcs12(struct kvm_vcpu *vcpu)
>> 		/* copy to memory all shadowed fields in case
>> 		   they were modified */
>> 		copy_shadow_to_vmcs12(vmx);
>> -		vmx->nested.need_vmcs12_to_shadow_sync = false;
>> 		vmx_disable_shadow_vmcs(vmx);
>> 	}
>> 	vmx->nested.posted_intr_nv = -1;
>> -- 
>> 1.8.3.1
>>
>