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

[Liran Alon]

> On 20 Jul 2019, at 1:21, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> On 20/07/19 00:06, Liran Alon wrote:
>> 
>> 
>>> On 20 Jul 2019, at 0:39, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxxx>
>>> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> 
>> 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() donât put NULL in cached_vmcs12 after kfree() it, I wonder if we shouldnât 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

ACK on all the above. :)
Reviewed-by:  Liran Alon <liran.alon@xxxxxxxxxx>

-Liran

> 
>> -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
>>> 
>> 
>