> 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() 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
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
>>>
>>
>
