Re: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset

[Bruce Rogers]

>>> On 4/28/2016 at 01:08 PM, Radim Kr*má*  wrote: 
> 2016-04-22 12:56-0600, Bruce Rogers:
>> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
>> allowing the current (old) cr0 value to mess up vcpu initialization.
>> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
>> kvm_mmu_reset_context(). Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
>> is completely redundant. Change the order back to ensure proper vcpu
>> initialization.
>> 
>> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
>> ept=N option being set results in a VM-entry failure. This patch fixes that.
> 
> Greg pointed out missing,
>   Cc: sta...@vger.kernel.org
> when sta...@vger.kernel.org was Cc'd. Adding
>   Fixes: d28bc9dd25ce ("KVM: x86: INIT and reset sequences are different")
> would be nice too (even when it is redundant).
> 
>> Signed-off-by: Bruce Rogers 
>> ---
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> @@ -5046,8 +5046,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool 
> init_event)
>>  cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
>> -vmx_set_cr0(vcpu, cr0); /* enter rmode */
>>  vmx->vcpu.arch.cr0 = cr0;
>> +vmx_set_cr0(vcpu, cr0); /* enter rmode */
> 
> So vmx_set_cr0() has a code that depends on vmx->vcpu.arch.cr0 being
> already set the to new value.  Do you know what function is it?

The one which I partly referred to above is the following:
vmx_set_cr0() ->
enter_rmode() ->
kvm_mmu_reset_context() ->
init_kvm_softmmu() ->
kvm_init_shadow_mmu() ->
is_write_protected()
which uses the CR0 WP bit.
There may be other problematic references. I haven't tried to do an
exhaustive search.

> 
> I think we better set vmx->vcpu.arch.cr0 early in vmx_set_cr0().
> Or do other callsites somehow depend on the old cr0 value?

You may be right that that is a better overall fix. I was simply trying
to undo the erroneous lines in the commit which broke things, partly
to have a patch better suited for applying to stable releases.

I'll send a v2 shortly with your suggested additions to the patch header.

Thanks,

Bruce

Re: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset

[Bruce Rogers]

>>> On 4/28/2016 at 01:08 PM, Radim Kr*má*  wrote: 
> 2016-04-22 12:56-0600, Bruce Rogers:
>> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
>> allowing the current (old) cr0 value to mess up vcpu initialization.
>> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
>> kvm_mmu_reset_context(). Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
>> is completely redundant. Change the order back to ensure proper vcpu
>> initialization.
>> 
>> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
>> ept=N option being set results in a VM-entry failure. This patch fixes that.
> 
> Greg pointed out missing,
>   Cc: sta...@vger.kernel.org
> when sta...@vger.kernel.org was Cc'd. Adding
>   Fixes: d28bc9dd25ce ("KVM: x86: INIT and reset sequences are different")
> would be nice too (even when it is redundant).
> 
>> Signed-off-by: Bruce Rogers 
>> ---
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> @@ -5046,8 +5046,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool 
> init_event)
>>  cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
>> -vmx_set_cr0(vcpu, cr0); /* enter rmode */
>>  vmx->vcpu.arch.cr0 = cr0;
>> +vmx_set_cr0(vcpu, cr0); /* enter rmode */
> 
> So vmx_set_cr0() has a code that depends on vmx->vcpu.arch.cr0 being
> already set the to new value.  Do you know what function is it?

The one which I partly referred to above is the following:
vmx_set_cr0() ->
enter_rmode() ->
kvm_mmu_reset_context() ->
init_kvm_softmmu() ->
kvm_init_shadow_mmu() ->
is_write_protected()
which uses the CR0 WP bit.
There may be other problematic references. I haven't tried to do an
exhaustive search.

> 
> I think we better set vmx->vcpu.arch.cr0 early in vmx_set_cr0().
> Or do other callsites somehow depend on the old cr0 value?

You may be right that that is a better overall fix. I was simply trying
to undo the erroneous lines in the commit which broke things, partly
to have a patch better suited for applying to stable releases.

I'll send a v2 shortly with your suggested additions to the patch header.

Thanks,

Bruce

Re: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset

[Radim Krčmář]

2016-04-22 12:56-0600, Bruce Rogers:
> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
> allowing the current (old) cr0 value to mess up vcpu initialization.
> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
> kvm_mmu_reset_context(). Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
> is completely redundant. Change the order back to ensure proper vcpu
> initialization.
> 
> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
> ept=N option being set results in a VM-entry failure. This patch fixes that.

Greg pointed out missing,
  Cc: sta...@vger.kernel.org
when sta...@vger.kernel.org was Cc'd. Adding
  Fixes: d28bc9dd25ce ("KVM: x86: INIT and reset sequences are different")
would be nice too (even when it is redundant).

> Signed-off-by: Bruce Rogers 
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -5046,8 +5046,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool 
> init_event)
>   cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
> - vmx_set_cr0(vcpu, cr0); /* enter rmode */
>   vmx->vcpu.arch.cr0 = cr0;
> + vmx_set_cr0(vcpu, cr0); /* enter rmode */

So vmx_set_cr0() has a code that depends on vmx->vcpu.arch.cr0 being
already set the to new value.  Do you know what function is it?

I think we better set vmx->vcpu.arch.cr0 early in vmx_set_cr0().
Or do other callsites somehow depend on the old cr0 value?

Thanks.

Re: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset

[Radim Krčmář]

2016-04-22 12:56-0600, Bruce Rogers:
> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
> allowing the current (old) cr0 value to mess up vcpu initialization.
> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
> kvm_mmu_reset_context(). Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
> is completely redundant. Change the order back to ensure proper vcpu
> initialization.
> 
> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
> ept=N option being set results in a VM-entry failure. This patch fixes that.

Greg pointed out missing,
  Cc: sta...@vger.kernel.org
when sta...@vger.kernel.org was Cc'd. Adding
  Fixes: d28bc9dd25ce ("KVM: x86: INIT and reset sequences are different")
would be nice too (even when it is redundant).

> Signed-off-by: Bruce Rogers 
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -5046,8 +5046,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool 
> init_event)
>   cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
> - vmx_set_cr0(vcpu, cr0); /* enter rmode */
>   vmx->vcpu.arch.cr0 = cr0;
> + vmx_set_cr0(vcpu, cr0); /* enter rmode */

So vmx_set_cr0() has a code that depends on vmx->vcpu.arch.cr0 being
already set the to new value.  Do you know what function is it?

I think we better set vmx->vcpu.arch.cr0 early in vmx_set_cr0().
Or do other callsites somehow depend on the old cr0 value?

Thanks.

Re: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset

[Greg KH]

On Fri, Apr 22, 2016 at 12:56:03PM -0600, Bruce Rogers wrote:
> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
> allowing the current (old) cr0 value to mess up vcpu initialization.
> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
> kvm_mmu_reset_context(). Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
> is completely redundant. Change the order back to ensure proper vcpu
> initialization.
> 
> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
> ept=N option being set results in a VM-entry failure. This patch fixes that.
> 
> Signed-off-by: Bruce Rogers 
> ---
>  arch/x86/kvm/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)



This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

Re: [PATCH] KVM: x86: fix ordering of cr0 initialization code in vmx_cpu_reset

[Greg KH]

On Fri, Apr 22, 2016 at 12:56:03PM -0600, Bruce Rogers wrote:
> Commit d28bc9dd25ce reversed the order of two lines which initialize cr0,
> allowing the current (old) cr0 value to mess up vcpu initialization.
> This was observed in the checks for cr0 X86_CR0_WP bit in the context of
> kvm_mmu_reset_context(). Besides, setting vcpu->arch.cr0 after vmx_set_cr0()
> is completely redundant. Change the order back to ensure proper vcpu
> initialization.
> 
> The combination of booting with ovmf firmware when guest vcpus > 1 and kvm's
> ept=N option being set results in a VM-entry failure. This patch fixes that.
> 
> Signed-off-by: Bruce Rogers 
> ---
>  arch/x86/kvm/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)



This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.