[PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Jan Kiszka]

In order to access the shadow VMCS, we need to load it. At this point,
vmx->loaded_vmcs->vmcs and the actually loaded one start to differ. If
we now get preempted by Linux, vmx_vcpu_put and, on return, the
vmx_vcpu_load will work against the wrong vmcs. That can cause
copy_shadow_to_vmcs12 to corrupt the vmcs12 state.

Fix the issue by disabling preemption during the copy operation.

copy_vmcs12_to_shadow is safe from this issue as it is executed by
vmx_vcpu_run when preemption is already disabled before vmentry.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

This fixes specifically Jailhouse in KVM on CPUs with shadow VMCS
support.

 arch/x86/kvm/vmx.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 04fa1b8..f3de106 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
 	const unsigned long *fields = shadow_read_write_fields;
 	const int num_fields = max_shadow_read_write_fields;
 
+	preempt_disable();
+
 	vmcs_load(shadow_vmcs);
 
 	for (i = 0; i < num_fields; i++) {
@@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
 
 	vmcs_clear(shadow_vmcs);
 	vmcs_load(vmx->loaded_vmcs->vmcs);
+
+	preempt_enable();
 }
 
 static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
-- 
1.8.4.5

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Re: [PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Bandan Das]

Jan Kiszka <jan.kiszka@siemens.com> writes:

> In order to access the shadow VMCS, we need to load it. At this point,
> vmx->loaded_vmcs->vmcs and the actually loaded one start to differ. If
> we now get preempted by Linux, vmx_vcpu_put and, on return, the
> vmx_vcpu_load will work against the wrong vmcs. That can cause
> copy_shadow_to_vmcs12 to corrupt the vmcs12 state.

Ouch! I apologize if I missed this in the previous discussion but why do
we never get into this condition while running a Linux guest ?

Will there be a performance impact of this change ? I hope it's 
negligible though..

> Fix the issue by disabling preemption during the copy operation.
>
> copy_vmcs12_to_shadow is safe from this issue as it is executed by
> vmx_vcpu_run when preemption is already disabled before vmentry.
>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>
> This fixes specifically Jailhouse in KVM on CPUs with shadow VMCS
> support.
>
>  arch/x86/kvm/vmx.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 04fa1b8..f3de106 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>  	const unsigned long *fields = shadow_read_write_fields;
>  	const int num_fields = max_shadow_read_write_fields;
>  
> +	preempt_disable();
> +
>  	vmcs_load(shadow_vmcs);
>  
>  	for (i = 0; i < num_fields; i++) {
> @@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>  
>  	vmcs_clear(shadow_vmcs);
>  	vmcs_load(vmx->loaded_vmcs->vmcs);
> +
> +	preempt_enable();
>  }
>  
>  static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)

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Re: [PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Jan Kiszka]

On 2014-10-08 21:07, Bandan Das wrote:
> Jan Kiszka <jan.kiszka@siemens.com> writes:
> 
>> In order to access the shadow VMCS, we need to load it. At this point,
>> vmx->loaded_vmcs->vmcs and the actually loaded one start to differ. If
>> we now get preempted by Linux, vmx_vcpu_put and, on return, the
>> vmx_vcpu_load will work against the wrong vmcs. That can cause
>> copy_shadow_to_vmcs12 to corrupt the vmcs12 state.
> 
> Ouch! I apologize if I missed this in the previous discussion but why do
> we never get into this condition while running a Linux guest ?

Well, you need high load on the host, preemption at the "wrong" time,
and some relevant difference between the still to-be-copied shadow vmcs
fields between the proper vmcs and the real one that was used by KVM at
that point. I don't think it is a Jailhouse-only issue, but other
hypervisors may be less sensitive.

> 
> Will there be a performance impact of this change ? I hope it's 
> negligible though..

The preemption counter is cheap, the only impact should be on scheduling
latency. But that is irrelevant as long as we have the counterpart under
the same preemption lock.

Jan

> 
>> Fix the issue by disabling preemption during the copy operation.
>>
>> copy_vmcs12_to_shadow is safe from this issue as it is executed by
>> vmx_vcpu_run when preemption is already disabled before vmentry.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>
>> This fixes specifically Jailhouse in KVM on CPUs with shadow VMCS
>> support.
>>
>>  arch/x86/kvm/vmx.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 04fa1b8..f3de106 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>>  	const unsigned long *fields = shadow_read_write_fields;
>>  	const int num_fields = max_shadow_read_write_fields;
>>  
>> +	preempt_disable();
>> +
>>  	vmcs_load(shadow_vmcs);
>>  
>>  	for (i = 0; i < num_fields; i++) {
>> @@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>>  
>>  	vmcs_clear(shadow_vmcs);
>>  	vmcs_load(vmx->loaded_vmcs->vmcs);
>> +
>> +	preempt_enable();
>>  }
>>  
>>  static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)

-- 
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Corporate Competence Center Embedded Linux

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Re: [PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Paolo Bonzini]

Il 08/10/2014 18:05, Jan Kiszka ha scritto:
> In order to access the shadow VMCS, we need to load it. At this point,
> vmx->loaded_vmcs->vmcs and the actually loaded one start to differ. If
> we now get preempted by Linux, vmx_vcpu_put and, on return, the
> vmx_vcpu_load will work against the wrong vmcs. That can cause
> copy_shadow_to_vmcs12 to corrupt the vmcs12 state.
> 
> Fix the issue by disabling preemption during the copy operation.
> 
> copy_vmcs12_to_shadow is safe from this issue as it is executed by
> vmx_vcpu_run when preemption is already disabled before vmentry.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> 
> This fixes specifically Jailhouse in KVM on CPUs with shadow VMCS
> support.

Thanks.  I'll add this above your SoB line:

This bug is exposed by running Jailhouse within KVM on CPUs with
shadow VMCS support.  Jailhouse never expects an interrupt pending
vmexit, but the bug can cause it if, after copy_shadow_to_vmcs12
is preempted, the active VMCS happens to have the virtual interrupt
pending flag set in the CPU-based execution controls.

Paolo

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Re: [PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Paolo Bonzini]

Il 08/10/2014 21:19, Jan Kiszka ha scritto:
> > Ouch! I apologize if I missed this in the previous discussion but why do
> > we never get into this condition while running a Linux guest ?
> 
> Well, you need high load on the host, preemption at the "wrong" time,
> and some relevant difference between the still to-be-copied shadow vmcs
> fields between the proper vmcs and the real one that was used by KVM at
> that point. I don't think it is a Jailhouse-only issue, but other
> hypervisors may be less sensitive.

Indeed; in the particular case of the virtual interrupt pending bit it's
easy to see why it is not a problem for Linux guests:
handle_interrupt_window hardly does anything, you will just have a
useless KVM_REQ_EVENT request.

Running many different L2 guests (with an overcommitted host) should
expose it.  Note that the memory map and selectors are relatively
similar among similar guests, and the most scary fields (RIP and RSP),
are copied first when preemption is less likely.  It's possible that
moving them last would increase the chance of breakage.

Paolo

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Re: [PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Wanpeng Li]

On Wed, Oct 08, 2014 at 06:05:39PM +0200, Jan Kiszka wrote:
>In order to access the shadow VMCS, we need to load it. At this point,
>vmx->loaded_vmcs->vmcs and the actually loaded one start to differ. If
>we now get preempted by Linux, vmx_vcpu_put and, on return, the
>vmx_vcpu_load will work against the wrong vmcs. That can cause
>copy_shadow_to_vmcs12 to corrupt the vmcs12 state.
>
>Fix the issue by disabling preemption during the copy operation.
>
>copy_vmcs12_to_shadow is safe from this issue as it is executed by
>vmx_vcpu_run when preemption is already disabled before vmentry.
>
>Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>---

Reviewed-by: Wanpeng Li <wanpeng.li@linux.intel.com>

Regards,
Wanpeng Li 

>
>This fixes specifically Jailhouse in KVM on CPUs with shadow VMCS
>support.
>
> arch/x86/kvm/vmx.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
>diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>index 04fa1b8..f3de106 100644
>--- a/arch/x86/kvm/vmx.c
>+++ b/arch/x86/kvm/vmx.c
>@@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
> 	const unsigned long *fields = shadow_read_write_fields;
> 	const int num_fields = max_shadow_read_write_fields;
> 
>+	preempt_disable();
>+
> 	vmcs_load(shadow_vmcs);
> 
> 	for (i = 0; i < num_fields; i++) {
>@@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
> 
> 	vmcs_clear(shadow_vmcs);
> 	vmcs_load(vmx->loaded_vmcs->vmcs);
>+
>+	preempt_enable();
> }
> 
> static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>-- 
>1.8.4.5

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