* [PATCH] VMX: disable SMEP feature when guest is in non-paging mode
@ 2013-01-30 9:26 dongxiao.xu
2013-01-30 17:54 ` Tim Deegan
0 siblings, 1 reply; 6+ messages in thread
From: dongxiao.xu @ 2013-01-30 9:26 UTC (permalink / raw)
To: xen-devel
From: Dongxiao Xu <dongxiao.xu@intel.com>
SMEP is disabled if CPU is in non-paging mode in hardware.
However Xen always uses paging mode to emulate guest non-paging
mode with HAP. To emulate this behavior, SMEP needs to be manually
disabled when guest switches to non-paging mode.
We met an issue that, SMP Linux guest with recent kernel (enable
SMEP support, for example, 3.5.3) would crash with triple fault if
setting unrestricted_guest=0 in grub. This is because Xen uses an
identity mapping page table to emulate the non-paging mode, where
the page table is set with USER flag. If SMEP is still enabled in
this case, guest will meet unhandlable page fault and then crash.
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
---
xen/arch/x86/hvm/vmx/vmx.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 4d7c93f..6347f66 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1167,6 +1167,14 @@ static void vmx_update_guest_cr(struct vcpu *v, unsigned int cr)
{
v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;
+ /*
+ * SMEP is disabled if CPU is in non-paging mode in hardware.
+ * However Xen always uses paging mode to emulate guest non-paging
+ * mode with HAP.
+ * To emulate this behavior, SMEP needs to be manually disabled when
+ * guest switches to non-paging mode.
+ */
+ v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_SMEP;
}
__vmwrite(GUEST_CR4, v->arch.hvm_vcpu.hw_cr[4]);
__vmwrite(CR4_READ_SHADOW, v->arch.hvm_vcpu.guest_cr[4]);
--
1.7.1
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH] VMX: disable SMEP feature when guest is in non-paging mode
2013-01-30 9:26 [PATCH] VMX: disable SMEP feature when guest is in non-paging mode dongxiao.xu
@ 2013-01-30 17:54 ` Tim Deegan
2013-01-30 18:26 ` Keir Fraser
0 siblings, 1 reply; 6+ messages in thread
From: Tim Deegan @ 2013-01-30 17:54 UTC (permalink / raw)
To: dongxiao.xu; +Cc: xen-devel
At 17:26 +0800 on 30 Jan (1359566815), dongxiao.xu@intel.com wrote:
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 4d7c93f..6347f66 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -1167,6 +1167,14 @@ static void vmx_update_guest_cr(struct vcpu *v, unsigned int cr)
> {
> v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
> v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;
> + /*
> + * SMEP is disabled if CPU is in non-paging mode in hardware.
> + * However Xen always uses paging mode to emulate guest non-paging
> + * mode with HAP.
> + * To emulate this behavior, SMEP needs to be manually disabled when
> + * guest switches to non-paging mode.
> + */
> + v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_SMEP;
Shouldn't this be handled on CR0 writes as well as CR4? The SMEP bit of
hw_cr[4] ought to be cleared and set whenever CR0_PG is changed.
Cheers,
Tim.
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] VMX: disable SMEP feature when guest is in non-paging mode
2013-01-30 17:54 ` Tim Deegan
@ 2013-01-30 18:26 ` Keir Fraser
2013-01-31 0:56 ` Xu, Dongxiao
0 siblings, 1 reply; 6+ messages in thread
From: Keir Fraser @ 2013-01-30 18:26 UTC (permalink / raw)
To: Tim Deegan, dongxiao.xu; +Cc: xen-devel
On 30/01/2013 09:54, "Tim Deegan" <tim@xen.org> wrote:
> At 17:26 +0800 on 30 Jan (1359566815), dongxiao.xu@intel.com wrote:
>> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
>> index 4d7c93f..6347f66 100644
>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>> @@ -1167,6 +1167,14 @@ static void vmx_update_guest_cr(struct vcpu *v,
>> unsigned int cr)
>> {
>> v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
>> v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;
>> + /*
>> + * SMEP is disabled if CPU is in non-paging mode in hardware.
>> + * However Xen always uses paging mode to emulate guest
>> non-paging
>> + * mode with HAP.
>> + * To emulate this behavior, SMEP needs to be manually disabled
>> when
>> + * guest switches to non-paging mode.
>> + */
>> + v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_SMEP;
>
> Shouldn't this be handled on CR0 writes as well as CR4? The SMEP bit of
> hw_cr[4] ought to be cleared and set whenever CR0_PG is changed.
That's what I would expect too.
-- Keir
> Cheers,
>
> Tim.
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] VMX: disable SMEP feature when guest is in non-paging mode
2013-01-30 18:26 ` Keir Fraser
@ 2013-01-31 0:56 ` Xu, Dongxiao
2013-01-31 1:11 ` Keir Fraser
0 siblings, 1 reply; 6+ messages in thread
From: Xu, Dongxiao @ 2013-01-31 0:56 UTC (permalink / raw)
To: Keir Fraser, Tim Deegan; +Cc: xen-devel@lists.xen.org
> -----Original Message-----
> From: Keir Fraser [mailto:keir.xen@gmail.com]
> Sent: Thursday, January 31, 2013 2:27 AM
> To: Tim Deegan; Xu, Dongxiao
> Cc: xen-devel@lists.xen.org
> Subject: Re: [Xen-devel] [PATCH] VMX: disable SMEP feature when guest is in
> non-paging mode
>
> On 30/01/2013 09:54, "Tim Deegan" <tim@xen.org> wrote:
>
> > At 17:26 +0800 on 30 Jan (1359566815), dongxiao.xu@intel.com wrote:
> >> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> >> index 4d7c93f..6347f66 100644
> >> --- a/xen/arch/x86/hvm/vmx/vmx.c
> >> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> >> @@ -1167,6 +1167,14 @@ static void vmx_update_guest_cr(struct vcpu
> *v,
> >> unsigned int cr)
> >> {
> >> v->arch.hvm_vcpu.hw_cr[4] |= X86_CR4_PSE;
> >> v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_PAE;
> >> + /*
> >> + * SMEP is disabled if CPU is in non-paging mode in
> hardware.
> >> + * However Xen always uses paging mode to emulate guest
> >> non-paging
> >> + * mode with HAP.
> >> + * To emulate this behavior, SMEP needs to be manually
> disabled
> >> when
> >> + * guest switches to non-paging mode.
> >> + */
> >> + v->arch.hvm_vcpu.hw_cr[4] &= ~X86_CR4_SMEP;
> >
> > Shouldn't this be handled on CR0 writes as well as CR4? The SMEP bit of
> > hw_cr[4] ought to be cleared and set whenever CR0_PG is changed.
>
> That's what I would expect too.
In the case 0 (CR0 update) in vmx_update_guest_cr(), it will call vmx_update_guest_cr(v, 4) to adjust CR4 related bits if certain bits (PG, PE) in CR0 changed.
Thanks,
Dongxiao
>
> -- Keir
>
> > Cheers,
> >
> > Tim.
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@lists.xen.org
> > http://lists.xen.org/xen-devel
>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] VMX: disable SMEP feature when guest is in non-paging mode
2013-01-31 0:56 ` Xu, Dongxiao
@ 2013-01-31 1:11 ` Keir Fraser
2013-01-31 1:28 ` Xu, Dongxiao
0 siblings, 1 reply; 6+ messages in thread
From: Keir Fraser @ 2013-01-31 1:11 UTC (permalink / raw)
To: Xu, Dongxiao, Tim Deegan; +Cc: xen-devel@lists.xen.org
On 30/01/2013 16:56, "Xu, Dongxiao" <dongxiao.xu@intel.com> wrote:
>>> Shouldn't this be handled on CR0 writes as well as CR4? The SMEP bit of
>>> hw_cr[4] ought to be cleared and set whenever CR0_PG is changed.
>>
>> That's what I would expect too.
>
> In the case 0 (CR0 update) in vmx_update_guest_cr(), it will call
> vmx_update_guest_cr(v, 4) to adjust CR4 related bits if certain bits (PG, PE)
> in CR0 changed.
Good point!
In fact we can make that cr4 update unconditional and simplify the cr0
handling a bit more.
-- Keir
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] VMX: disable SMEP feature when guest is in non-paging mode
2013-01-31 1:11 ` Keir Fraser
@ 2013-01-31 1:28 ` Xu, Dongxiao
0 siblings, 0 replies; 6+ messages in thread
From: Xu, Dongxiao @ 2013-01-31 1:28 UTC (permalink / raw)
To: Keir Fraser, Tim Deegan; +Cc: xen-devel@lists.xen.org
> -----Original Message-----
> From: Keir Fraser [mailto:keir.xen@gmail.com]
> Sent: Thursday, January 31, 2013 9:12 AM
> To: Xu, Dongxiao; Tim Deegan
> Cc: xen-devel@lists.xen.org
> Subject: Re: [Xen-devel] [PATCH] VMX: disable SMEP feature when guest is in
> non-paging mode
>
> On 30/01/2013 16:56, "Xu, Dongxiao" <dongxiao.xu@intel.com> wrote:
>
> >>> Shouldn't this be handled on CR0 writes as well as CR4? The SMEP bit of
> >>> hw_cr[4] ought to be cleared and set whenever CR0_PG is changed.
> >>
> >> That's what I would expect too.
> >
> > In the case 0 (CR0 update) in vmx_update_guest_cr(), it will call
> > vmx_update_guest_cr(v, 4) to adjust CR4 related bits if certain bits (PG, PE)
> > in CR0 changed.
>
> Good point!
>
> In fact we can make that cr4 update unconditional and simplify the cr0
> handling a bit more.
Yes, I saw your changes, which makes the code more neat.
Thanks,
Dongxiao
>
> -- Keir
>
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2013-01-31 1:28 UTC | newest]
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2013-01-30 9:26 [PATCH] VMX: disable SMEP feature when guest is in non-paging mode dongxiao.xu
2013-01-30 17:54 ` Tim Deegan
2013-01-30 18:26 ` Keir Fraser
2013-01-31 0:56 ` Xu, Dongxiao
2013-01-31 1:11 ` Keir Fraser
2013-01-31 1:28 ` Xu, Dongxiao
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