[Xen-devel] [PATCH] VMX: disable SMEP feature when guest is in non-paging mode

[dongxiao.xu@xxxxxxxxx]

From: Dongxiao Xu <dongxiao.xu@xxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Xiantao Zhang <xiantao.zhang@xxxxxxxxx>
---
 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


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