SMEP is disabled if CPU is in non-paging mode in hardware.
However KVM 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. This is because KVM 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>
---
arch/x86/kvm/vmx.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 9120ae1..e82f20d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3155,6 +3155,14 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
if (!is_paging(vcpu)) {
hw_cr4 &= ~X86_CR4_PAE;
hw_cr4 |= X86_CR4_PSE;
+ /*
+ * SMEP is disabled if CPU is in non-paging mode in
+ * hardware. However KVM 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.
+ */
+ hw_cr4 &= ~X86_CR4_SMEP;
} else if (!(cr4 & X86_CR4_PAE)) {
hw_cr4 &= ~X86_CR4_PAE;
}
--
1.7.1
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