[Xen-changelog] [xen stable-4.2] VMX: disable SMEP feature when guest is in non-paging mode

[patchbot@xxxxxxx]

commit 485f374230d39e153d7b9786e3d0336bd52ee661
Author:     Dongxiao Xu <dongxiao.xu@xxxxxxxxx>
AuthorDate: Tue Feb 12 13:44:02 2013 +0100
Commit:     Dongxiao Xu <dongxiao.xu@xxxxxxxxx>
CommitDate: Tue Feb 12 13:44:02 2013 +0100

    VMX: disable SMEP feature when guest is in non-paging mode
    
    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-unstable changeset: 26502:d1bf3b21f783
    xen-unstable date: Wed Jan 30 17:17:30 UTC 2013
---
 xen/arch/x86/hvm/vmx/vmx.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index b8f0336..ca92b86 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1227,6 +1227,13 @@ 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]);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.2

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