[PATCH 1/2] KVM: MMU: fix ept=0/pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo

From: Paolo Bonzini
Date: Tue Mar 08 2016 - 06:46:55 EST

Yes, all of these are needed. :) This is admittedly a bit odd, but
kvm-unit-tests access.flat tests this if you run it with "-cpu host"
and of course ept=0.

KVM handles supervisor writes of a pte.u=0/pte.w=0/CR0.WP=0 page by
setting U=0 and W=1 in the shadow PTE. This will cause a user write
to fault and a supervisor write to succeed (which is correct because
CR0.WP=0). A user read instead will flip U=0 to 1 and W=1 back to 0.
This enables user reads; it also disables supervisor writes, the next
of which will then flip the bits again.

When SMEP is in effect, however, U=0 will enable kernel execution of
this page. To avoid this, KVM also sets NX=1 in the shadow PTE together
with U=0. If the guest has not enabled NX, the result is a continuous
stream of page faults due to the NX bit being reserved.

The fix is to force EFER.NX=1 even if the CPU is taking care of the EFER

There is another bug in the reserved bit check, which I've split to a
separate patch for easier application to stable kernels.

Cc: stable@xxxxxxxxxxxxxxx
Cc: Xiao Guangrong <guangrong.xiao@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Fixes: f6577a5fa15d82217ca73c74cd2dcbc0f6c781dd
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Documentation/virtual/kvm/mmu.txt | 3 ++-
arch/x86/kvm/vmx.c | 25 +++++++++++++++----------
2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index daf9c0f742d2..c81731096a43 100644
--- a/Documentation/virtual/kvm/mmu.txt
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -358,7 +358,8 @@ In the first case there are two additional complications:
- if CR4.SMEP is enabled: since we've turned the page into a kernel page,
the kernel may now execute it. We handle this by also setting spte.nx.
If we get a user fetch or read fault, we'll change spte.u=1 and
- spte.nx=gpte.nx back.
+ spte.nx=gpte.nx back. For this to work, KVM forces EFER.NX to 1 when
+ shadow paging is in use.
- if CR4.SMAP is disabled: since the page has been changed to a kernel
page, it can not be reused when CR4.SMAP is enabled. We set
CR4.SMAP && !CR0.WP into shadow page's role to avoid this case. Note,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 6e51493ff4f9..91830809d837 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1863,20 +1863,20 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
guest_efer = vmx->vcpu.arch.efer;

- * NX is emulated; LMA and LME handled by hardware; SCE meaningless
- * outside long mode
+ * LMA and LME handled by hardware; SCE meaningless outside long mode.
- ignore_bits = EFER_NX | EFER_SCE;
+ ignore_bits = EFER_SCE;
#ifdef CONFIG_X86_64
ignore_bits |= EFER_LMA | EFER_LME;
/* SCE is meaningful only in long mode on Intel */
if (guest_efer & EFER_LMA)
ignore_bits &= ~(u64)EFER_SCE;
- guest_efer &= ~ignore_bits;
- guest_efer |= host_efer & ignore_bits;
- vmx->guest_msrs[efer_offset].data = guest_efer;
- vmx->guest_msrs[efer_offset].mask = ~ignore_bits;
+ /* NX is needed to handle CR0.WP=1, CR4.SMEP=1. */
+ if (!enable_ept) {
+ guest_efer |= EFER_NX;
+ ignore_bits |= EFER_NX;
+ }

clear_atomic_switch_msr(vmx, MSR_EFER);

@@ -1887,16 +1887,21 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
if (cpu_has_load_ia32_efer ||
(enable_ept && ((vmx->vcpu.arch.efer ^ host_efer) & EFER_NX))) {
- guest_efer = vmx->vcpu.arch.efer;
if (!(guest_efer & EFER_LMA))
guest_efer &= ~EFER_LME;
if (guest_efer != host_efer)
add_atomic_switch_msr(vmx, MSR_EFER,
guest_efer, host_efer);
return false;
- }
+ } else {
+ guest_efer &= ~ignore_bits;
+ guest_efer |= host_efer & ignore_bits;

- return true;
+ vmx->guest_msrs[efer_offset].data = guest_efer;
+ vmx->guest_msrs[efer_offset].mask = ~ignore_bits;
+ return true;
+ }

static unsigned long segment_base(u16 selector)