Patch "KVM: MMU: fix reserved bit check for ept=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

KVM: MMU: fix reserved bit check for ept=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     kvm-mmu-fix-reserved-bit-check-for-ept-0-cr0.wp-0-cr4.smep-1-efer.nx-0.patch

and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paolo Bonzini <[email protected]>
Date: Wed, 9 Mar 2016 14:28:02 +0100
Subject: KVM: MMU: fix reserved bit check for ept=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0

From: Paolo Bonzini <[email protected]>

commit 5f0b819995e172f48fdcd91335a2126ba7d9deae upstream.

KVM has special logic to handle pages with pte.u=1 and pte.w=0 when CR0.WP=1. These pages' SPTEs flip continuously between two states:
U=1/W=0 (user and supervisor reads allowed, supervisor writes not allowed) and U=0/W=1 (supervisor reads and writes allowed, user writes not allowed).

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, making the two states U=1/W=0/NX=gpte.NX and U=0/W=1/NX=1. When guest EFER has the NX bit cleared, the reserved bit check thinks that the latter state is invalid; teach it that the smep_andnot_wp case will also use the NX bit of SPTEs.

---
 arch/x86/kvm/mmu.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -3754,13 +3754,15 @@ static void reset_rsvds_bits_mask_ept(st
 void
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
+	bool uses_nx = context->nx || context->base_role.smep_andnot_wp;
+
 	/*
 	 * Passing "true" to the last argument is okay; it adds a check
 	 * on bit 8 of the SPTEs which KVM doesn't use anyway.
 	 */
 	__reset_rsvds_bits_mask(vcpu, &context->shadow_zero_check,
 				boot_cpu_data.x86_phys_bits,
-				context->shadow_root_level, context->nx,
+				context->shadow_root_level, uses_nx,
 				guest_cpuid_has_gbpages(vcpu), is_pse(vcpu),
 				true);
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/kvm-cap-halt-polling-at-exactly-halt_poll_ns.patch queue-4.4/kvm-vmx-disable-pebs-before-a-guest-entry.patch queue-4.4/kvm-mmu-fix-reserved-bit-check-for-ept-0-cr0.wp-0-cr4.smep-1-efer.nx-0.patch queue-4.4/kvm-mmu-fix-ept-0-pte.u-1-pte.w-0-cr0.wp-0-cr4.smep-1-efer.nx-0-combo.patch queue-4.4/kvm-s390-correct-fprs-on-sigp-stop-and-store-status.patch