KVM: MMU: Adjust shadow paging to work when SMEP=1 and CR0.WP=0

When CR0.WP=0, we sometimes map user pages as kernel pages (to allow
the kernel to write to them).  Unfortunately this also allows the kernel
to fetch from these pages, even if CR4.SMEP is set.

Adjust for this by also setting NX on the spte in these circumstances.
Signed-off-by: Avi Kivity <avi@redhat.com>

Documentation/virtual/kvm/mmu.txt

@@ -165,6 +165,10 @@ Shadow pages contain the following information:
     Contains the value of efer.nxe for which the page is valid.
   role.cr0_wp:
     Contains the value of cr0.wp for which the page is valid.
+  role.smep_andnot_wp:
+    Contains the value of cr4.smep && !cr0.wp for which the page is valid
+    (pages for which this is true are different from other pages; see the
+    treatment of cr0.wp=0 below).
   gfn:
     Either the guest page table containing the translations shadowed by this
     page, or the base page frame for linear translations.  See role.direct.
@@ -317,6 +321,20 @@ on fault type:
 
 (user write faults generate a #PF)
 
+In the first case there is an additional complication 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.
+
+To prevent an spte that was converted into a kernel page with cr0.wp=0
+from being written by the kernel after cr0.wp has changed to 1, we make
+the value of cr0.wp part of the page role.  This means that an spte created
+with one value of cr0.wp cannot be used when cr0.wp has a different value -
+it will simply be missed by the shadow page lookup code.  A similar issue
+exists when an spte created with cr0.wp=0 and cr4.smep=0 is used after
+changing cr4.smep to 1.  To avoid this, the value of !cr0.wp && cr4.smep
+is also made a part of the page role.
+
 Large pages
 ===========
 

arch/x86/include/asm/kvm_host.h

@@ -205,6 +205,7 @@ union kvm_mmu_page_role {
 		unsigned invalid:1;
 		unsigned nxe:1;
 		unsigned cr0_wp:1;
+		unsigned smep_andnot_wp:1;
 	};
 };
 

arch/x86/kvm/mmu.c

@@ -1985,8 +1985,17 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		spte |= PT_WRITABLE_MASK;
 
 		if (!vcpu->arch.mmu.direct_map
-		    && !(pte_access & ACC_WRITE_MASK))
+		    && !(pte_access & ACC_WRITE_MASK)) {
 			spte &= ~PT_USER_MASK;
+			/*
+			 * If we converted a user page to a kernel page,
+			 * so that the kernel can write to it when cr0.wp=0,
+			 * then we should prevent the kernel from executing it
+			 * if SMEP is enabled.
+			 */
+			if (kvm_read_cr4_bits(vcpu, X86_CR4_SMEP))
+				spte |= PT64_NX_MASK;
+		}
 
 		/*
 		 * Optimization: for pte sync, if spte was writable the hash
@@ -2955,6 +2964,7 @@ static int init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
 int kvm_init_shadow_mmu(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
 	int r;
+	bool smep = kvm_read_cr4_bits(vcpu, X86_CR4_SMEP);
 	ASSERT(vcpu);
 	ASSERT(!VALID_PAGE(vcpu->arch.mmu.root_hpa));
 
@@ -2969,6 +2979,8 @@ int kvm_init_shadow_mmu(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 
 	vcpu->arch.mmu.base_role.cr4_pae = !!is_pae(vcpu);
 	vcpu->arch.mmu.base_role.cr0_wp  = is_write_protection(vcpu);
+	vcpu->arch.mmu.base_role.smep_andnot_wp
+		= smep && !is_write_protection(vcpu);
 
 	return r;
 }