KVM: VMX: Make CR0.WP a guest owned bit

Guests like grsecurity that make heavy use of CR0.WP to implement kernel
level W^X will suffer from the implied VMEXITs.

With EPT there is no need to intercept a guest change of CR0.WP, so
simply make it a guest owned bit if we can do so.

This implies that a read of a guest's CR0.WP bit might need a VMREAD.
However, the only potentially affected user seems to be kvm_init_mmu()
which is a heavy operation to begin with. But also most callers already
cache the full value of CR0 anyway, so no additional VMREAD is needed.
The only exception is nested_vmx_load_cr3().

This change is VMX-specific, as SVM has no such fine grained control
register intercept control.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20230322013731.102955-7-minipli@grsecurity.net
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/kvm_cache_regs.h

@@ -4,7 +4,7 @@
 
 #include <linux/kvm_host.h>
 
-#define KVM_POSSIBLE_CR0_GUEST_BITS X86_CR0_TS
+#define KVM_POSSIBLE_CR0_GUEST_BITS	(X86_CR0_TS | X86_CR0_WP)
 #define KVM_POSSIBLE_CR4_GUEST_BITS				  \
 	(X86_CR4_PVI | X86_CR4_DE | X86_CR4_PCE | X86_CR4_OSFXSR  \
 	 | X86_CR4_OSXMMEXCPT | X86_CR4_PGE | X86_CR4_TSD | X86_CR4_FSGSBASE)

arch/x86/kvm/vmx/nested.c

@@ -4481,7 +4481,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
 	 * CR0_GUEST_HOST_MASK is already set in the original vmcs01
 	 * (KVM doesn't change it);
 	 */
-	vcpu->arch.cr0_guest_owned_bits = KVM_POSSIBLE_CR0_GUEST_BITS;
+	vcpu->arch.cr0_guest_owned_bits = vmx_l1_guest_owned_cr0_bits();
 	vmx_set_cr0(vcpu, vmcs12->host_cr0);
 
 	/* Same as above - no reason to call set_cr4_guest_host_mask().  */
@@ -4632,7 +4632,7 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu)
 	 */
 	vmx_set_efer(vcpu, nested_vmx_get_vmcs01_guest_efer(vmx));
 
-	vcpu->arch.cr0_guest_owned_bits = KVM_POSSIBLE_CR0_GUEST_BITS;
+	vcpu->arch.cr0_guest_owned_bits = vmx_l1_guest_owned_cr0_bits();
 	vmx_set_cr0(vcpu, vmcs_readl(CR0_READ_SHADOW));
 
 	vcpu->arch.cr4_guest_owned_bits = ~vmcs_readl(CR4_GUEST_HOST_MASK);

arch/x86/kvm/vmx/vmx.c

@@ -4790,7 +4790,7 @@ static void init_vmcs(struct vcpu_vmx *vmx)
 	/* 22.2.1, 20.8.1 */
 	vm_entry_controls_set(vmx, vmx_vmentry_ctrl());
 
-	vmx->vcpu.arch.cr0_guest_owned_bits = KVM_POSSIBLE_CR0_GUEST_BITS;
+	vmx->vcpu.arch.cr0_guest_owned_bits = vmx_l1_guest_owned_cr0_bits();
 	vmcs_writel(CR0_GUEST_HOST_MASK, ~vmx->vcpu.arch.cr0_guest_owned_bits);
 
 	set_cr4_guest_host_mask(vmx);

arch/x86/kvm/vmx/vmx.h

@@ -640,6 +640,24 @@ BUILD_CONTROLS_SHADOW(tertiary_exec, TERTIARY_VM_EXEC_CONTROL, 64)
 				(1 << VCPU_EXREG_EXIT_INFO_1) | \
 				(1 << VCPU_EXREG_EXIT_INFO_2))
 
+static inline unsigned long vmx_l1_guest_owned_cr0_bits(void)
+{
+	unsigned long bits = KVM_POSSIBLE_CR0_GUEST_BITS;
+
+	/*
+	 * CR0.WP needs to be intercepted when KVM is shadowing legacy paging
+	 * in order to construct shadow PTEs with the correct protections.
+	 * Note!  CR0.WP technically can be passed through to the guest if
+	 * paging is disabled, but checking CR0.PG would generate a cyclical
+	 * dependency of sorts due to forcing the caller to ensure CR0 holds
+	 * the correct value prior to determining which CR0 bits can be owned
+	 * by L1.  Keep it simple and limit the optimization to EPT.
+	 */
+	if (!enable_ept)
+		bits &= ~X86_CR0_WP;
+	return bits;
+}
+
 static __always_inline struct kvm_vmx *to_kvm_vmx(struct kvm *kvm)
 {
 	return container_of(kvm, struct kvm_vmx, kvm);