KVM: x86/mmu: Refresh CR0.WP prior to checking for emulated permission faults

Refresh the MMU's snapshot of the vCPU's CR0.WP prior to checking for
permission faults when emulating a guest memory access and CR0.WP may be
guest owned.  If the guest toggles only CR0.WP and triggers emulation of
a supervisor write, e.g. when KVM is emulating UMIP, KVM may consume a
stale CR0.WP, i.e. use stale protection bits metadata.

Note, KVM passes through CR0.WP if and only if EPT is enabled as CR0.WP
is part of the MMU role for legacy shadow paging, and SVM (NPT) doesn't
support per-bit interception controls for CR0.  Don't bother checking for
EPT vs. NPT as the "old == new" check will always be true under NPT, i.e.
the only cost is the read of vcpu->arch.cr4 (SVM unconditionally grabs CR0
from the VMCB on VM-Exit).

Reported-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lkml.kernel.org/r/677169b4-051f-fcae-756b-9a3e1bb9f8fe%40grsecurity.net
Fixes: fb509f76acc8 ("KVM: VMX: Make CR0.WP a guest owned bit")
Tested-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20230405002608.418442-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

2 files changed, 40 insertions, 1 deletions

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 89f532516a45..92d5a1924fc1 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -113,6 +113,8 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,
 bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu);
 int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
 				u64 fault_address, char *insn, int insn_len);
+void __kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu,
+					struct kvm_mmu *mmu);
 
 int kvm_mmu_load(struct kvm_vcpu *vcpu);
 void kvm_mmu_unload(struct kvm_vcpu *vcpu);
@@ -153,6 +155,24 @@ static inline void kvm_mmu_load_pgd(struct kvm_vcpu *vcpu)
 					  vcpu->arch.mmu->root_role.level);
 }
 
+static inline void kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu,
+						    struct kvm_mmu *mmu)
+{
+	/*
+	 * When EPT is enabled, KVM may passthrough CR0.WP to the guest, i.e.
+	 * @mmu's snapshot of CR0.WP and thus all related paging metadata may
+	 * be stale.  Refresh CR0.WP and the metadata on-demand when checking
+	 * for permission faults.  Exempt nested MMUs, i.e. MMUs for shadowing
+	 * nEPT and nNPT, as CR0.WP is ignored in both cases.  Note, KVM does
+	 * need to refresh nested_mmu, a.k.a. the walker used to translate L2
+	 * GVAs to GPAs, as that "MMU" needs to honor L2's CR0.WP.
+	 */
+	if (!tdp_enabled || mmu == &vcpu->arch.guest_mmu)
+		return;
+
+	__kvm_mmu_refresh_passthrough_bits(vcpu, mmu);
+}
+
 /*
  * Check if a given access (described through the I/D, W/R and U/S bits of a
  * page fault error code pfec) causes a permission fault with the given PTE
@@ -184,8 +204,12 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
 	u64 implicit_access = access & PFERR_IMPLICIT_ACCESS;
 	bool not_smap = ((rflags & X86_EFLAGS_AC) | implicit_access) == X86_EFLAGS_AC;
 	int index = (pfec + (not_smap << PFERR_RSVD_BIT)) >> 1;
-	bool fault = (mmu->permissions[index] >> pte_access) & 1;
 	u32 errcode = PFERR_PRESENT_MASK;
+	bool fault;
+
+	kvm_mmu_refresh_passthrough_bits(vcpu, mmu);
+
+	fault = (mmu->permissions[index] >> pte_access) & 1;
 
 	WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
 	if (unlikely(mmu->pkru_mask)) {
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 144c5a01cd77..4544605272e2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5112,6 +5112,21 @@ kvm_calc_cpu_role(struct kvm_vcpu *vcpu, const struct kvm_mmu_role_regs *regs)
 	return role;
 }
 
+void __kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu,
+					struct kvm_mmu *mmu)
+{
+	const bool cr0_wp = kvm_is_cr0_bit_set(vcpu, X86_CR0_WP);
+
+	BUILD_BUG_ON((KVM_MMU_CR0_ROLE_BITS & KVM_POSSIBLE_CR0_GUEST_BITS) != X86_CR0_WP);
+	BUILD_BUG_ON((KVM_MMU_CR4_ROLE_BITS & KVM_POSSIBLE_CR4_GUEST_BITS));
+
+	if (is_cr0_wp(mmu) == cr0_wp)
+		return;
+
+	mmu->cpu_role.base.cr0_wp = cr0_wp;
+	reset_guest_paging_metadata(vcpu, mmu);
+}
+
 static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
 {
 	/* tdp_root_level is architecture forced level, use it if nonzero */