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

Sean Christopherson
2023-04-05T00:26:16Z

If CR0.WP may be guest-owned, i.e. TDP is enabled, refresh the MMU's
snapshot of the guest's CR0.WP prior to checking for permission faults
when emulating a guest memory access. 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.

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")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu.h | 26 +++++++++++++++++++++++++-
arch/x86/kvm/mmu/mmu.c | 15 +++++++++++++++
2 files changed, 40 insertions(+), 1 deletion(-)

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 4c874d4ec68f..47269d50e98d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5186,6 +5186,21 @@ static union kvm_cpu_role kvm_calc_cpu_role(struct kvm_vcpu *vcpu,
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 */

base-commit: 27d6845d258b67f4eb3debe062b7dacc67e0c393
--
2.40.0.348.gf938b09366-goog