[PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit

Mathias Krause
2023-03-22T01:34:54Z

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-and-co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
---
arch/x86/kvm/kvm_cache_regs.h | 2 +-
arch/x86/kvm/vmx/nested.c | 4 ++--
arch/x86/kvm/vmx/vmx.c | 2 +-
arch/x86/kvm/vmx/vmx.h | 18 ++++++++++++++++++
4 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
index 4c91f626c058..e50d353b5c1c 100644
--- a/arch/x86/kvm/kvm_cache_regs.h
+++ b/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)
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index f63b28f46a71..61d940fc91ba 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/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);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 8fc1a0c7856f..e501f6864a72 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/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);
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 2acdc54bc34b..423e9d3c9c40 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/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);
--
2.39.2