From: Sean Christopherson seanjc@google.com
[ Upstream commit 470750b3425513b9f63f176e564e63e0e7998afc ]
Keep CR3 load/store exiting enable as needed when running L2 in order to honor L1's desires. This fixes a largely theoretical bug where L1 could intercept CR3 but not CR0.PG and end up not getting the desired CR3 exits when L2 enables paging. In other words, the existing !is_paging() check inadvertantly handles the normal case for L2 where vmx_set_cr0() is called during VM-Enter, which is guaranteed to run with paging enabled, and thus will never clear the bits.
Removing the !is_paging() check will also allow future consolidation and cleanup of the related code. From a performance perspective, this is all a nop, as the VMCS controls shadow will optimize away the VMWRITE when the controls are in the desired state.
Add a comment explaining why CR3 is intercepted, with a big disclaimer about not querying the old CR3. Because vmx_set_cr0() is used for flows that are not directly tied to MOV CR3, e.g. vCPU RESET/INIT and nested VM-Enter, it's possible that is_paging() is not synchronized with CR3 load/store exiting. This is actually guaranteed in the current code, as KVM starts with CR3 interception disabled. Obviously that can be fixed, but there's no good reason to play whack-a-mole, and it tends to end poorly, e.g. descriptor table exiting for UMIP emulation attempted to be precise in the past and ended up botching the interception toggling.
Fixes: fe3ef05c7572 ("KVM: nVMX: Prepare vmcs02 from vmcs01 and vmcs12") Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20210713163324.627647-25-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Stable-dep-of: c4abd7352023 ("KVM: VMX: Don't fudge CR0 and CR4 for restricted L2 guest") Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kvm/vmx/vmx.c | 46 +++++++++++++++++++++++++++++++++--------- 1 file changed, 37 insertions(+), 9 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index b9abe08c9d590..ca51df0df3f94 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -3063,10 +3063,14 @@ void ept_save_pdptrs(struct kvm_vcpu *vcpu) kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR); }
+#define CR3_EXITING_BITS (CPU_BASED_CR3_LOAD_EXITING | \ + CPU_BASED_CR3_STORE_EXITING) + void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0) { struct vcpu_vmx *vmx = to_vmx(vcpu); unsigned long hw_cr0; + u32 tmp;
hw_cr0 = (cr0 & ~KVM_VM_CR0_ALWAYS_OFF); if (is_unrestricted_guest(vcpu)) @@ -3093,18 +3097,42 @@ void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0) #endif
if (enable_ept && !is_unrestricted_guest(vcpu)) { + /* + * Ensure KVM has an up-to-date snapshot of the guest's CR3. If + * the below code _enables_ CR3 exiting, vmx_cache_reg() will + * (correctly) stop reading vmcs.GUEST_CR3 because it thinks + * KVM's CR3 is installed. + */ if (!kvm_register_is_available(vcpu, VCPU_EXREG_CR3)) vmx_cache_reg(vcpu, VCPU_EXREG_CR3); + + /* + * When running with EPT but not unrestricted guest, KVM must + * intercept CR3 accesses when paging is _disabled_. This is + * necessary because restricted guests can't actually run with + * paging disabled, and so KVM stuffs its own CR3 in order to + * run the guest when identity mapped page tables. + * + * Do _NOT_ check the old CR0.PG, e.g. to optimize away the + * update, it may be stale with respect to CR3 interception, + * e.g. after nested VM-Enter. + * + * Lastly, honor L1's desires, i.e. intercept CR3 loads and/or + * stores to forward them to L1, even if KVM does not need to + * intercept them to preserve its identity mapped page tables. + */ if (!(cr0 & X86_CR0_PG)) { - /* From paging/starting to nonpaging */ - exec_controls_setbit(vmx, CPU_BASED_CR3_LOAD_EXITING | - CPU_BASED_CR3_STORE_EXITING); - vcpu->arch.cr0 = cr0; - vmx_set_cr4(vcpu, kvm_read_cr4(vcpu)); - } else if (!is_paging(vcpu)) { - /* From nonpaging to paging */ - exec_controls_clearbit(vmx, CPU_BASED_CR3_LOAD_EXITING | - CPU_BASED_CR3_STORE_EXITING); + exec_controls_setbit(vmx, CR3_EXITING_BITS); + } else if (!is_guest_mode(vcpu)) { + exec_controls_clearbit(vmx, CR3_EXITING_BITS); + } else { + tmp = exec_controls_get(vmx); + tmp &= ~CR3_EXITING_BITS; + tmp |= get_vmcs12(vcpu)->cpu_based_vm_exec_control & CR3_EXITING_BITS; + exec_controls_set(vmx, tmp); + } + + if (!is_paging(vcpu) != !(cr0 & X86_CR0_PG)) { vcpu->arch.cr0 = cr0; vmx_set_cr4(vcpu, kvm_read_cr4(vcpu)); }