2024-05-07 16:13:06

by Paolo Bonzini

Subject: [PATCH 10/17] KVM: x86/mmu: Move private vs. shared check above slot validity checks

From: Sean Christopherson <[email protected]>

Prioritize private vs. shared gfn attribute checks above slot validity
checks to ensure a consistent userspace ABI. E.g. as is, KVM will exit to
userspace if there is no memslot, but emulate accesses to the APIC access
page even if the attributes mismatch.

Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
Cc: Yu Zhang <[email protected]>
Cc: Chao Peng <[email protected]>
Cc: Fuad Tabba <[email protected]>
Cc: Michael Roth <[email protected]>
Cc: Isaku Yamahata <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: Kai Huang <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
---
arch/x86/kvm/mmu/mmu.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0d884d0b0f35..ba50b93e93ed 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4317,11 +4317,6 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
return RET_PF_EMULATE;
}

- if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
- kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
- return -EFAULT;
- }
-
if (fault->is_private)
return kvm_faultin_pfn_private(vcpu, fault);

@@ -4359,9 +4354,24 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
{
int ret;

+ /*
+ * Note that the mmu_invalidate_seq also serves to detect a concurrent
+ * change in attributes. is_page_fault_stale() will detect an
+ * invalidation relate to fault->fn and resume the guest without
+ * installing a mapping in the page tables.
+ */
fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
smp_rmb();

+ /*
+ * Now that we have a snapshot of mmu_invalidate_seq we can check for a
+ * private vs. shared mismatch.
+ */
+ if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
+ kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
+ return -EFAULT;
+ }
+
/*
* Check for a relevant mmu_notifier invalidation event before getting
* the pfn from the primary MMU, and before acquiring mmu_lock.
--
2.43.0

2024-05-13 06:22:40

by Xiaoyao Li

Subject: Re: [PATCH 10/17] KVM: x86/mmu: Move private vs. shared check above slot validity checks

On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
> From: Sean Christopherson <[email protected]>
>
> Prioritize private vs. shared gfn attribute checks above slot validity
> checks to ensure a consistent userspace ABI. E.g. as is, KVM will exit to
> userspace if there is no memslot, but emulate accesses to the APIC access
> page even if the attributes mismatch.
>
> Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
> Cc: Yu Zhang <[email protected]>
> Cc: Chao Peng <[email protected]>
> Cc: Fuad Tabba <[email protected]>
> Cc: Michael Roth <[email protected]>
> Cc: Isaku Yamahata <[email protected]>
> Signed-off-by: Sean Christopherson <[email protected]>
> Reviewed-by: Kai Huang <[email protected]>
> Message-ID: <[email protected]>
> Signed-off-by: Paolo Bonzini <[email protected]>

Reviewed-by: Xiaoyao Li <[email protected]>

> ---
> arch/x86/kvm/mmu/mmu.c | 20 +++++++++++++++-----
> 1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0d884d0b0f35..ba50b93e93ed 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4317,11 +4317,6 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> return RET_PF_EMULATE;
> }
>
> - if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> - kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> - return -EFAULT;
> - }
> -
> if (fault->is_private)
> return kvm_faultin_pfn_private(vcpu, fault);
>
> @@ -4359,9 +4354,24 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> {
> int ret;
>
> + /*
> + * Note that the mmu_invalidate_seq also serves to detect a concurrent
> + * change in attributes. is_page_fault_stale() will detect an
> + * invalidation relate to fault->fn and resume the guest without

s/fault->fn/fault->gfn/

> + * installing a mapping in the page tables.
> + */
> fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> smp_rmb();
>
> + /*
> + * Now that we have a snapshot of mmu_invalidate_seq we can check for a
> + * private vs. shared mismatch.
> + */
> + if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> + kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> + return -EFAULT;
> + }
> +
> /*
> * Check for a relevant mmu_notifier invalidation event before getting
> * the pfn from the primary MMU, and before acquiring mmu_lock.