On Tue, Feb 27, 2024 at 06:41:40PM -0800, Sean Christopherson wrote:
> 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 <yu.c.zhang@xxxxxxxxxxxxxxx>
> Cc: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> Cc: Fuad Tabba <tabba@xxxxxxxxxx>
> Cc: Michael Roth <michael.roth@xxxxxxx>
> Cc: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/mmu.c | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 9206cfa58feb..58c5ae8be66c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4365,11 +4365,6 @@ 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);
>
> @@ -4410,6 +4405,16 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> smp_rmb();
>
> + /*
> + * Check for a private vs. shared mismatch *after* taking a snapshot of
> + * mmu_invalidate_seq, as changes to gfn attributes are guarded by the
> + * invalidation notifier.
I didn't see how mmu_invalidate_seq influences gfn attribute judgement.
And there is no synchronization between the below check and
kvm_vm_set_mem_attributes(), the gfn attribute could still be changing
after the snapshot. So why this comment?
Thanks,
Yilun
> + */
> + 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.
> --
> 2.44.0.278.ge034bb2e1d-goog
>
>
