* [PATCH] KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest
@ 2017-08-07 19:11 Brijesh Singh
2017-08-08 10:06 ` Paolo Bonzini
0 siblings, 1 reply; 2+ messages in thread
From: Brijesh Singh @ 2017-08-07 19:11 UTC (permalink / raw)
To: kvm, linux-kernel
Cc: Brijesh Singh, Paolo Bonzini, Radim Krčmář,
Thomas Lendacky
Commit: 1472775 (kvm: svm: Add support for additional SVM NPF error codes)
added new error code to aid nested page fault handling. The commit
unprotect (kvm_mmu_unprotect_page) the page when we get a NFP due to
guest page table walk where the page was marked RO.
Paolo highlighted a use case, where an L0->L2 shadow nested page table
is marked read-only, in particular when a page is read only in L1's nested
page table. If such a page is accessed by L2 while walking page tables
it can cause a nested page fault (page table walks are write accessed).
However, after kvm_mmu_unprotect_page we may get another page fault, and
again in an endless stream.
To cover this use case, we qualify the new error_code check with
vcpu->arch.mmu_direct_map so that the error_code check would run on L1
guest, and not the L2 guest. This would restrict it avoid hitting the above
use case.
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
See http://marc.info/?l=kvm&m=150153155519373&w=2 for detail discussion on the use case and code flow.
arch/x86/kvm/mmu.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 9b1dd11..4aaa4aa 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4839,7 +4839,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
* Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
* in PFERR_NEXT_GUEST_PAGE)
*/
- if (error_code == PFERR_NESTED_GUEST_PAGE) {
+ if (vcpu->arch.mmu.direct_map &&
+ (error_code == PFERR_NESTED_GUEST_PAGE)) {
kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
return 1;
}
--
2.9.4
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest
2017-08-07 19:11 [PATCH] KVM: SVM: Limit PFERR_NESTED_GUEST_PAGE error_code check to L1 guest Brijesh Singh
@ 2017-08-08 10:06 ` Paolo Bonzini
0 siblings, 0 replies; 2+ messages in thread
From: Paolo Bonzini @ 2017-08-08 10:06 UTC (permalink / raw)
To: Brijesh Singh, kvm, linux-kernel
Cc: Radim Krčmář, Thomas Lendacky
On 07/08/2017 21:11, Brijesh Singh wrote:
> Commit: 1472775 (kvm: svm: Add support for additional SVM NPF error codes)
> added new error code to aid nested page fault handling. The commit
> unprotect (kvm_mmu_unprotect_page) the page when we get a NFP due to
> guest page table walk where the page was marked RO.
>
> Paolo highlighted a use case, where an L0->L2 shadow nested page table
> is marked read-only, in particular when a page is read only in L1's nested
> page table. If such a page is accessed by L2 while walking page tables
> it can cause a nested page fault (page table walks are write accessed).
> However, after kvm_mmu_unprotect_page we may get another page fault, and
> again in an endless stream.
>
> To cover this use case, we qualify the new error_code check with
> vcpu->arch.mmu_direct_map so that the error_code check would run on L1
> guest, and not the L2 guest. This would restrict it avoid hitting the above
> use case.
>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> Cc: Thomas Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>
> See http://marc.info/?l=kvm&m=150153155519373&w=2 for detail discussion on the use case and code flow.
>
> arch/x86/kvm/mmu.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 9b1dd11..4aaa4aa 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4839,7 +4839,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
> * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
> * in PFERR_NEXT_GUEST_PAGE)
> */
> - if (error_code == PFERR_NESTED_GUEST_PAGE) {
> + if (vcpu->arch.mmu.direct_map &&
> + (error_code == PFERR_NESTED_GUEST_PAGE)) {
> kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
> return 1;
> }
>
Thanks, queued for 4.14.
Paolo
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