* [PATCH] KVM: nSVM: call nested_svm_load_cr3 on nested state load
@ 2021-02-10 15:59 Maxim Levitsky
2021-02-10 17:38 ` Paolo Bonzini
0 siblings, 1 reply; 2+ messages in thread
From: Maxim Levitsky @ 2021-02-10 15:59 UTC (permalink / raw)
To: kvm
Cc: Sean Christopherson,
open list:X86 ARCHITECTURE (32-BIT AND 64-BIT),
Paolo Bonzini, H. Peter Anvin,
maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT),
Jim Mattson, Borislav Petkov, Wanpeng Li, Thomas Gleixner,
Ingo Molnar, Joerg Roedel, Vitaly Kuznetsov, Maxim Levitsky
While KVM's MMU should be fully reset by loading of nested CR0/CR3/CR4
by KVM_SET_SREGS, we are not in nested mode yet when we do it and therefore
only root_mmu is reset.
On regular nested entries we call nested_svm_load_cr3 which both updates the
guest's CR3 in the MMU when it is needed, and it also initializes
the mmu again which makes it initialize the walk_mmu as well when nested
paging is enabled in both host and guest.
Since we don't call nested_svm_load_cr3 on nested state load,
the walk_mmu can be left uninitialized, which can lead to a NULL pointer
dereference while accessing it if we happen to get a nested page fault
right after entering the nested guest first time after the migration and
we decide to emulate it, which leads to emulator trying to access
walk_mmu->gva_to_gpa which is NULL.
Therefore we should call this function on nested state load as well.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/svm/nested.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 519fe84f2100..c209f1232928 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1282,6 +1282,14 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
nested_vmcb02_prepare_control(svm);
+ ret = nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
+ nested_npt_enabled(svm));
+
+ if (ret) {
+ svm_leave_nested(svm);
+ goto out_free;
+ }
+
kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
ret = 0;
out_free:
--
2.26.2
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] KVM: nSVM: call nested_svm_load_cr3 on nested state load
2021-02-10 15:59 [PATCH] KVM: nSVM: call nested_svm_load_cr3 on nested state load Maxim Levitsky
@ 2021-02-10 17:38 ` Paolo Bonzini
0 siblings, 0 replies; 2+ messages in thread
From: Paolo Bonzini @ 2021-02-10 17:38 UTC (permalink / raw)
To: Maxim Levitsky, kvm
Cc: Sean Christopherson,
open list:X86 ARCHITECTURE (32-BIT AND 64-BIT),
H. Peter Anvin, maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT),
Jim Mattson, Borislav Petkov, Wanpeng Li, Thomas Gleixner,
Ingo Molnar, Joerg Roedel, Vitaly Kuznetsov
On 10/02/21 16:59, Maxim Levitsky wrote:
> While KVM's MMU should be fully reset by loading of nested CR0/CR3/CR4
> by KVM_SET_SREGS, we are not in nested mode yet when we do it and therefore
> only root_mmu is reset.
>
> On regular nested entries we call nested_svm_load_cr3 which both updates the
> guest's CR3 in the MMU when it is needed, and it also initializes
> the mmu again which makes it initialize the walk_mmu as well when nested
> paging is enabled in both host and guest.
>
> Since we don't call nested_svm_load_cr3 on nested state load,
> the walk_mmu can be left uninitialized, which can lead to a NULL pointer
> dereference while accessing it if we happen to get a nested page fault
> right after entering the nested guest first time after the migration and
> we decide to emulate it, which leads to emulator trying to access
> walk_mmu->gva_to_gpa which is NULL.
>
> Therefore we should call this function on nested state load as well.
>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
> arch/x86/kvm/svm/nested.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 519fe84f2100..c209f1232928 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1282,6 +1282,14 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>
> nested_vmcb02_prepare_control(svm);
>
> + ret = nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
> + nested_npt_enabled(svm));
> +
> + if (ret) {
> + svm_leave_nested(svm);
> + goto out_free;
> + }
> +
> kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
> ret = 0;
> out_free:
>
I think you have to delay this to KVM_REQ_GET_NESTED_STATE_PAGES,
because the !nested_npt case can be accessing memory before the VM is
started (PDPTRs!).
In fact the same is true for VMX: this code
/* Shadow page tables on either EPT or shadow page tables. */
if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3,
nested_cpu_has_ept(vmcs12),
entry_failure_code))
return -EINVAL;
must be moved from prepare_vmcs02 to both nested_vmx_enter_non_root_mode
and nested_get_vmcs12_pages.
Thanks,
Paolo
^ permalink raw reply [flat|nested] 2+ messages in thread
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