[v2] KVM: x86: nSVM: Improve PAT virtualization

[PATCH v2 2/8] KVM: x86: nSVM: Cache g_pat in vmcb_save_area_cached

Posted by Jim Mattson 3 weeks, 2 days ago

To avoid TOCTTOU issues, all fields in the vmcb12 save area that are
subject to validation must be copied to svm->nested.save prior to
validation, since vmcb12 is writable by the guest. Add g_pat to this set in
preparation for validting it.

Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/kvm/svm/nested.c | 2 ++
 arch/x86/kvm/svm/svm.h    | 1 +
 2 files changed, 3 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index f295a41ec659..07a57a43fc3b 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -506,6 +506,8 @@ static void __nested_copy_vmcb_save_to_cache(struct vmcb_save_area_cached *to,
 
 	to->dr6 = from->dr6;
 	to->dr7 = from->dr7;
+
+	to->g_pat = from->g_pat;
 }
 
 void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 7d28a739865f..39138378531e 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -145,6 +145,7 @@ struct vmcb_save_area_cached {
 	u64 cr0;
 	u64 dr7;
 	u64 dr6;
+	u64 g_pat;
 };
 
 struct vmcb_ctrl_area_cached {
-- 
2.52.0.457.g6b5491de43-goog

Re: [PATCH v2 2/8] KVM: x86: nSVM: Cache g_pat in vmcb_save_area_cached

Posted by Yosry Ahmed 2 weeks, 3 days ago

On Thu, Jan 15, 2026 at 03:21:41PM -0800, Jim Mattson wrote:
> To avoid TOCTTOU issues, all fields in the vmcb12 save area that are
> subject to validation must be copied to svm->nested.save prior to
> validation, since vmcb12 is writable by the guest. Add g_pat to this set in
> preparation for validting it.
> 
> Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")

g_pat is not currently used from VMCB12, so this patch isn't technically
fixing an issue, right? I suspect this only applies to patch 3.

Anyway, I think it's probably best to squash this into patch 3. Also
maybe CC stable?

> Signed-off-by: Jim Mattson <jmattson@google.com>

If you decide to keep this as an individual patch, feel free to add:

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>

> ---
>  arch/x86/kvm/svm/nested.c | 2 ++
>  arch/x86/kvm/svm/svm.h    | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index f295a41ec659..07a57a43fc3b 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -506,6 +506,8 @@ static void __nested_copy_vmcb_save_to_cache(struct vmcb_save_area_cached *to,
>  
>  	to->dr6 = from->dr6;
>  	to->dr7 = from->dr7;
> +
> +	to->g_pat = from->g_pat;
>  }
>  
>  void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 7d28a739865f..39138378531e 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -145,6 +145,7 @@ struct vmcb_save_area_cached {
>  	u64 cr0;
>  	u64 dr7;
>  	u64 dr6;
> +	u64 g_pat;
>  };
>  
>  struct vmcb_ctrl_area_cached {
> -- 
> 2.52.0.457.g6b5491de43-goog
>

Re: [PATCH v2 2/8] KVM: x86: nSVM: Cache g_pat in vmcb_save_area_cached

Posted by Jim Mattson 5 days, 15 hours ago

On Wed, Jan 21, 2026 at 5:28 PM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:
>
> On Thu, Jan 15, 2026 at 03:21:41PM -0800, Jim Mattson wrote:
> > To avoid TOCTTOU issues, all fields in the vmcb12 save area that are
> > subject to validation must be copied to svm->nested.save prior to
> > validation, since vmcb12 is writable by the guest. Add g_pat to this set in
> > preparation for validting it.
> >
> > Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
>
> g_pat is not currently used from VMCB12, so this patch isn't technically
> fixing an issue, right? I suspect this only applies to patch 3.

The fact that VMCB12.g_pat isn't used *is* the issue. This patch is
part of the fix. To make the fix easier to review, it's broken into
several pieces.

> Anyway, I think it's probably best to squash this into patch 3. Also
> maybe CC stable?

I will squash it in the next version.

> > Signed-off-by: Jim Mattson <jmattson@google.com>
>
> If you decide to keep this as an individual patch, feel free to add:
>
> Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
>
> > ---
> >  arch/x86/kvm/svm/nested.c | 2 ++
> >  arch/x86/kvm/svm/svm.h    | 1 +
> >  2 files changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index f295a41ec659..07a57a43fc3b 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -506,6 +506,8 @@ static void __nested_copy_vmcb_save_to_cache(struct vmcb_save_area_cached *to,
> >
> >       to->dr6 = from->dr6;
> >       to->dr7 = from->dr7;
> > +
> > +     to->g_pat = from->g_pat;
> >  }
> >
> >  void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
> > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > index 7d28a739865f..39138378531e 100644
> > --- a/arch/x86/kvm/svm/svm.h
> > +++ b/arch/x86/kvm/svm/svm.h
> > @@ -145,6 +145,7 @@ struct vmcb_save_area_cached {
> >       u64 cr0;
> >       u64 dr7;
> >       u64 dr6;
> > +     u64 g_pat;
> >  };
> >
> >  struct vmcb_ctrl_area_cached {
> > --
> > 2.52.0.457.g6b5491de43-goog
> >