On Fri, Feb 06, 2026, Jim Mattson wrote:
> On Fri, Feb 6, 2026 at 10:23 AM Yosry Ahmed <[email protected]> wrote:
> >
> > February 6, 2026 at 10:19 AM, "Sean Christopherson" <[email protected]>
> > wrote:
> >
> >
> > >
> > > On Thu, Feb 05, 2026, Jim Mattson wrote:
> > >
> > > >
> > > > Cache g_pat from vmcb12 in svm->nested.gpat to avoid TOCTTOU issues, and
> > > > add a validity check so that when nested paging is enabled for vmcb12,
> > > > an
> > > > invalid g_pat causes an immediate VMEXIT with exit code
> > > > VMEXIT_INVALID, as
> > > > specified in the APM, volume 2: "Nested Paging and VMRUN/VMEXIT."
> > > >
> > > > Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
> > > > Signed-off-by: Jim Mattson <[email protected]>
> > > > ---
> > > > arch/x86/kvm/svm/nested.c | 4 +++-
> > > > arch/x86/kvm/svm/svm.h | 3 +++
> > > > 2 files changed, 6 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > > > index f72dbd10dcad..1d4ff6408b34 100644
> > > > --- a/arch/x86/kvm/svm/nested.c
> > > > +++ b/arch/x86/kvm/svm/nested.c
> > > > @@ -1027,9 +1027,11 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
> > > >
> > > > nested_copy_vmcb_control_to_cache(svm, &vmcb12->control);
> > > > nested_copy_vmcb_save_to_cache(svm, &vmcb12->save);
> > > > + svm->nested.gpat = vmcb12->save.g_pat;
> > > >
> > > > if (!nested_vmcb_check_save(vcpu) ||
> > > > - !nested_vmcb_check_controls(vcpu)) {
> > > > + !nested_vmcb_check_controls(vcpu) ||
> > > > + (nested_npt_enabled(svm) && !kvm_pat_valid(svm->nested.gpat))) {
> > > > vmcb12->control.exit_code = SVM_EXIT_ERR;
> > > > vmcb12->control.exit_info_1 = 0;
> > > > vmcb12->control.exit_info_2 = 0;
> > > > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > > > index 986d90f2d4ca..42a4bf83b3aa 100644
> > > > --- a/arch/x86/kvm/svm/svm.h
> > > > +++ b/arch/x86/kvm/svm/svm.h
> > > > @@ -208,6 +208,9 @@ struct svm_nested_state {
> > > > */
> > > > struct vmcb_save_area_cached save;
> > > >
> > > > + /* Cached guest PAT from vmcb12.save.g_pat */
> > > > + u64 gpat;
> > > >
> > > Shouldn't this go in vmcb_save_area_cached?
> >
> > I believe Jim changed it after this discussion on v2:
> > https://lore.kernel.org/kvm/[email protected]/.
LOL, oh the irony:
I'm going to cache it on its own to avoid confusion.
> Right. The two issues with putting it in vmcb_save_area_cached were:
>
> 1. Checking all of vmcb_save_area_cached requires access to the
> corresponding control area (or at least the boolean, "NTP enabled.")
Checking the control area seems like the right answer (I went down that path
before reading this).
> 2. In the nested state serialization payload, everything else in the
> vmcb_save_area_cached comes from L1 (host state to be restored at
> emulated #VMEXIT.)
Hmm, right, but *because* it's ignored, that gives us carte blanche to clobber
it.
More below.
> The first issue was a little messy, but not that distasteful.
I actually find it the opposite of distasteful. KVM definitely _should_ be
checking the controls, not the vCPU state. If it weren't for needing to get at
MAXPHYADDR in CPUID, I'd push to drop @vcpu entirely.
> The second issue was really a mess.
I'd rather have the mess contained and document though. Caching g_pat outside
of vmcb_save_area_cached bleeds the mess into all of the relevant nSVM code, and
doesn't leave any breadcrumbs in the code/comments to explain that it "needs" to
be kept separate.
AFAICT, the only "problem" is that g_pat in the serialization payload will be
garbage when restoring state from an older KVM. But that's totally fine,
precisely
because L1's PAT isn't restored from vmcb01 on nested #VMEXIT, it's always
resident
in vcpu->arch.pat. So can't we just do this to avoid a spurious -EINVAL?
/*
* Validate host state saved from before VMRUN (see
* nested_svm_check_permissions).
*/
__nested_copy_vmcb_save_to_cache(&save_cached, save);
/*
* Stuff gPAT in L1's save state, as older KVM may not have saved L1's
* gPAT. L1's PAT, i.e. hPAT for the vCPU, is *always* tracked in
* vcpu->arch.pat, i.e. gPAT is a reflection of vcpu->arch.pat, not the
* other way around.
*/
save_cached.g_pat = vcpu->arch.pat;
if (!(save->cr0 & X86_CR0_PG) ||
!(save->cr0 & X86_CR0_PE) ||
(save->rflags & X86_EFLAGS_VM) ||
!nested_vmcb_check_save(vcpu, &ctl_cached, &save_cached))
goto out_free;
Oh, and if we do plumb in @ctrl to __nested_vmcb_check_save(), I vote to
opportunistically drop the useless single-use wrappers (probably in a standalone
patch to plumb in @ctrl). E.g. (completely untested)
---
arch/x86/kvm/svm/nested.c | 71 ++++++++++++++++++---------------------
arch/x86/kvm/svm/svm.c | 2 +-
arch/x86/kvm/svm/svm.h | 6 ++--
3 files changed, 35 insertions(+), 44 deletions(-)
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index a7d6fc1382a7..a429947c8966 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -339,8 +339,8 @@ static bool nested_svm_check_bitmap_pa(struct kvm_vcpu
*vcpu, u64 pa, u32 size)
kvm_vcpu_is_legal_gpa(vcpu, addr + size - 1);
}
-static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
- struct vmcb_ctrl_area_cached *control)
+static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
+ struct vmcb_ctrl_area_cached *control)
{
if (CC(!vmcb12_is_intercept(control, INTERCEPT_VMRUN)))
return false;
@@ -367,8 +367,9 @@ static bool __nested_vmcb_check_controls(struct kvm_vcpu
*vcpu,
}
/* Common checks that apply to both L1 and L2 state. */
-static bool __nested_vmcb_check_save(struct kvm_vcpu *vcpu,
- struct vmcb_save_area_cached *save)
+static bool nested_vmcb_check_save(struct kvm_vcpu *vcpu,
+ struct vmcb_ctrl_area_cached *ctrl,
+ struct vmcb_save_area_cached *save)
{
if (CC(!(save->efer & EFER_SVME)))
return false;
@@ -399,25 +400,13 @@ static bool __nested_vmcb_check_save(struct kvm_vcpu
*vcpu,
if (CC(!kvm_valid_efer(vcpu, save->efer)))
return false;
+ if (CC(ctrl->nested_ctl & SVM_NESTED_CTL_NP_ENABLE) &&
+ !kvm_pat_valid(save->g_pat))
+ return false;
+
return true;
}
-static bool nested_vmcb_check_save(struct kvm_vcpu *vcpu)
-{
- struct vcpu_svm *svm = to_svm(vcpu);
- struct vmcb_save_area_cached *save = &svm->nested.save;
-
- return __nested_vmcb_check_save(vcpu, save);
-}
-
-static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu)
-{
- struct vcpu_svm *svm = to_svm(vcpu);
- struct vmcb_ctrl_area_cached *ctl = &svm->nested.ctl;
-
- return __nested_vmcb_check_controls(vcpu, ctl);
-}
-
/*
* If a feature is not advertised to L1, clear the corresponding vmcb12
* intercept.
@@ -504,6 +493,9 @@ 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,
@@ -644,17 +636,14 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm
*svm, struct vmcb *vmcb12
svm->nested.force_msr_bitmap_recalc = true;
}
- if (npt_enabled) {
- if (nested_npt_enabled(svm)) {
- if (unlikely(new_vmcb12 ||
- vmcb_is_dirty(vmcb12, VMCB_NPT))) {
- vmcb02->save.g_pat = svm->nested.gpat;
- vmcb_mark_dirty(vmcb02, VMCB_NPT);
- }
- } else {
- vmcb02->save.g_pat = vcpu->arch.pat;
+ if (nested_npt_enabled(svm)) {
+ if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_NPT))) {
+ vmcb02->save.g_pat = svm->nested.save.g_pat;
vmcb_mark_dirty(vmcb02, VMCB_NPT);
}
+ } else if (npt_enabled) {
+ vmcb02->save.g_pat = vcpu->arch.pat;
+ vmcb_mark_dirty(vmcb02, VMCB_NPT);
}
if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_SEG))) {
@@ -1028,11 +1017,9 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
nested_copy_vmcb_control_to_cache(svm, &vmcb12->control);
nested_copy_vmcb_save_to_cache(svm, &vmcb12->save);
- svm->nested.gpat = vmcb12->save.g_pat;
- if (!nested_vmcb_check_save(vcpu) ||
- !nested_vmcb_check_controls(vcpu) ||
- (nested_npt_enabled(svm) && !kvm_pat_valid(svm->nested.gpat))) {
+ if (!nested_vmcb_check_save(vcpu, &svm->nested.ctl, &svm->nested.save)
||
+ !nested_vmcb_check_controls(vcpu, &svm->nested.ctl)) {
vmcb12->control.exit_code = SVM_EXIT_ERR;
vmcb12->control.exit_info_1 = 0;
vmcb12->control.exit_info_2 = 0;
@@ -1766,7 +1753,7 @@ static int svm_get_nested_state(struct kvm_vcpu *vcpu,
kvm_state.hdr.svm.vmcb_pa = svm->nested.vmcb12_gpa;
if (nested_npt_enabled(svm)) {
kvm_state.hdr.svm.flags |= KVM_STATE_SVM_VALID_GPAT;
- kvm_state.hdr.svm.gpat = svm->nested.gpat;
+ kvm_state.hdr.svm.gpat = svm->nested.save.g_pat;
}
kvm_state.size += KVM_STATE_NESTED_SVM_VMCB_SIZE;
kvm_state.flags |= KVM_STATE_NESTED_GUEST_MODE;
@@ -1871,7 +1858,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
ret = -EINVAL;
__nested_copy_vmcb_control_to_cache(vcpu, &ctl_cached, ctl);
- if (!__nested_vmcb_check_controls(vcpu, &ctl_cached))
+ if (!nested_vmcb_check_controls(vcpu, &ctl_cached))
goto out_free;
/*
@@ -1887,15 +1874,21 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
* nested_svm_check_permissions).
*/
__nested_copy_vmcb_save_to_cache(&save_cached, save);
+
+ /*
+ * Stuff gPAT in L1's save state, as older KVM may not have saved L1's
+ * gPAT. L1's PAT, i.e. hPAT for the vCPU, is *always* tracked in
+ * vcpu->arch.pat, i.e. hPAT is a reflection of vcpu->arch.pat, not the
+ * other way around.
+ */
+ save_cached.g_pat = vcpu->arch.pat;
+
if (!(save->cr0 & X86_CR0_PG) ||
!(save->cr0 & X86_CR0_PE) ||
(save->rflags & X86_EFLAGS_VM) ||
- !__nested_vmcb_check_save(vcpu, &save_cached))
+ !nested_vmcb_check_save(vcpu, &ctl_cached, &save_cached))
goto out_free;
- /*
- * Validate gPAT, if provided.
- */
if ((kvm_state->hdr.svm.flags & KVM_STATE_SVM_VALID_GPAT) &&
!kvm_pat_valid(kvm_state->hdr.svm.gpat))
goto out_free;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a6a44deec82b..bf8562a5f655 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2862,7 +2862,7 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct
msr_data *msr_info)
WARN_ON_ONCE(msr_info->host_initiated && vcpu->wants_to_run);
if (!msr_info->host_initiated && is_guest_mode(vcpu) &&
nested_npt_enabled(svm))
- msr_info->data = svm->nested.gpat;
+ msr_info->data = svm->nested.save.g_pat;
else
msr_info->data = vcpu->arch.pat;
break;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index a559cd45c8a9..6f07d8e3f06e 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -146,6 +146,7 @@ struct vmcb_save_area_cached {
u64 cr0;
u64 dr7;
u64 dr6;
+ u64 g_pat;
};
struct vmcb_ctrl_area_cached {
@@ -208,9 +209,6 @@ struct svm_nested_state {
*/
struct vmcb_save_area_cached save;
- /* Cached guest PAT from vmcb12.save.g_pat */
- u64 gpat;
-
bool initialized;
/*
@@ -599,7 +597,7 @@ static inline bool nested_npt_enabled(struct vcpu_svm *svm)
static inline void svm_set_gpat(struct vcpu_svm *svm, u64 data)
{
- svm->nested.gpat = data;
+ svm->nested.save.g_pat = data;
svm_set_vmcb_gpat(svm->nested.vmcb02.ptr, data);
}
base-commit: 6461c50e232d6f81d5b9604236f7ee3df870e932
--
