When emulating L2 instructions, svm_check_intercept() checks whether a
write to CR0 should trigger a synthesized #VMEXIT with
SVM_EXIT_CR0_SEL_WRITE. However, it does not check whether L1 enabled
the intercept for SVM_EXIT_WRITE_CR0, which has higher priority
according to the APM (24593—Rev. 3.42—March 2024, Table 15-7):
When both selective and non-selective CR0-write
intercepts are active at the same time, the non-selective
intercept takes priority. With respect to exceptions, the
priority of this inter
Make sure L1 does NOT intercept SVM_EXIT_WRITE_CR0 before checking if
SVM_EXIT_CR0_SEL_WRITE needs to be injected.
Fixes: cfec82cb7d31 ("KVM: SVM: Add intercept check for emulated cr accesses")
Cc: stable@vger.kernel
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
arch/x86/kvm/svm/svm.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 9ea0ff136e299..4f79c4d837535 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4533,12 +4533,22 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
if (info->intercept == x86_intercept_cr_write)
icpt_info.exit_code += info->modrm_reg;
+ /*
+ * If the write is indeed to CR0, check whether the exit_code
+ * needs to be converted to SVM_EXIT_CR0_SEL_WRITE. Intercepting
+ * SVM_EXIT_WRITE_CR0 has higher priority than
+ * SVM_EXIT_CR0_SEL_WRITE, so this is only relevant if L1 sets
+ * INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE.
+ */
if (icpt_info.exit_code != SVM_EXIT_WRITE_CR0 ||
- info->intercept == x86_intercept_clts)
+ vmcb12_is_intercept(&svm->nested.ctl,
+ INTERCEPT_CR0_WRITE) ||
+ !(vmcb12_is_intercept(&svm->nested.ctl,
+ INTERCEPT_SELECTIVE_CR0)))
break;
- if (!(vmcb12_is_intercept(&svm->nested.ctl,
- INTERCEPT_SELECTIVE_CR0)))
+ /* CLTS never triggers INTERCEPT_SELECTIVE_CR0 */
+ if (info->intercept == x86_intercept_clts)
break;
/* LMSW always triggers INTERCEPT_SELECTIVE_CR0 */
--
2.51.1.821.gb6fe4d2222-goog
On Fri, Oct 24, 2025, Yosry Ahmed wrote:
> When emulating L2 instructions, svm_check_intercept() checks whether a
> write to CR0 should trigger a synthesized #VMEXIT with
> SVM_EXIT_CR0_SEL_WRITE. However, it does not check whether L1 enabled
> the intercept for SVM_EXIT_WRITE_CR0, which has higher priority
> according to the APM (24593—Rev. 3.42—March 2024, Table 15-7):
>
> When both selective and non-selective CR0-write
> intercepts are active at the same time, the non-selective
> intercept takes priority. With respect to exceptions, the
> priority of this inter
>
> Make sure L1 does NOT intercept SVM_EXIT_WRITE_CR0 before checking if
> SVM_EXIT_CR0_SEL_WRITE needs to be injected.
>
> Fixes: cfec82cb7d31 ("KVM: SVM: Add intercept check for emulated cr accesses")
> Cc: stable@vger.kernel
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
> arch/x86/kvm/svm/svm.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 9ea0ff136e299..4f79c4d837535 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4533,12 +4533,22 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
> if (info->intercept == x86_intercept_cr_write)
> icpt_info.exit_code += info->modrm_reg;
>
> + /*
> + * If the write is indeed to CR0, check whether the exit_code
> + * needs to be converted to SVM_EXIT_CR0_SEL_WRITE. Intercepting
> + * SVM_EXIT_WRITE_CR0 has higher priority than
> + * SVM_EXIT_CR0_SEL_WRITE, so this is only relevant if L1 sets
> + * INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE.
> + */
> if (icpt_info.exit_code != SVM_EXIT_WRITE_CR0 ||
Oof, the existing is all kinds of confusing. Even with your comment, it took me
a few seconds to understand how/where the exit_code is being modified. Eww.
Any objection to opportunistically fixing this up to the (completely untested)
below when applying?
/*
* Adjust the exit code accordingly if a CR other than CR0 is
* being written, and skip straight to the common handling as
* only CR0 has an additional selective intercept.
*/
if (info->intercept == x86_intercept_cr_write && info->modrm_reg) {
icpt_info.exit_code += info->modrm_reg;
break;
}
/*
* Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if L1 set
* INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE, as the
* unconditional intercept has higher priority.
*/
if (vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_CR0_WRITE) ||
!(vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_SELECTIVE_CR0)))
break;
> - info->intercept == x86_intercept_clts)
> + vmcb12_is_intercept(&svm->nested.ctl,
> + INTERCEPT_CR0_WRITE) ||
> + !(vmcb12_is_intercept(&svm->nested.ctl,
> + INTERCEPT_SELECTIVE_CR0)))
Let these poke out.
> break;
>
> - if (!(vmcb12_is_intercept(&svm->nested.ctl,
> - INTERCEPT_SELECTIVE_CR0)))
> + /* CLTS never triggers INTERCEPT_SELECTIVE_CR0 */
> + if (info->intercept == x86_intercept_clts)
> break;
>
> /* LMSW always triggers INTERCEPT_SELECTIVE_CR0 */
> --
> 2.51.1.821.gb6fe4d2222-goog
>
On Wed, Nov 05, 2025 at 11:48:27AM -0800, Sean Christopherson wrote:
> On Fri, Oct 24, 2025, Yosry Ahmed wrote:
> > When emulating L2 instructions, svm_check_intercept() checks whether a
> > write to CR0 should trigger a synthesized #VMEXIT with
> > SVM_EXIT_CR0_SEL_WRITE. However, it does not check whether L1 enabled
> > the intercept for SVM_EXIT_WRITE_CR0, which has higher priority
> > according to the APM (24593—Rev. 3.42—March 2024, Table 15-7):
> >
> > When both selective and non-selective CR0-write
> > intercepts are active at the same time, the non-selective
> > intercept takes priority. With respect to exceptions, the
> > priority of this inter
> >
> > Make sure L1 does NOT intercept SVM_EXIT_WRITE_CR0 before checking if
> > SVM_EXIT_CR0_SEL_WRITE needs to be injected.
> >
> > Fixes: cfec82cb7d31 ("KVM: SVM: Add intercept check for emulated cr accesses")
> > Cc: stable@vger.kernel
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> > ---
> > arch/x86/kvm/svm/svm.c | 16 +++++++++++++---
> > 1 file changed, 13 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 9ea0ff136e299..4f79c4d837535 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4533,12 +4533,22 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
> > if (info->intercept == x86_intercept_cr_write)
> > icpt_info.exit_code += info->modrm_reg;
> >
> > + /*
> > + * If the write is indeed to CR0, check whether the exit_code
> > + * needs to be converted to SVM_EXIT_CR0_SEL_WRITE. Intercepting
> > + * SVM_EXIT_WRITE_CR0 has higher priority than
> > + * SVM_EXIT_CR0_SEL_WRITE, so this is only relevant if L1 sets
> > + * INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE.
> > + */
> > if (icpt_info.exit_code != SVM_EXIT_WRITE_CR0 ||
>
> Oof, the existing is all kinds of confusing. Even with your comment, it took me
> a few seconds to understand how/where the exit_code is being modified. Eww.
>
> Any objection to opportunistically fixing this up to the (completely untested)
> below when applying?
Looks good with a minor nit:
>
> /*
> * Adjust the exit code accordingly if a CR other than CR0 is
> * being written, and skip straight to the common handling as
> * only CR0 has an additional selective intercept.
> */
> if (info->intercept == x86_intercept_cr_write && info->modrm_reg) {
> icpt_info.exit_code += info->modrm_reg;
> break;
> }
>
> /*
> * Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if L1 set
> * INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE, as the
> * unconditional intercept has higher priority.
> */
We only convert the exict_code to SVM_EXIT_CR0_SEL_WRITE if other
conditions are true below. So maybe "Check if the exit_code needs to be
converted to.."?
> if (vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_CR0_WRITE) ||
> !(vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_SELECTIVE_CR0)))
> break;
>
>
> > - info->intercept == x86_intercept_clts)
> > + vmcb12_is_intercept(&svm->nested.ctl,
> > + INTERCEPT_CR0_WRITE) ||
> > + !(vmcb12_is_intercept(&svm->nested.ctl,
> > + INTERCEPT_SELECTIVE_CR0)))
>
> Let these poke out.
Sure. Do you prefer a new version with this + your fixup above, or will
you fix them up while applying?
>
> > break;
> >
> > - if (!(vmcb12_is_intercept(&svm->nested.ctl,
> > - INTERCEPT_SELECTIVE_CR0)))
> > + /* CLTS never triggers INTERCEPT_SELECTIVE_CR0 */
> > + if (info->intercept == x86_intercept_clts)
> > break;
> >
> > /* LMSW always triggers INTERCEPT_SELECTIVE_CR0 */
> > --
> > 2.51.1.821.gb6fe4d2222-goog
> >
On Wed, Nov 05, 2025, Yosry Ahmed wrote:
> On Wed, Nov 05, 2025 at 11:48:27AM -0800, Sean Christopherson wrote:
> > On Fri, Oct 24, 2025, Yosry Ahmed wrote:
> Looks good with a minor nit:
>
> >
> > /*
> > * Adjust the exit code accordingly if a CR other than CR0 is
> > * being written, and skip straight to the common handling as
> > * only CR0 has an additional selective intercept.
> > */
> > if (info->intercept == x86_intercept_cr_write && info->modrm_reg) {
> > icpt_info.exit_code += info->modrm_reg;
> > break;
> > }
> >
> > /*
> > * Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if L1 set
> > * INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE, as the
> > * unconditional intercept has higher priority.
> > */
>
> We only convert the exict_code to SVM_EXIT_CR0_SEL_WRITE if other
> conditions are true below. So maybe "Check if the exit_code needs to be
> converted to.."?
Ouch, good point. I keep forgetting that the common code below this needs to
check the exit_code against the intercept enables. How about this?
/*
* Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if a
* selective CR0 intercept is triggered (the common logic will
* treat the selective intercept as being enabled). Note, the
* unconditional intercept has higher priority, i.e. this is
* only relevant if *only* the selective intercept is enabled.
*/
>
> > if (vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_CR0_WRITE) ||
> > !(vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_SELECTIVE_CR0)))
> > break;
> >
> >
> > > - info->intercept == x86_intercept_clts)
> > > + vmcb12_is_intercept(&svm->nested.ctl,
> > > + INTERCEPT_CR0_WRITE) ||
> > > + !(vmcb12_is_intercept(&svm->nested.ctl,
> > > + INTERCEPT_SELECTIVE_CR0)))
> >
> > Let these poke out.
>
> Sure. Do you prefer a new version with this + your fixup above, or will
> you fix them up while applying?
If you're happy with it, I'll just fixup when applying.
On Wed, Nov 05, 2025 at 12:37:37PM -0800, Sean Christopherson wrote:
> On Wed, Nov 05, 2025, Yosry Ahmed wrote:
> > On Wed, Nov 05, 2025 at 11:48:27AM -0800, Sean Christopherson wrote:
> > > On Fri, Oct 24, 2025, Yosry Ahmed wrote:
> > Looks good with a minor nit:
> >
> > >
> > > /*
> > > * Adjust the exit code accordingly if a CR other than CR0 is
> > > * being written, and skip straight to the common handling as
> > > * only CR0 has an additional selective intercept.
> > > */
> > > if (info->intercept == x86_intercept_cr_write && info->modrm_reg) {
> > > icpt_info.exit_code += info->modrm_reg;
> > > break;
> > > }
> > >
> > > /*
> > > * Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if L1 set
> > > * INTERCEPT_SELECTIVE_CR0 but not INTERCEPT_CR0_WRITE, as the
> > > * unconditional intercept has higher priority.
> > > */
> >
> > We only convert the exict_code to SVM_EXIT_CR0_SEL_WRITE if other
> > conditions are true below. So maybe "Check if the exit_code needs to be
> > converted to.."?
>
> Ouch, good point. I keep forgetting that the common code below this needs to
> check the exit_code against the intercept enables. How about this?
Looks good.
>
> /*
> * Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if a
> * selective CR0 intercept is triggered (the common logic will
> * treat the selective intercept as being enabled). Note, the
> * unconditional intercept has higher priority, i.e. this is
> * only relevant if *only* the selective intercept is enabled.
> */
>
> >
> > > if (vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_CR0_WRITE) ||
> > > !(vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_SELECTIVE_CR0)))
> > > break;
> > >
> > >
> > > > - info->intercept == x86_intercept_clts)
> > > > + vmcb12_is_intercept(&svm->nested.ctl,
> > > > + INTERCEPT_CR0_WRITE) ||
> > > > + !(vmcb12_is_intercept(&svm->nested.ctl,
> > > > + INTERCEPT_SELECTIVE_CR0)))
> > >
> > > Let these poke out.
> >
> > Sure. Do you prefer a new version with this + your fixup above, or will
> > you fix them up while applying?
>
> If you're happy with it, I'll just fixup when applying.
More than happy :)
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