Re: [PATCH v2 12/13] KVM: nSVM: Sanitize control fields copied from VMCB12

Sean Christopherson
2025-12-09T16:19:04Z

On Mon, Nov 10, 2025, Yosry Ahmed wrote:
> Make sure all fields used from VMCB12 in creating the VMCB02 are
> sanitized, such no unhandled or reserved bits end up in the VMCB02.
>
> The following control fields are read from VMCB12 and have bits that are
> either reserved or not handled/advertised by KVM: tlb_ctl, int_ctl,
> int_state, int_vector, event_inj, misc_ctl, and misc_ctl2.
>
> The following fields do not require any extra sanitizing:
> - int_ctl: bits from VMCB12 are copied bit-by-bit as needed.
> - misc_ctl: only used in consistency checks (particularly NP_ENABLE).
> - misc_ctl2: bits from VMCB12 are copied bit-by-bit as needed.
>
> For the remaining fields, make sure only defined bits are copied from
> VMCB12 by defining appropriate masks where needed. The only exception is
> tlb_ctl, which is unused, so remove it.
>
> Opportunisitcally move some existing definitions in svm.h around such

Opportunistically. But moot point, because please put such cleanups in a separate
patch. There are so many opportunistic cleanups in this patch that I genuinely
can't see what's changing, and I don't have the patience right now to stare hard.

Cleanups will making *related* changes are totally fine, e.g. bundling the use
of PAGE_MASK in conjuction with changing the code to do "from->iopm_base_pa & ..."
instead of "to->msrpm_base_pa &= ..." is fine, but those changes have nothing to
do with the rest of the patch.

> that they are ordered by bit position, and cleanup ignoring the lower
> bits of {io/msr}pm_base_pa in __nested_copy_vmcb_control_to_cache() by
> using PAGE_MASK. Also, expand the comment about the ASID being copied
> only for consistency checks.
>
> Suggested-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
> arch/x86/include/asm/svm.h | 11 ++++++++---
> arch/x86/kvm/svm/nested.c | 26 ++++++++++++++------------
> arch/x86/kvm/svm/svm.h | 1 -
> 3 files changed, 22 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index a842018952d2c..44f2cfcd8d4ff 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -213,11 +213,13 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
> #define V_NMI_ENABLE_SHIFT 26
> #define V_NMI_ENABLE_MASK (1 << V_NMI_ENABLE_SHIFT)
>
> +#define X2APIC_MODE_SHIFT 30
> +#define X2APIC_MODE_MASK (1 << X2APIC_MODE_SHIFT)
> +
> #define AVIC_ENABLE_SHIFT 31
> #define AVIC_ENABLE_MASK (1 << AVIC_ENABLE_SHIFT)
>
> -#define X2APIC_MODE_SHIFT 30
> -#define X2APIC_MODE_MASK (1 << X2APIC_MODE_SHIFT)
> +#define SVM_INT_VECTOR_MASK (0xff)
>
> #define SVM_INTERRUPT_SHADOW_MASK BIT_ULL(0)
> #define SVM_GUEST_INTERRUPT_MASK BIT_ULL(1)
> @@ -626,8 +628,11 @@ static inline void __unused_size_checks(void)
> #define SVM_EVTINJ_TYPE_EXEPT (3 << SVM_EVTINJ_TYPE_SHIFT)
> #define SVM_EVTINJ_TYPE_SOFT (4 << SVM_EVTINJ_TYPE_SHIFT)
>
> -#define SVM_EVTINJ_VALID (1 << 31)
> #define SVM_EVTINJ_VALID_ERR (1 << 11)
> +#define SVM_EVTINJ_VALID (1 << 31)

If you want to do cleanup, these should all use BIT()...

> +
> +#define SVM_EVTINJ_RESERVED_BITS ~(SVM_EVTINJ_VEC_MASK | SVM_EVTINJ_TYPE_MASK | \
> + SVM_EVTINJ_VALID_ERR | SVM_EVTINJ_VALID)

Because then I don't have to think hard about what exactly this will generate.

> #define SVM_EXITINTINFO_VEC_MASK SVM_EVTINJ_VEC_MASK
> #define SVM_EXITINTINFO_TYPE_MASK SVM_EVTINJ_TYPE_MASK
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 89830380cebc5..503cb7f5a4c5f 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -479,10 +479,11 @@ void __nested_copy_vmcb_control_to_cache(struct kvm_vcpu *vcpu,
> for (i = 0; i < MAX_INTERCEPT; i++)
> to->intercepts[i] = from->intercepts[i];
>
> - to->iopm_base_pa = from->iopm_base_pa;
> - to->msrpm_base_pa = from->msrpm_base_pa;
> + /* Lower bits of IOPM_BASE_PA and MSRPM_BASE_PA are ignored */
> + to->iopm_base_pa = from->iopm_base_pa & PAGE_MASK;
> + to->msrpm_base_pa = from->msrpm_base_pa & PAGE_MASK;
> +
> to->tsc_offset = from->tsc_offset;
> - to->tlb_ctl = from->tlb_ctl;
> to->int_ctl = from->int_ctl;
> to->int_vector = from->int_vector;
> to->int_state = from->int_state;
> @@ -492,19 +493,21 @@ void __nested_copy_vmcb_control_to_cache(struct kvm_vcpu *vcpu,
> to->exit_info_2 = from->exit_info_2;
> to->exit_int_info = from->exit_int_info;
> to->exit_int_info_err = from->exit_int_info_err;
> - to->misc_ctl = from->misc_ctl;
> + to->misc_ctl = from->misc_ctl;
> to->event_inj = from->event_inj;
> to->event_inj_err = from->event_inj_err;
> to->next_rip = from->next_rip;
> to->nested_cr3 = from->nested_cr3;
> - to->misc_ctl2 = from->misc_ctl2;
> + to->misc_ctl2 = from->misc_ctl2;
> to->pause_filter_count = from->pause_filter_count;
> to->pause_filter_thresh = from->pause_filter_thresh;
>
> - /* Copy asid here because nested_vmcb_check_controls will check it. */
> + /*
> + * Copy asid here because nested_vmcb_check_controls() will check it.
> + * The ASID could be invalid, or conflict with another VM's ASID , so it
> + * should never be used directly to run L2.
> + */
> to->asid = from->asid;
> - to->msrpm_base_pa &= ~0x0fffULL;
> - to->iopm_base_pa &= ~0x0fffULL;
>
> #ifdef CONFIG_KVM_HYPERV
> /* Hyper-V extensions (Enlightened VMCB) */
> @@ -890,9 +893,9 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
> (svm->nested.ctl.int_ctl & int_ctl_vmcb12_bits) |
> (vmcb01->control.int_ctl & int_ctl_vmcb01_bits);
>
> - vmcb02->control.int_vector = svm->nested.ctl.int_vector;
> - vmcb02->control.int_state = svm->nested.ctl.int_state;
> - vmcb02->control.event_inj = svm->nested.ctl.event_inj;
> + vmcb02->control.int_vector = svm->nested.ctl.int_vector & SVM_INT_VECTOR_MASK;
> + vmcb02->control.int_state = svm->nested.ctl.int_state & SVM_INTERRUPT_SHADOW_MASK;
> + vmcb02->control.event_inj = svm->nested.ctl.event_inj & ~SVM_EVTINJ_RESERVED_BITS;
> vmcb02->control.event_inj_err = svm->nested.ctl.event_inj_err;
>
> /*
> @@ -1774,7 +1777,6 @@ static void nested_copy_vmcb_cache_to_control(struct vmcb_control_area *dst,
> dst->msrpm_base_pa = from->msrpm_base_pa;
> dst->tsc_offset = from->tsc_offset;
> dst->asid = from->asid;
> - dst->tlb_ctl = from->tlb_ctl;
> dst->int_ctl = from->int_ctl;
> dst->int_vector = from->int_vector;
> dst->int_state = from->int_state;
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index ef6bdce630dc0..c8d43793aa9d6 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -178,7 +178,6 @@ struct vmcb_ctrl_area_cached {
> u64 msrpm_base_pa;
> u64 tsc_offset;
> u32 asid;
> - u8 tlb_ctl;
> u32 int_ctl;
> u32 int_vector;
> u32 int_state;
> --
> 2.51.2.1041.gc1ab5b90ca-goog
>