Re: [PATCH v5 09/37] KVM: arm64: Extend masking facility to arbitrary registers

Joey Gouly
2024-10-24T10:38:29Z

On Wed, Oct 23, 2024 at 03:53:17PM +0100, Marc Zyngier wrote:
> We currently only use the masking (RES0/RES1) facility for VNCR
> registers, as they are memory-based and thus easy to sanitise.
>
> But we could apply the same thing to other registers if we:
>
> - split the sanitisation from __VNCR_START__
> - apply the sanitisation when reading from a HW register
>
> This involves a new "marker" in the vcpu_sysreg enum, which
> defines the point at which the sanitisation applies (the VNCR
> registers being of course after this marker).
>
> Whle we are at it, rename kvm_vcpu_sanitise_vncr_reg() to
> kvm_vcpu_apply_reg_masks(), which is vaguely more explicit,
> and harden set_sysreg_masks() against setting masks for
> random registers...
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
> arch/arm64/include/asm/kvm_host.h | 19 +++++++++++++------
> arch/arm64/kvm/nested.c | 12 ++++++++----
> arch/arm64/kvm/sys_regs.c | 3 +++
> 3 files changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 1adf68971bb17..7f409dfc5cd4a 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -367,7 +367,7 @@ struct kvm_arch {
>
> u64 ctr_el0;
>
> - /* Masks for VNCR-baked sysregs */
> + /* Masks for VNCR-backed and general EL2 sysregs */
> struct kvm_sysreg_masks *sysreg_masks;
>
> /*
> @@ -401,6 +401,9 @@ struct kvm_vcpu_fault_info {
> r = __VNCR_START__ + ((VNCR_ ## r) / 8), \
> __after_##r = __MAX__(__before_##r - 1, r)
>
> +#define MARKER(m) \
> + m, __after_##m = m - 1
> +
> enum vcpu_sysreg {
> __INVALID_SYSREG__, /* 0 is reserved as an invalid value */
> MPIDR_EL1, /* MultiProcessor Affinity Register */
> @@ -487,7 +490,11 @@ enum vcpu_sysreg {
> CNTHV_CTL_EL2,
> CNTHV_CVAL_EL2,
>
> - __VNCR_START__, /* Any VNCR-capable reg goes after this point */
> + /* Anything from this can be RES0/RES1 sanitised */
> + MARKER(__SANITISED_REG_START__),
> +
> + /* Any VNCR-capable reg goes after this point */
> + MARKER(__VNCR_START__),
>
> VNCR(SCTLR_EL1),/* System Control Register */
> VNCR(ACTLR_EL1),/* Auxiliary Control Register */
> @@ -547,7 +554,7 @@ struct kvm_sysreg_masks {
> struct {
> u64 res0;
> u64 res1;
> - } mask[NR_SYS_REGS - __VNCR_START__];
> + } mask[NR_SYS_REGS - __SANITISED_REG_START__];
> };
>
> struct kvm_cpu_context {
> @@ -995,13 +1002,13 @@ static inline u64 *___ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
>
> #define ctxt_sys_reg(c,r) (*__ctxt_sys_reg(c,r))
>
> -u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *, enum vcpu_sysreg);
> +u64 kvm_vcpu_apply_reg_masks(const struct kvm_vcpu *, enum vcpu_sysreg, u64);
> #define __vcpu_sys_reg(v,r) \
> (*({ \
> const struct kvm_cpu_context *ctxt = &(v)->arch.ctxt; \
> u64 *__r = __ctxt_sys_reg(ctxt, (r)); \
> - if (vcpu_has_nv((v)) && (r) >= __VNCR_START__) \
> - *__r = kvm_vcpu_sanitise_vncr_reg((v), (r)); \
> + if (vcpu_has_nv((v)) && (r) >= __SANITISED_REG_START__) \
> + *__r = kvm_vcpu_apply_reg_masks((v), (r), *__r);\
> __r; \
> }))
>
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index f9e30dd34c7a1..b20b3bfb9caec 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -908,15 +908,15 @@ static void limit_nv_id_regs(struct kvm *kvm)
> kvm_set_vm_id_reg(kvm, SYS_ID_AA64DFR0_EL1, val);
> }
>
> -u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *vcpu, enum vcpu_sysreg sr)
> +u64 kvm_vcpu_apply_reg_masks(const struct kvm_vcpu *vcpu,
> + enum vcpu_sysreg sr, u64 v)
> {
> - u64 v = ctxt_sys_reg(&vcpu->arch.ctxt, sr);
> struct kvm_sysreg_masks *masks;
>
> masks = vcpu->kvm->arch.sysreg_masks;
>
> if (masks) {
> - sr -= __VNCR_START__;
> + sr -= __SANITISED_REG_START__;
>
> v &= ~masks->mask[sr].res0;
> v |= masks->mask[sr].res1;
> @@ -927,7 +927,11 @@ u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *vcpu, enum vcpu_sysreg sr)
>
> static void set_sysreg_masks(struct kvm *kvm, int sr, u64 res0, u64 res1)
> {
> - int i = sr - __VNCR_START__;
> + int i = sr - __SANITISED_REG_START__;
> +
> + BUILD_BUG_ON(!__builtin_constant_p(sr));
> + BUILD_BUG_ON(sr < __SANITISED_REG_START__);
> + BUILD_BUG_ON(sr >= NR_SYS_REGS);
>
> kvm->arch.sysreg_masks->mask[i].res0 = res0;
> kvm->arch.sysreg_masks->mask[i].res1 = res1;
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 932d2fb7a52a0..d9c20563cae93 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -189,6 +189,9 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
>
> /* Get the current version of the EL1 counterpart. */
> WARN_ON(!__vcpu_read_sys_reg_from_cpu(el1r, &val));
> + if (reg >= __SANITISED_REG_START__)
> + val = kvm_vcpu_apply_reg_masks(vcpu, reg, val);
> +
> return val;
> }
>

Reviewed-by: Joey Gouly <joey.gouly@arm.com>