Re: [PATCH v1 1/3] KVM: arm64: nv: Don't advertise FEAT_LPA2 for guest stage-1

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Equality checks for TGRAN4 and TGRAN16 fail to mask future architectural enum values, leaking unsupported hardware capabilities to NV guests.
--

commit 6c2ebbb5542b3e22eaee90b7cf8d8cbbc9704e7c
Author: Wei-Lin Chang <weilin.chang@arm.com>

KVM: arm64: nv: Don't advertise FEAT_LPA2 for guest stage-1

There is no FEAT_LPA2 support for NV guests as of now, but we missed
limiting it for the non-stage-2 granule size fields (TGRAN4, TGRAN16) in
ID_AA64MMFR0_EL1. Add the sanitisation.

> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 17123f0b6daba..4d1884b32fc48 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -1731,6 +1731,19 @@ u64 limit_nv_id_reg(struct kvm *kvm, u32 reg, u64 val)
>  		/* Hide CNTPOFF if present */
>  		val = ID_REG_LIMIT_FIELD_ENUM(val, ID_AA64MMFR0_EL1, ECV, IMP);
>  
> +		/* NV does not support FEAT_LPA2. */
> +		if (SYS_FIELD_GET(ID_AA64MMFR0_EL1, TGRAN4, val) ==
> +		    ID_AA64MMFR0_EL1_TGRAN4_52_BIT) {

[Severity: Medium]
Will this exact equality check fail to mask future architectural enum values?

If future hardware implements a capability enum value greater than 52_BIT for
TGRAN4, this check will evaluate to false, leaking the unsupported capability
to NV guests.

Could this be capped instead, perhaps using inequalities or the
ID_REG_LIMIT_FIELD_ENUM macro, to maintain forward compatibility?

> +			val &= ~ID_AA64MMFR0_EL1_TGRAN4;
> +			val |= SYS_FIELD_PREP_ENUM(ID_AA64MMFR0_EL1, TGRAN4, IMP);
> +		}
> +
> +		if (SYS_FIELD_GET(ID_AA64MMFR0_EL1, TGRAN16, val) ==
> +		    ID_AA64MMFR0_EL1_TGRAN16_52_BIT) {

[Severity: Medium]
Similarly, will this exact equality check fail to mask future unsigned capability
values greater than 52_BIT for TGRAN16?

> +			val &= ~ID_AA64MMFR0_EL1_TGRAN16;
> +			val |= SYS_FIELD_PREP_ENUM(ID_AA64MMFR0_EL1, TGRAN16, IMP);
> +		}
> +
>  		/* Disallow unsupported S2 page sizes */
>  		switch (PAGE_SIZE) {
>  		case SZ_64K:

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909222015.3046432-1-weilin.chang@arm.com?part=1