Re: [PATCH 03/28] KVM: arm64: Merge guest's HCRX_EL2 using NV_HCRX_GUEST_EXCLUDE

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Missing RES0 Sanitization / Unsafe Register Merge for HCRX_EL2
--

commit a8dae7a323ff7e33355d5cb4c144294087a9bbfa
Author: Marc Zyngier <maz@kernel.org>
KVM: arm64: Merge guest's HCRX_EL2 using NV_HCRX_GUEST_EXCLUDE

This commit changes how the guest-provided HCRX_EL2 value is merged with
the host's value. It replaces positive masking with an exclusion list
of bits that the guest is not allowed to override.

> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 4bf624a49591d..8e5f492f39086 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h

[ ... ]

> @@ -350,8 +368,8 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu)
>  		u64 hcrx = vcpu->arch.hcrx_el2;
>  		if (is_nested_ctxt(vcpu)) {
>  			u64 val = __vcpu_sys_reg(vcpu, HCRX_EL2);
> -			hcrx |= val & __HCRX_EL2_MASK;
> -			hcrx &= ~(~val & __HCRX_EL2_nMASK);
> +			hcrx |= (val & ~NV_HCRX_GUEST_EXCLUDE);
> +			hcrx &= ~(~val & ~NV_HCRX_GUEST_EXCLUDE);

[Severity: High]
Does this allow an untrusted L1 guest to write garbage to the
architecturally RES0 bits of the physical HCRX_EL2 register?

By switching from a positive mask to a negative exclusion mask, any bits
not explicitly excluded are now merged into the host value.

If the guest modifies its VNCR page at the VNCR_HCRX_EL2 offset and sets
RES0 bits, these bits would be passed through. Since hcrx_feat_map lacks
FORCE_RES0(HCRX_EL2_RES0), KVM's register mask application in
kvm_vcpu_apply_reg_masks() will fail to clear them before they reach this
merge point in __activate_traps_common().

Could this lead to unpredictable behavior if the physical hardware
register is written with garbage in those RES0 bits?

>  		}
>  
>  		ctxt_sys_reg(hctxt, HCRX_EL2) = read_sysreg_s(SYS_HCRX_EL2);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260702160248.1377250-1-maz@kernel.org?part=3