Re: [PATCH] KVM: arm64: Reject attempts to set invalid debug arch version

[Marc Zyngier <maz@kernel.org>]

On Fri, 23 Jun 2023 21:52:32 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> The debug architecture is mandatory in ARMv8, so KVM should not allow
> userspace to configure a vCPU with less than that. Of course, this isn't
> handled elegantly by the generic ID register plumbing, as the respective
> ID register fields have a nonzero starting value.
> 
> Add an explicit check for debug versions less than v8 of the
> architecture.
> 
> Fixes: c118cead07a7 ("KVM: arm64: Use generic sanitisation for ID_(AA64)DFR0_EL1")
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/sys_regs.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 1a13bab1a06c..5b25053a8e04 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1482,6 +1482,7 @@ static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
>  			       const struct sys_reg_desc *rd,
>  			       u64 val)
>  {
> +	u8 debugver = SYS_FIELD_GET(ID_AA64DFR0_EL1, DebugVer, val);
>  	u8 pmuver = SYS_FIELD_GET(ID_AA64DFR0_EL1, PMUVer, val);
>  
>  	/*
> @@ -1501,6 +1502,13 @@ static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
>  	if (pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF)
>  		val &= ~ID_AA64DFR0_EL1_PMUVer_MASK;
>  
> +	/*
> +	 * ID_AA64DFR0_EL1.DebugVer is one of those awkward fields with a
> +	 * nonzero minimum safe value.
> +	 */
> +	if (debugver < ID_AA64DFR0_EL1_DebugVer_IMP)
> +		return -EINVAL;
> +

Why isn't that caught by the check at the end of arm64_check_features
which says that for RO fields, the only safe value is the sanitised
version?

Am I missing something obvious (full disclosure, I've had a single
coffee at this time of the day...)?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.