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

[Oliver Upton <oliver.upton@linux.dev>]

On Mon, Jun 26, 2023 at 10:23:24AM +0100, Marc Zyngier wrote:
> 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?

You're right, I was mistaken in thinking the field was already writable.
Now, having said that, it _is_ an issue with Jing's series to make DFR0
fully writable.

https://lore.kernel.org/kvmarm/20230607194554.87359-2-jingzhangos@google.com/

-- 
Thanks,
Oliver