[PATCH v5 4/7] KVM: arm64: Fix missing traps of guest accesses to the MPAM registers

[Joey Gouly]

On Wed, Oct 16, 2024 at 05:10:17PM -0700, Oliver Upton wrote:
> Hi Joey,
> 
> On Tue, Oct 15, 2024 at 02:39:20PM +0100, Joey Gouly wrote:
> > +static inline void  __activate_traps_mpam(struct kvm_vcpu *vcpu)
> > +{
> > +	u64 r = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1;
> > +
> > +	if (!cpus_support_mpam())
> > +		return;
> > +
> > +	/* trap guest access to MPAMIDR_EL1 */
> > +	if (mpam_cpus_have_mpam_hcr()) {
> > +		write_sysreg_s(MPAMHCR_EL2_TRAP_MPAMIDR_EL1, SYS_MPAMHCR_EL2);
> > +	} else {
> > +		/* From v1.1 TIDR can trap MPAMIDR, set it unconditionally */
> > +		r |= MPAM2_EL2_TIDR;
> > +	}
> > +
> > +	write_sysreg_s(r, SYS_MPAM2_EL2);
> > +}
> > +
> > +static inline void __deactivate_traps_mpam(void)
> > +{
> > +	if (!cpus_support_mpam())
> > +		return;
> > +
> > +	write_sysreg_s(0, SYS_MPAM2_EL2);
> > +
> > +	if (mpam_cpus_have_mpam_hcr())
> > +		write_sysreg_s(MPAMHCR_HOST_FLAGS, SYS_MPAMHCR_EL2);
> > +}
> 
> TBH, I think our trap configuration should *not* be conditioned on
> CONFIG_ARM64_MPAM. Otherwise we're silently allowing the guest to change
> things under the nose of KVM/host kernel, assuming an unkind firmware
> that left the EL2 trap configuration in a permissive state.
> 
> WDYT about detecting the feature && enforcing traps regardless of the Kconfig?

I had actually thought about the same thing. I spoke with James and he agrees,
so I will look into removing that.

I will probably end up removing the Kconfig entirely, it can be added back in
later, when actual support for MPAM is added.

Thanks,
Joey

> 
> -- 
> Thanks,
> Oliver