Re: [PATCH 10/11] arm64/kvm: context-switch ptrauth registers

[Mark Rutland]

On Tue, Aug 01, 2017 at 01:00:14PM +0200, Christoffer Dall wrote:
> On Wed, Jul 19, 2017 at 05:01:31PM +0100, Mark Rutland wrote:
> > When pointer authentication is supported, a guest may wish to use it.
> > This patch adds the necessary KVM infrastructure for this to work, with
> > a semi-lazy context switch of the pointer auth state.
> > 
> > When we schedule a vcpu, we disable guest usage of pointer
> > authentication instructions and accesses to the keys. While these are
> > disabled, we avoid context-switching the keys. When we trap the guest
> > trying to use pointer authentication functionality, we change to eagerly
> > context-switching the keys, and enable the feature. The next time the
> > vcpu is scheduled out/in, we start again.
> > 
> > This is sufficient for systems with uniform pointer authentication
> > support. For systems with mismatched support, it will be necessary to
> 
> What is mismatched support?  You mean systems where one CPU has ptrauth
> and another one doesn't (or if they both have it but in different ways)?

Both! Any case where the support is not uniform across all CPUs.

A CPU can implement address auth and/or generic auth, and either may use
an architected algorithm or an IMP DEF algorithm.

Even if all CPUs report an IMP DEF algorithm, the particular algorithm
may differ across CPUs.

> > hide the feature from the guest's view of the ID registers.
> 
> I think the work Drew is pondering to allow userspace more control of
> what we emulate to the guest can semi-automatically take care of this as
> well.

I'll take a look.

[...]

> > +static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)
> > +{
> > +	kvm_arm_vcpu_ptrauth_disable(vcpu);
> > +}
> > +
> 
> why sched_in and not vcpu_load?  (vcpu_load happens whenever you're
> returning from userspace for example, and not only when you've been
> scheduled away and are coming back).

I think this is the result of going searching for similar lazy context
switching, and stumbling on some (fairly different) code on x86.

It looks like I can use vcpu_load() instead, so I'll give that a shot
for v2.

> And why do we want to 'reset' the behavior of KVM when the host
> schedules a VCPU thread?
> 
> If I understand the logic correctly, what you're establishing with the
> appraoch of initially trapping use of ptrauth is to avoid
> saving/restoring the state if the guest dosn't use ptrauth, but then if
> the guest starts using it, it's likely to keep using it, and therefore
> we start saving/restoring the registers.

Yes.

> Why is the host's decision to schedule something else on this physical
> CPU a good indication that we should no longer save/restore these
> registers for the VCPU?

I guess it's not.

Initially, I was followed the same approach we take for fpsimd, leaving
the feature trapped until we take a shallow trap to hyp. Handing all the
sysreg traps in hyp was unwieldy, so I moved that down to the kernel
proper. That meant I couldn't enable the trap when switching from host
to guest, and doing so in the regular context switch seemed like the
next best option.

> Wouldn't it make more sense to have a flag on the VCPU, and
> potentially a counter, so that if you switch X times without ever
> touching the registers again, you can stop saving/restoring the state,
> if that's even something we care about?

Something like that could make more sense. It should be fairly simple to
implement a decaying counter to determine when to trap.

I'd steered clear of optimising the lazy heuristic as I'm testing with
models, and I don't have numbers that would be representative of real
HW.

> Another thing that comes to mind; does the kernel running a KVM's VCPU
> thread (and handling softirqs, ISRs, etc.) ever use the ptrauth system,
> or does that only happen when we go to userspace? 

Today, only userspace uses pointer auth, and the kernel does not.
However, in-kernel usage is on the cards.

> If the latter, we could handle the save/restore entirely in
> vcpu_put/vcpu_load instead. I don't mind picking up that bit as part
> of my ongoing optimization work later though, if you're eager to get
> these patches merged.

I'd avoided that so far, since it would be undone when in-kernel usage
is implemented. If you prefer, I can implement that for now.

[...]

> > +/*
> > + * Handle the guest trying to use a ptrauth instruction, or trying to access a
> > + * ptrauth register.
> > + */
> > +void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu)
> > +{
> > +	if (cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH) &&
> > +	    cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH)) {
> > +		kvm_arm_vcpu_ptrauth_enable(vcpu);
> > +	} else {
> > +		kvm_inject_undefined(vcpu);
> 
> How can we trap here without having ARM64_HAS_[ADDRESS,GENERIC]_AUTH ?

We'll trap if the current CPU supports one of the two (with an
architected algorithm), but some other CPU does not (or uses an IMP DEF
algorithm). Note that we're checking that all CPUs have the feature.

> If this to cover the case if we only have one of the two or is there a
> case where we trap ptrauth registers/instructions even though the CPU
> doesn't have the support?

It's there to cater for the case that some CPUs lack a feature that
others have, so that we expose a uniform view to guests.

There's a single control to trap both address/generic authentication, so
it has to check that support is uniform for both. 

I'd meant to fix this up to not be so pessimistic -- we could support
one without that other, so long as it is uniformly absent. e.g. if all
CPUs support address auth and all CPUs have no support for generic auth.

I'll need to add negative cpucaps to detect that. I'll try to sort that
out for v2.

[...]

> > +static void __hyp_text __ptrauth_save_state(struct kvm_cpu_context *ctxt)
> > +{
> > +	if (cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH)) {
> > +		__ptrauth_save_key(ctxt->sys_regs, APIA);
> > +		__ptrauth_save_key(ctxt->sys_regs, APIB);
> > +		__ptrauth_save_key(ctxt->sys_regs, APDA);
> > +		__ptrauth_save_key(ctxt->sys_regs, APDB);
> > +	}
> > +
> > +	if (cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH)) {
> > +		__ptrauth_save_key(ctxt->sys_regs, APGA);
> > +	}
> 
> Aren't we ever only enabling the save/restore if we have both caps, so
> why are we checking it here again?

Sorry; I'd meant to fix things up so that we could support one without
the other. Likewise for __ptrauth_restore_state().

I'll try to fix this up for v2.

Thanks,
Mark.