Re: [PATCH v6 3/6] arm64/kvm: context-switch ptrauth registers

[Dave Martin]

On Thu, Feb 21, 2019 at 12:29:42PM +0000, Mark Rutland wrote:
> On Tue, Feb 19, 2019 at 02:54:28PM +0530, Amit Daniel Kachhap wrote:
> > From: Mark Rutland <mark.rutland@xxxxxxx>
> > 
> > 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.
> > 
> > Pointer authentication feature is only enabled when VHE is built
> > in the kernel and present into CPU implementation so only VHE code
> > paths are modified.
> 
> Nit: s/into/in the/
> 
> > 
> > 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. However the host key registers
> > are saved in vcpu load stage as they remain constant for each vcpu
> > schedule.
> > 
> > Pointer authentication consists of address authentication and generic
> > authentication, and CPUs in a system might have varied support for
> > either. Where support for either feature is not uniform, it is hidden
> > from guests via ID register emulation, as a result of the cpufeature
> > framework in the host.
> > 
> > Unfortunately, address authentication and generic authentication cannot
> > be trapped separately, as the architecture provides a single EL2 trap
> > covering both. If we wish to expose one without the other, we cannot
> > prevent a (badly-written) guest from intermittently using a feature
> > which is not uniformly supported (when scheduled on a physical CPU which
> > supports the relevant feature). Hence, this patch expects both type of
> > authentication to be present in a cpu.
> > 
> > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > [Only VHE, key switch from from assembly, kvm_supports_ptrauth
> > checks, save host key in vcpu_load]
> 
> Hmm, why do we need to do the key switch in assembly, given it's not
> used in-kernel right now?
> 
> Is that in preparation for in-kernel pointer auth usage? If so, please
> call that out in the commit message.

[...]

> Huh, so we're actually doing the switch in C code...
> 
> >  # KVM code is run at a different exception code with a different map, so
> >  # compiler instrumentation that inserts callbacks or checks into the code may
> > diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> > index 675fdc1..b78cc15 100644
> > --- a/arch/arm64/kvm/hyp/entry.S
> > +++ b/arch/arm64/kvm/hyp/entry.S
> > @@ -64,6 +64,12 @@ ENTRY(__guest_enter)
> >  
> >  	add	x18, x0, #VCPU_CONTEXT
> >  
> > +#ifdef	CONFIG_ARM64_PTR_AUTH
> > +	// Prepare parameter for __ptrauth_switch_to_guest(vcpu, host, guest).
> > +	mov	x2, x18
> > +	bl	__ptrauth_switch_to_guest
> > +#endif
> 
> ... and conditionally *calling* that switch code from assembly ...
> 
> > +
> >  	// Restore guest regs x0-x17
> >  	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
> >  	ldp	x2, x3,   [x18, #CPU_XREG_OFFSET(2)]
> > @@ -118,6 +124,17 @@ ENTRY(__guest_exit)
> >  
> >  	get_host_ctxt	x2, x3
> >  
> > +#ifdef	CONFIG_ARM64_PTR_AUTH
> > +	// Prepare parameter for __ptrauth_switch_to_host(vcpu, guest, host).
> > +	// Save x0, x2 which are used later in callee saved registers.
> > +	mov	x19, x0
> > +	mov	x20, x2
> > +	sub	x0, x1, #VCPU_CONTEXT
> > +	ldr	x29, [x2, #CPU_XREG_OFFSET(29)]
> > +	bl	__ptrauth_switch_to_host
> > +	mov	x0, x19
> > +	mov	x2, x20
> > +#endif
> 
> ... which adds a load of boilerplate for no immediate gain.
> 
> Do we really need to do this in assembly today?

If we will need to move this to assembly when we add in-kernel ptrauth
support, it may be best to have it in assembly from the start, to reduce
unnecessary churn.

But having a mix of C and assembly is likely to make things more
complicated: we should go with one or the other IMHO.

Cheers
---Dave