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

[Amit Daniel Kachhap]

On 2/21/19 9:21 PM, Dave Martin wrote:
> 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.
ok, I will check on this.

Thanks,
Amit D
> Cheers
> ---Dave