Re: [PATCH v5 11/17] arm64: docs: document pointer authentication

[Ramana Radhakrishnan]

On 19/10/2018 12:35, Catalin Marinas wrote:
> On Tue, Oct 16, 2018 at 05:14:39PM +0100, Kristina Martsenko wrote:
> > On 05/10/2018 10:04, Ramana Radhakrishnan wrote:
> > > On 05/10/2018 09:47, Kristina Martsenko wrote:
> > > > +Virtualization
> > > > +--------------
> > > > +
> > > > +Pointer authentication is not currently supported in KVM guests. KVM
> > > > +will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of
> > > > +the feature will result in an UNDEFINED exception being injected into
> > > > +the guest.
> > >
> > > However applications using instructions from the hint space will
> > > continue to work albeit without any protection (as they would just be
> > > nops) ?
> >
> > Mostly, yes. If the guest leaves SCTLR_EL1.EnIA unset (and
> > EnIB/EnDA/EnDB), then PAC* and AUT* instructions in the HINT space will
> > execute as NOPs. If the guest sets EnIA, then PAC*/AUT* instructions
> > will trap and KVM will inject an "Unknown reason" exception into the
> > guest (which will cause a Linux guest to send a SIGILL to the application).
>
> I think that part is fine. If KVM (a fairly recent version with CPUID
> sanitisation) does not enable ptr auth, the CPUID should not advertise
> this feature either so the guest kernel should not enable it. For the
> above instructions in the HINT space, they will just be NOPs. If the
> guest kernel enables the feature regardless of the CPUID information, it
> deserves to get an "Unknown reason" exception.
>
> > In the latter case we could instead pretend the instruction was a NOP
> > and not inject an exception, but trapping twice per every function would
> > probably be terrible for performance. The guest shouldn't be setting
> > EnIA anyway if ID_AA64ISAR1_EL1 reports that pointer authentication is
> > not present (because KVM has hidden it).
>
> I don't think we should. The SCTLR_EL1 bits are RES0 unless you know
> that the feature is present via CPUID.
>
> > The other special case is the XPACLRI instruction, which is also in the
> > HINT space. Currently it will trap and KVM will inject an exception into
> > the guest. We should probably change this to NOP instead, as that's what
> > applications will expect. Unfortunately there is no EnIA-like control to
> > make it NOP.
>
> Very good catch. Basically if EL2 doesn't know about ptr auth (older
> distro), EL1 may or may not know but leaves SCTLR_EL1 disabled (based on
> CPUID), the default HCR_EL2 is to trap (I'm ignoring EL3 as that's like
> to have ptr auth enabled, being built for the specific HW). So a user
> app considering XPACLRI a NOP (or inoffensive) will get a SIGILL
> (injected by the guest kernel following the injection of "Unknown
> reason" exception by KVM).
>
> Ramana, is XPACLRI commonly generated by gcc and expects it to be a NOP?
> Could we restrict it to only being used at run-time if the corresponding
> HWCAP is set? This means redefining this instruction as no longer in the
> NOP space.

Sorry to have missed this - I'm still catching up on email.

XPACLRI is used in the unwinder in exactly 2 places but not for 
unwinding itself but for storing the actual return address in the data 
structures, its not something I expect to be used very commonly so a 
check there seems reasonable. The xpaclri is considered a nop in the 
architecture as it is defined today. I don't like the idea of redefining 
instructions as not in the nop space after it's been defined as being 
so. We could investigate guarding the XPACLRI with a check with the 
HWCAP. How many unwinders would you like us to fix ?



> > One option is for KVM to pretend the instruction was a NOP and return to
> > the guest. But if XPACLRI gets executed frequently, then the constant
> > trapping might hurt performance. I don't know how frequently it might
> > get used, as I don't know of any applications currently using it. From
> > what I understand, it may be used by userspace stack unwinders.

Yep. Probably one instruction per frame being unwound during exception 
unwinding. And no trapping will be expensive even though it's *only* in 
the exception unwind case.

> > (Also worth noting - as far as I can tell there is no easy way for KVM
> > to know which pointer authentication instruction caused the trap, so we
> > may have to do something unusual like use "at s12e1r" to read guest
> > memory and check for XPACLRI.)
>
> Indeed, it's not an easy fix. As discussed (in the office), we can't
> even guarantee that the guest stage 1 translation is stable and points
> to the actual XPACLRI instruction.
>
> > The other option is to turn off trapping entirely. However then on a
> > big.LITTLE system with mismatched pointer authentication support
> > instructions will work intermittently on some CPUs but not others.
>
> That's another case but let's assume we never see such configurations ;).

That's a broken system by design :) !

Ramana
>