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

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