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    SubjectRe: [PATCH v6 04/13] arm64/kvm: hide ptrauth from guests
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    On 09/12/2018 14:53, Richard Henderson wrote:
    > On 12/7/18 12:39 PM, Kristina Martsenko wrote:
    >> From: Mark Rutland <mark.rutland@arm.com>
    >>
    >> In subsequent patches we're going to expose ptrauth to the host kernel
    >> and userspace, but things are a bit trickier for guest kernels. For the
    >> time being, let's hide ptrauth from KVM guests.
    >>
    >> Regardless of how well-behaved the guest kernel is, guest userspace
    >> could attempt to use ptrauth instructions, triggering a trap to EL2,
    >> resulting in noise from kvm_handle_unknown_ec(). So let's write up a
    >> handler for the PAC trap, which silently injects an UNDEF into the
    >> guest, as if the feature were really missing.
    >
    > Reviewing the long thread that accompanied v5, I thought we were *not* going to
    > trap PAuth instructions from the guest.
    >
    > In particular, the OS distribution may legitimately be built to include
    > hint-space nops. This includes XPACLRI, which is used by the C++ exception
    > unwinder and not controlled by SCTLR_EL1.EnI{A,B}.

    The plan was to disable trapping, yes. However, after that thread there
    was a retrospective change applied to the architecture, such that the
    XPACLRI (and XPACD/XPACI) instructions are no longer trapped by
    HCR_EL2.API. (The public documentation on this has not been updated
    yet.) This means that no HINT-space instructions should trap anymore.
    (The guest is expected to not set SCTLR_EL1.EnI{A,B} since
    ID_AA64ISAR1_EL1.{APA,API} read as 0.)

    > It seems like the header comment here, and
    Sorry, which header comment?

    >> +/*
    >> + * Guest usage of a ptrauth instruction (which the guest EL1 did not turn into
    >> + * a NOP).
    >> + */
    >> +static int kvm_handle_ptrauth(struct kvm_vcpu *vcpu, struct kvm_run *run)
    >> +
    >
    > here, need updating.

    Changed it to "a trapped ptrauth instruction".

    Kristina

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