Re: [RESEND PATCH v6 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.

[Kyle Huey <me@kylehuey.com>]

On Thu, Nov 10, 2022 at 5:38 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 11/10/22 16:03, Kyle Huey wrote:
> > On Tue, Nov 8, 2022 at 10:23 AM Dave Hansen <dave.hansen@intel.com> wrote:
> ...
> >> At a high level, this patch does a *LOT*.  Generally, it's nice when
> >> bugfixes can be encapsulted in one patch, but I think there's too much
> >> going on here for one patch.
> >
> > Ok. How about I break the first part into two pieces, one that changes the
> > signatures of copy_uabi_from_kernel_to_xstate() and
> > copy_sigframe_from_user_to_xstate(), and one that moves the relevant
> > KVM code from fpu_copy_uabi_to_guest_fpstate() to copy_uabi_to_xstate()
> > and deals with the edge case behavior of the mask?
>
> Sounds like a good start.  My gut says there's another patch or two that
> could be broken out, but that sounds like a reasonable next step.
>
> >>> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> >>> index 3b28c5b25e12..c273669e8a00 100644
> >>> --- a/arch/x86/kernel/fpu/core.c
> >>> +++ b/arch/x86/kernel/fpu/core.c
> >>> @@ -391,8 +391,6 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
> >>>  {
> >>>       struct fpstate *kstate = gfpu->fpstate;
> >>>       const union fpregs_state *ustate = buf;
> >>> -     struct pkru_state *xpkru;
> >>> -     int ret;
> >>>
> >>>       if (!cpu_feature_enabled(X86_FEATURE_XSAVE)) {
> >>>               if (ustate->xsave.header.xfeatures & ~XFEATURE_MASK_FPSSE)
> >>> @@ -406,16 +404,16 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
> >>>       if (ustate->xsave.header.xfeatures & ~xcr0)
> >>>               return -EINVAL;
> >>>
> >>> -     ret = copy_uabi_from_kernel_to_xstate(kstate, ustate);
> >>> -     if (ret)
> >>> -             return ret;
> >>> +     /*
> >>> +      * Nullify @vpkru to preserve its current value if PKRU's bit isn't set
> >>> +      * in the header.  KVM's odd ABI is to leave PKRU untouched in this
> >>> +      * case (all other components are eventually re-initialized).
> >>> +      * (Not clear that this is actually necessary for compat).
> >>> +      */
> >>> +     if (!(ustate->xsave.header.xfeatures & XFEATURE_MASK_PKRU))
> >>> +             vpkru = NULL;
> >>
> >> I'm not a big fan of hunks that are part of bugfixes where it is not
> >> clear that the hunk is necessary.
> >
> > This is necessary to avoid changing KVM's behavior at the same time
> > that we change
> > ptrace, since KVM doesn't want the same behavior as ptrace.
>
> Your "This is necessary" doesn't really match with "Not clear that this
> is actually necessary" from the comment, right?
>
> Rather than claim whether it is necessary or not, maybe just say why
> it's there: it's there to preserve wonky KVM behavior.
>
> BTW, I'd love to know if KVM *REALLY* depends on this.  It'd be nice to
> kill if not.

qemu didn't appear to (it treats the KVM_GET_XSAVE2/KVM_SET_XSAVE
buffers as opaque blobs afaict) but it's of course not the only KVM
application out there.

> >> Would something like this be more clear?
> >>
> >>         if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
> >>                 struct pkru_state *xpkru;
> >>
> >>                 xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);
> >>                 *pkru = xpkru->pkru;
> >>         } else {
> >>                 /*
> >>                  * KVM may pass a NULL 'pkru' to indicate
> >>                  * that it does not need PKRU updated.
> >>                  */
> >>                 if (pkru)
> >>                         *pkru = 0;
> >>         }
> >
> > Yeah, Sean Christopherson suggested this (with the else and if
> > collapsed into a single level) when I submitted this previously.
>
> I generally agree with Sean, but he's also been guilty of an atrocity or
> two over the years.  :)  While I generally like low levels of
> indentation I also think my version is much more clear in this case.
>

- Kyle