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

From: Kyle Huey <me@kylehuey.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	linux-kselftest@vger.kernel.org,
	"Robert O'Callahan" <robert@ocallahan.org>,
	David Manouchehri <david.manouchehri@riseup.net>,
	Borislav Petkov <bp@suse.de>,
	stable@vger.kernel.org
Subject: Re: [PATCH v6 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.
Date: Fri, 2 Sep 2022 07:55:51 -0700	[thread overview]
Message-ID: <CAP045Aqc7T8in-MQz0hj4dOZ7TU6oAKuBNFtH57KttZ8Ueng-g@mail.gmail.com> (raw)
In-Reply-To: <YxDP6jie4cwzZIHp@google.com>

On Thu, Sep 1, 2022 at 8:29 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Aug 29, 2022, Kyle Huey wrote:
> > @@ -1246,6 +1246,21 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
> >               }
> >       }
> >
> > +     /*
> > +      * Update the user protection key storage. Allow KVM to
> > +      * pass in a NULL pkru pointer if the mask bit is unset
> > +      * for its legacy ABI behavior.
> > +      */
> > +     if (pkru)
> > +             *pkru = 0;
> > +
> > +     if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
> > +             struct pkru_state *xpkru;
> > +
> > +             xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);
> > +             *pkru = xpkru->pkru;
> > +     }
>
> What about writing this as:
>
>         if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
>                 ...
>
>                 *pkru = xpkru->pkru;
>         } else if (pkru) {
>                 *pkru = 0;
>         }
>
> to make it slightly more obvious that @pkru must be non-NULL if the feature flag
> is enabled?

tglx didn't seem to like the branchiness before but maybe he'll change
his mind since we have to have the `if (pkru)` now anyways.

> Or we could be paranoid, though I'm not sure this is worthwhile.
>
>         if ((hdr.xfeatures & XFEATURE_MASK_PKRU) &&
>             !WARN_ON_ONCE(!pkru)) {
>                 ...
>
>                 *pkru = xpkru->pkru;
>         } else if (pkru) {
>                 *pkru = 0;
>         }

I don't feel strongly about this.

> Otherwise, looks good from a KVM perspective.  Thanks!

Great.

- Kyle