linux-kernel - Re: [patch V2 00/14] x86/fpu: Mop up XSAVES and related damage

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Message-ID: <87h7i9zv3r.ffs@nanos.tec.linutronix.de>
Date:   Mon, 07 Jun 2021 15:02:00 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch V2 00/14] x86/fpu: Mop up XSAVES and related damage

On Sun, Jun 06 2021 at 01:47, Thomas Gleixner wrote:
> P.S: I picked up Dave's PKRU series from

and while looking deeper at that I found the following existing
inconsistency vs. PKRU:

On exec() the kernel sets PKRU to the default value which is as
restrictive as possible.

But on sigrestore PKRU when the init optimization is used PKRU is reset
to the hardware default (0), which is as permissive as possible:

  1) On XRSTOR from user and the signal handler cleared the PKRU feature
     bit in xsave.header.xfeatures. That's true for the fast and the
     slow path.

  2) For the fx_only case which loads the init_fpstate (with my fixes
     applied that's not longer the case)

So that's inconsistent at best.

As the kernel defines the "init" behaviour of PKRU as non-permissive
on exec() it should do so consequently in sigrestore as well.

Thoughts?

Thanks,

        tglx