linux-kernel - Re: [PATCH stable 5.4,5.10] x86/fpu: Correct pkru/xstate inconsistency

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Date:   Thu, 17 Feb 2022 08:44:17 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Willis Kung <williskung@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        "# v4 . 10+" <stable@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH stable 5.4,5.10] x86/fpu: Correct pkru/xstate
 inconsistency

On 2/17/22 05:31, Brian Geffon wrote:
> How would you and Greg KH like to proceed with this? I'm happy to help
> however I can.

If I could wave a magic wand, I'd just apply the whole FPU rewrite to
stable.

My second choice would be to stop managing PKRU with XSAVE.
x86_pkru_load() uses WRPKRU instead of XSAVE and keeps the task's PKRU
in task->pkru instead of the XSAVE buffer.  Doing that will take some
care, including pulling XFEATURE_PKRU out of the feature mask (RFBM) at
XRSTOR.  I _think_ that can be done in a manageable set of patches which
 will keep stable close to mainline.  I recognize that more bugs might
get introduced in the process which are unique to stable.

If you give that a shot and realize that it's not feasible to do a
subset, then we can fall back to the minimal fix.  I'm not asking for a
multi-month engineering effort here.  Maybe an hour or two to see if
it's really as scary as it looks.