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

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Date:   Wed, 16 Feb 2022 07:16:13 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Brian Geffon <bgeffon@...gle.com>
Cc:     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 2/16/22 02:05, Greg KH wrote:
>>> How was this tested, and what do the maintainers of this subsystem
>>> think?  And will you be around to fix the bugs in this when they are
>>> found?
>> This has been trivial to reproduce, I've used a small repro which I've
>> put here: https://gist.github.com/bgaff/9f8cbfc8dd22e60f9492e4f0aff8f04f
>> , I also was able to reproduce this using the protection_keys self
>> tests on a 11th Gen Core i5-1135G7. I'm happy to commit to addressing
>> any bugs that may appear. I'll see what the maintainers say, but there
>> is also a smaller fix that just involves using this_cpu_read() in
>> switch_fpu_finish() for this specific issue, although that approach
>> isn't as clean.
> Can you add the test to the in-kernel tests so that we make sure it is
> fixed and never comes back?

It would be great if Brian could confirm this.  But, I'm 99% sure that
this can be reproduced in the vm/protection_keys.c selftest, if you run
it for long enough.

The symptom here is corruption of the PKRU register.  I created *lots*
of bugs like this during protection keys development so the selftest
keeps a shadow copy of the register to specifically watch for corruption.

It's _plausible_ that no one ever ran the pkey selftests with a
clang-compiled kernel for long enough to hit this issue.