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

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Date:   Tue, 08 Jun 2021 14:27:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>,
        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 Tue, Jun 08 2021 at 13:17, Thomas Gleixner wrote:
> On Mon, Jun 07 2021 at 09:38, Dave Hansen wrote:
>> On 6/7/21 7:08 AM, Thomas Gleixner wrote:
>>>> By the way, are you talking specifically about the _error_ paths where
>>>> the kernel is unable to XRSTOR the signal XSAVE buffer for some reason,
>>>> and tries to apply either init_fpu or the hardware init state instead?
>>> 
>>> 1) Successful XRSTOR from user if the PKRU feature bit in the
>>>    sigframe xsave.header.xfeatures is cleared. Both fast and slow path.
>>
>> It seems like the suggestion here is to inject 'init_pkru_value' in all
>> cases where the kernel would be injecting the hardware init value.  I
>> don't think we should go that far.
>>
>> If a signal handler sets xsave.header.xfeatures[PKRU]=0, I can't imagine
>> any other intent than wanting the hardware init value.
>
> Fine. But PKRU=0 is broken today...
>
> T1 in user space
>      wrpkru(0)

It's actually not wrpkru(0), but a XRSTOR with the
xsave.header.xfeatures[PKRU] bit cleared. But the rest is the same...