linux-kernel - Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

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Date:   Tue, 29 Jun 2021 12:26:57 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state

On 6/29/21 12:13 PM, Bae, Chang Seok wrote:
>>
>> Is it actually important to make sure that they are dynamic features?
>> Is there *any* case where a feature (dynamic or not) can have XFD armed
>> and be out of its init state?
> In this AMX series, XFD is only used for the xstate buffer management. The
> code is made in a such way that XFD and dynamic states are a bit coupled.
> 
> But I think XFD can be extended for other usages in the future. Then, yes.
> (This warning is also for future code changes.)
> 
> So, reading the MSR is just simple and clean here, but it consumes cycles. Or,
> a task may have a field for XFD value per se unless this conversion is
> acceptable.

I'm not following.

All that I see here is that you made what could be a very generic check:

	Thou shalt never XSAVE a xfeature which is XFD-armed

and made it more specific:

	Thou shalt never XSAVE a *dynamic* xfeature which is XFD-armed

I'm just saying that I don't see the value in a less-broad check.  Let's
make the sanity check as broad as possible.  That actually makes the
code simpler.