linux-kernel - Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state

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Message-ID: <FC0176FB-32CD-4E7D-8AC7-17452E40FDFE@intel.com>
Date:   Wed, 18 Aug 2021 21:37:58 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     "Macieira, Thiago" <thiago.macieira@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to
 protect dynamic user state

On Aug 18, 2021, at 14:17, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Aug 18, 2021 at 08:43:50PM +0000, Bae, Chang Seok wrote:
>> Maybe I’m missing something, but I wonder what’s the difference
>> from reading XCR0.
> 
> Wny, because adding another prctl() is too damn hard?

Well, IIUC, merely XGETBV(0) in the kernel instead of from userspace.

> What if this modus operandi of features userspace can use with kernel
> assistance but need an explicit request and are off otherwise, gets
> extended beyond XSAVE-managed features?

What if it never happens? It will be just the same as XGETBV(0). I think on
the flip side there is also a benefit of maintaining a simple API as possible.

Thanks,
Chang