linux-kernel - Re: [PATCH v5 15/28] x86/arch_prctl: Create ARCH_GET_XSTATE/ARCH_PUT

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Date:   Tue, 25 May 2021 10:33:25 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>, Len Brown <lenb@...nel.org>
Cc:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 15/28] x86/arch_prctl: Create
 ARCH_GET_XSTATE/ARCH_PUT_XSTATE

On 5/25/21 10:27 AM, Borislav Petkov wrote:
> On Mon, May 24, 2021 at 07:10:57PM -0400, Len Brown wrote:
>> 1. CPUID has AMX
>> 2. XCR0 has AMX
>> 3. Linux permission has been requested and granted to this process
> Actually, you want *only* 3 as 1 is a bad idea - we're in this mess
> because userspace does feature detection on its own even when kernel
> support is needed.
> 
> When Linux grants the permission, 1 and 2 should be implicitly given.

We did this for pkeys.  Essentially, we said, "don't bother with
CPUID.OSPKE and XCRO[PKRU] checking".  Just call alloc_pkey() and we'll
do the checking for you.  One stop shopping.

So far, it's worked well.