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From:   Keno Fischer <keno@juliacomputing.com>
Date:   Wed, 15 Apr 2020 21:14:50 -0400
Message-ID: <CABV8kRzbGgF4Uc9+VyzBUiH-kGfMALd8tDtjE3hjyE2Z5VD3-g@mail.gmail.com>
Subject: Re: [RFC PATCH v2] x86/arch_prctl: Add ARCH_SET_XCR0 to set XCR0 per-thread
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andi Kleen <andi@firstfloor.org>,
        Kyle Huey <khuey@kylehuey.com>,
        "Robert O'Callahan" <robert@ocallahan.org>
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> Would it make matters easier if tasks with nonstandard XCR0 were not
> allowed to use ptrace() at all?  And if ARCH_SET_XCR0 were disallowed
> if the caller is tracing anyone?

That would be fine by me (as long as you're still allowed to ptrace them of
course). I do think that using the kernel XCR0 is the best choice, but since
I don't really have a use case for it, I'm happy to disallow that and
let anybody who does have a use case come back here and
argue for it one way or the other.