linux-kernel - Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

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Message-ID: <20190815210547.GL15313@zn.tnic>
Date:   Thu, 15 Aug 2019 23:05:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, Chen Yu <yu.c.chen@...el.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

On Thu, Aug 15, 2019 at 09:59:03PM +0100, Andrew Cooper wrote:
> If you're virtualised, the write to MSR_AMD64_CPUID_FN_1 almost
> certainly won't take effect, which means userspace will still be able to
> see the bit.

msr_clear_bit() has a return value. We should check it before
doing anything further. I hope the HV actually signals the write
success/failure properly so that we get a correct return value.

-- 
Regards/Gruss,
    Boris.

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