All of lore.kernel.org
 help / color / mirror / Atom feed
From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Pavel Machek <pavel@ucw.cz>, Chen Yu <yu.c.chen@intel.com>,
	Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h
Date: Thu, 15 Aug 2019 21:59:03 +0100	[thread overview]
Message-ID: <a24a2c7d-cfab-a049-37e8-7260a9063a7c@citrix.com> (raw)
In-Reply-To: <776cb5c2d33e7fd0d2893904724c0e52b394f24a.1565817448.git.thomas.lendacky@amd.com>

On 14/08/2019 22:17, Lendacky, Thomas wrote:
> +static void init_hide_rdrand(struct cpuinfo_x86 *c)
> +{
> +	/*
> +	 * The nordrand option can clear X86_FEATURE_RDRAND, so check for
> +	 * RDRAND support using the CPUID function directly.
> +	 */
> +	if (!(cpuid_ecx(1) & BIT(30)) || rdrand_force)
> +		return;
> +
> +	msr_clear_bit(MSR_AMD64_CPUID_FN_00000001, 62);
> +	clear_cpu_cap(c, X86_FEATURE_RDRAND);
> +	pr_info_once("hiding RDRAND via CPUID\n");

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.

Best to leave everything untouched if you can't actually clear the bit. 
All you can do is trust that your hypervisor knows what it is doing.

~Andrew

  parent reply	other threads:[~2019-08-15 20:59 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-14 21:17 [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h Lendacky, Thomas
2019-08-14 23:24 ` Non-random RDRAND " Pavel Machek
2019-08-14 23:38   ` Pavel Machek
2019-08-15 13:01   ` Lendacky, Thomas
2019-08-15 15:12   ` Theodore Y. Ts'o
2019-08-16  9:07     ` Pavel Machek
2019-08-16 14:42     ` Neil Horman
2019-08-15  7:21 ` Borislav Petkov
2019-08-15 13:47   ` Lendacky, Thomas
2019-08-15 15:34     ` Borislav Petkov
2019-08-15 20:14       ` Thomas Gleixner
2019-08-15 20:59 ` Andrew Cooper [this message]
2019-08-15 21:04   ` Thomas Gleixner
2019-08-15 21:05   ` Borislav Petkov
2019-08-15 21:25     ` Andrew Cooper
2019-08-17  8:44       ` Borislav Petkov
2019-08-17 11:43         ` Andrew Cooper
2019-08-18 16:32           ` Thomas Gleixner
2019-08-16 15:19 ` Andy Lutomirski

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=a24a2c7d-cfab-a049-37e8-7260a9063a7c@citrix.com \
    --to=andrew.cooper3@citrix.com \
    --cc=Thomas.Lendacky@amd.com \
    --cc=bp@alien8.de \
    --cc=corbet@lwn.net \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-pm@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=pavel@ucw.cz \
    --cc=rjw@rjwysocki.net \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.org \
    --cc=yu.c.chen@intel.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.