linux-kernel - Re: [PATCH v2] x86: use fixed read-only IDT

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Date:	Tue, 9 Apr 2013 17:29:22 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Alex Shi <alex.shi@...el.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>,
	Eric Northup <digitaleric@...gle.com>
Subject: Re: [PATCH v2] x86: use fixed read-only IDT

On Tue, Apr 9, 2013 at 5:14 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 04/09/2013 09:39 AM, Kees Cook wrote:
>> -
>>  static void __cpuinit intel_smp_check(struct cpuinfo_x86 *c)
>>  {
>>       /* calling is from identify_secondary_cpu() ? */
>> @@ -206,8 +192,7 @@ static void __cpuinit intel_workarounds(struct cpuinfo_x86 *c)
>>       /*
>>        * All current models of Pentium and Pentium with MMX technology CPUs
>>        * have the F0 0F bug, which lets nonprivileged users lock up the
>> -      * system.
>> -      * Note that the workaround only should be initialized once...
>> +      * system. Announce that the fault handler will be checking for it.
>>        */
>>       c->f00f_bug = 0;
>>       if (!paravirt_enabled() && c->x86 == 5) {
>> @@ -215,7 +200,6 @@ static void __cpuinit intel_workarounds(struct cpuinfo_x86 *c)
>>
>>               c->f00f_bug = 1;
>>               if (!f00f_workaround_enabled) {
>> -                     trap_init_f00f_bug();
>>                       printk(KERN_NOTICE "Intel Pentium with F0 0F bug - workaround enabled.\n");
>>                       f00f_workaround_enabled = 1;
>>               }
>
> Why do we care about this message anymore?  It provides no relevant user
> information, the flag itself is already in /proc/cpuinfo, and the
> message is likely to be wrong since all it does is look for an Intel CPU
> with family == 5.

I have no objection to removing it, but with CONFIG_F00F_BUG, the trap
handler does still do some checking, and I figured this message was
there to notify people about it.

-Kees

--
Kees Cook
Chrome OS Security
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