linux-kernel - Re: [PATCH] x86: make IDT read-only

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Message-ID: <CAGXu5j+fWeinnBr3JLkLt-M4N8uSXzP9MiRF20h37_WABQ1Rrw@mail.gmail.com>
Date:	Mon, 8 Apr 2013 16:05:54 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.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>,
	Borislav Petkov <borislav.petkov@....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] x86: make IDT read-only

On Mon, Apr 8, 2013 at 3:56 PM, Maciej W. Rozycki <macro@...ux-mips.org> wrote:
> On Mon, 8 Apr 2013, Kees Cook wrote:
>
>> This makes the IDT unconditionally read-only. This primarily removes
>> the IDT from being a target for arbitrary memory write attacks. It has
>> an added benefit of also not leaking (via the "sidt" instruction) the
>> kernel base offset, if it has been relocated.
> [...]
>> --- a/arch/x86/kernel/cpu/intel.c
>> +++ b/arch/x86/kernel/cpu/intel.c
>> @@ -215,7 +201,6 @@ 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;
>>               }
>
>  FWIW the change looks reasonable to me, however given that that it makes
> the arrangement unconditional and there is no longer a workaround to
> enable I think it would make sense to remove the conditional block quoted
> above altogether, along with the f00f_workaround_enabled variable itself
> (alternatively "Intel Pentium with F0 0F bug" alone could be printed
> instead and the name of the variable adjusted to make sense with the new
> meaning -- up to you to decide).

Actually, I take it back. The other portion of the workaround is still
active (in mm/fault.c), and this chunk announces it, so I'm going to
leave it as-is.

-Kees

--
Kees Cook
Chrome OS Security
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