linux-kernel - Re: [PATCH] x86, traps: maps all IDTs to fixmap area.

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Date:	Fri, 27 Feb 2015 09:41:07 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] x86, traps: maps all IDTs to fixmap area.

On Thu, Feb 26, 2015 at 9:16 PM, Wang Nan <wangnan0@...wei.com> wrote:
> On 2015/2/27 2:31, Andy Lutomirski wrote:
>> On Thu, Feb 26, 2015 at 8:45 AM, Kees Cook <keescook@...omium.org> wrote:
>>> On Thu, Feb 26, 2015 at 7:17 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> On Wed, Feb 25, 2015 at 11:06 PM, Wang Nan <wangnan0@...wei.com> wrote:
>>>>> The reason why mapping idt_table to fixmap area should also be applied
>>>>> to debug_idt_table and trace_idt_table. This patch does same thing for
>>>>> all IDTs.
>>>>>
>>>>> Signed-off-by: Wang Nan <wangnan0@...wei.com>
>>>>> ---
>>>>>
>>>>> I believe trace_idt_table and debug_idt_table should be symmetrical with
>>>>> idt_table. However, Like my previous patch 'x86, traps: install gates
>>>>> using IST after cpu_init()', I'm not sure whether this is a practical
>>>>> fix.
>>>>
>>>> It shouldn't matter, since we should never enter userspace with these
>>>> IDTs loaded.
>>>>
>>>> --Andy
>>>>
>>>> [patch kept below for Kees' benefit]
>>>
>>> Is there a reason to use fixmap entries for these IDTs? Or rather, is
>>> there a situation where these IDTs are ever visible to userspace? (The
>>> reason to use the fixmap is to hide their "true" location from
>>> userspace.)
>>
>> There's also the F00F workaround, which IIRC we get for free by using
>> the fixmap, but that also shouldn't matter here.
>>
>
> What about a flaw module triggering the F00F bug in kernel space? Instead of
> kernel panic, the system will hang. I think tis should be a case for which
> my patch can help. However, the trigger condition is critical.

If it solves a real problem, I have no objection. I just wanted to be
sure we weren't trivially using up a fixmap entry.

-Kees

-- 
Kees Cook
Chrome OS Security
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