linux-kernel - Re: [PATCH v8 1/4] x86/KASLR: Clarify identity map interface

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Message-ID: <CAGXu5j+4N0YSkEb54k6Wj6oZW8OKTwS12LtajTnerw03knRimw@mail.gmail.com>
Date:	Wed, 11 May 2016 08:23:45 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Borislav Petkov <bp@...e.de>, Yinghai Lu <yinghai@...nel.org>,
	Baoquan He <bhe@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Lasse Collin <lasse.collin@...aani.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <dyoung@...hat.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 1/4] x86/KASLR: Clarify identity map interface

On Tue, May 10, 2016 at 11:24 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Kees Cook <keescook@...omium.org> wrote:
>
>> +/*
>> + * Mapping information structure passed to kernel_ident_mapping_init().
>> + * Due to relocation, pointers must be assigned at run time not build time.
>> + */
>> +static struct x86_mapping_info mapping_info = {
>> +     .pmd_flag       = __PAGE_KERNEL_LARGE_EXEC,
>> +};
>
>> +void initialize_identity_maps(void)
>>  {
>> +     /* Init mapping_info with run-time function/buffer pointers. */
>> +     mapping_info.alloc_pgt_page = alloc_pgt_page;
>> +     mapping_info.context = &pgt_data;
>
> Could you please outline the precise failure mode? What gets executed when, which
> pointer gets relocated and which not, and exactly when does it pose a problem,
> etc.

It's the issue described at the top of misc.c:

/*
 * WARNING!!
 * This code is compiled with -fPIC and it is relocated dynamically at
 * run time, but no relocation processing is performed. This means that
 * it is not safe to place pointers in static structures.
 */

Should this be repeated in each .c file maybe as a reminder?

If the rest of the patches look good, we could clean this up as a
following patch? What do you think?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security