linux-kernel - Re: [PATCH] kallsyms: fix absolute addresses for kASLR

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Message-ID: <CAGXu5j+iwHryXB9sQrT4agTa=GQ-OwoPLS256=cBuQoqnT3fhA@mail.gmail.com>
Date:	Tue, 25 Feb 2014 22:15:25 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	LKML <linux-kernel@...r.kernel.org>, Michal Marek <mmarek@...e.cz>,
	Andi Kleen <andi@...stfloor.org>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Ming Lei <tom.leiming@...il.com>,
	Andy Honig <ahonig@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, Vivek Goyal <vgoyal@...ibm.com>
Subject: Re: [PATCH] kallsyms: fix absolute addresses for kASLR

On Mon, Feb 24, 2014 at 5:29 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> Kees Cook <keescook@...omium.org> writes:
>> From: Andy Honig <ahonig@...gle.com>
>>
>> Currently symbols that are absolute addresses are incorrectly
>> displayed in /proc/kallsyms if the kernel is loaded with kASLR.
>>
>> The problem was that the scripts/kallsyms.c file which generates
>> the array of symbol names and addresses uses an relocatable value
>> for all symbols, even absolute symbols.  This patch fixes that.
>
> Hi Andy, Kees,
>
>         This is not a good patch.  See the commit where this was
> introduced:
>
> [PATCH] relocatable kernel: Fix kallsyms on avr32 after relocatable kernel changes
>
>     o On some platforms like avr32, section init comes before .text and
>       not necessarily a symbol's relative position w.r.t _text is positive.
>       In such cases assembler detects the overflow and emits warning. This
>       patch fixes it.
>
> Did you just break avr32?
>
> And absolute symbols are supposed to be handled in the other branch:
>
>         for (i = 0; i < table_cnt; i++) {
>                 if (toupper(table[i].sym[0]) != 'A') {
>                         if (_text <= table[i].addr)
>                                 printf("\tPTR\t_text + %#llx\n",
>                                         table[i].addr - _text);
>                         else
>                                 printf("\tPTR\t_text - %#llx\n",
>                                         _text - table[i].addr);
>                 } else {
>                         printf("\tPTR\t%#llx\n", table[i].addr);
>                 }
>         }
>
> __per_cpu_start is not an absolute symbol anyway.
>
> You need to fix this properly.
> Rusty.

Hm, yeah, it seems we need another class of variable. The per_cpu
stuff is technically relative, but it's not relocated, since it's not
relative to the text location. We'll see how to do this more sanely.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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