linux-kernel - Re: [PATCH] Arm: mm: ftrace: Only set text back to ro after kernel has been marked ro

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Date:   Tue, 5 Dec 2017 12:14:46 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Stefan Wahren <stefan.wahren@...e.com>,
        Eric Anholt <eric@...olt.net>,
        Phil Elwell <phil@...pberrypi.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Matthias Reichl <hias@...us.com>,
        linux-rpi-kernel@...ts.infradead.org,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] Arm: mm: ftrace: Only set text back to ro after kernel
 has been marked ro

On Tue, Dec 5, 2017 at 12:09 PM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Tue, Dec 05, 2017 at 11:35:59AM -0800, Kees Cook wrote:
>> We don't _need_ to, but they're all contiguous, so the ro_perms array
>> used by set_kernel_text_*() is actually only a single entry:
>>
>> static struct section_perm ro_perms[] = {
>>         /* Make kernel code and rodata RX (set RO). */
>>         {
>>                 .name   = "text/rodata RO",
>>                 .start  = (unsigned long)_stext,
>>                 .end    = (unsigned long)__init_begin,
>> ...
>
> Well, they may not be contiguous - it depends on DEBUG_ALIGN_RODATA.

Maybe I'm picking a slightly wrong word. I guess I meant adjacent. The
range _stext to __init_begin is all read-only, though there may be
padding (controlled by DEBUG_ALIGN_RODATA), to allow a split for NX
markings on rodata.

> Either way, we have __start_rodata_section_aligned, which is either
> the start of the read-only data section, or the start of the first
> section beyond __start_rodata if DEBUG_ALIGN_RODATA is not set.
>
> Given that __start_rodata_section_aligned will always be less than
> __init_begin, is there any reason not to make the above end at
> __start_rodata_section_aligned, thereby allowing more of the read-only
> data (in the case of DEBUG_ALIGN_RODATA=n) or all of the read-only
> data (in the case of DEBUG_ALIGN_RODATA=y) to remain write-protected?

Sure, there's no reason not to split this into two entries. It'll
require some reworking of the function calls to get it right,
obviously.

-Kees

-- 
Kees Cook
Pixel Security