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

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Message-ID: <CAGXu5jJb0PCSuC4DuOkh5-901iVLcmmPpn1G7UBq3-r9DaeMvw@mail.gmail.com>
Date:   Tue, 5 Dec 2017 11:35:59 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Phil Elwell <phil@...pberrypi.org>,
        Matthias Reichl <hias@...us.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Anholt <eric@...olt.net>,
        Stefan Wahren <stefan.wahren@...e.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 5:36 AM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Tue, Dec 05, 2017 at 01:30:11PM +0000, Phil Elwell wrote:
>> This was my initial explanation:
>>
>> 1. Data which is marked __ro_after_init is initially writeable.
>>
>> 2. The ro_perms data covers kernel text, read-only data and __ro_after_init data.
>>
>> 3. set_kernel_text_rw marks everything in ro_perms as writeable.
>>
>> 4. set_kernel_text_ro marks everything in ro_perms as read-only, including the __ro_after_init data.
>>
>> 5. Using the function tracing code involves code modification, resulting in calls to
>>    __ftrace_modify_code and set_kernel_text_ro.
>>
>> 6. Therefore if function tracing is enabled before kernel_init has completed then the __ro_after_init
>>    data is made read-only prematurely.
>
> My question still stands, but let me rephrase.  Do we need
> set_kernel_text_*() to touch the read-only data?

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,
...


-Kees

-- 
Kees Cook
Pixel Security