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

Kees Cook keescook at chromium.org
Tue Dec 5 11:35:59 PST 2017

On Tue, Dec 5, 2017 at 5:36 AM, Russell King - ARM Linux
<linux at armlinux.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 Cook
Pixel Security

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