[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
[off-list ref] wrote:

On Tue, Dec 05, 2017 at 01:30:11PM +0000, Phil Elwell wrote:

quoted

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