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

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Date:   Wed, 23 Aug 2017 15:03:51 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Eric Anholt <eric@...olt.net>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Phil Elwell <phil@...pberrypi.org>,
        linux-rpi-kernel@...ts.infradead.org,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Matthias Reichl <hias@...us.com>
Subject: Re: [PATCH] Arm: mm: ftrace: Only set text back to ro after kernel
 has been marked ro

On Wed, 23 Aug 2017 11:48:13 -0700
Kees Cook <keescook@...omium.org> wrote:

> > diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> > index ad80548..fd75f38 100644
> > --- a/arch/arm/mm/init.c
> > +++ b/arch/arm/mm/init.c
> > @@ -745,19 +745,29 @@ static int __mark_rodata_ro(void *unused)
> >         return 0;
> >  }
> >
> > +static int kernel_set_to_readonly;  
> 
> Adding a comment here might be a good idea, something like:
> 
> /* Has system boot-up reached mark_rodata_ro() yet? */

I don't mind adding a comment, but the above is rather self explanatory
(one can easily see that it is set in mark_rodata_ro() with a simple
search).

If a comment is to be added, something a bit more descriptive of the
functionality of the variable would be appropriate:

/*
 * Ignore modifying kernel text permissions until the kernel core calls
 * make_rodata_ro() at system start up.
 */

I can resend with the comment, or whoever takes this could add it
themselves.

-- Steve


> 
> Otherwise:
> 
> Acked-by: Kees Cook <keescook@...omium.org>
> 
> > +
> >  void mark_rodata_ro(void)
> >  {
> > +       kernel_set_to_readonly = 1;
> > +
> >         stop_machine(__mark_rodata_ro, NULL, NULL);
> >  }
> >
> >  void set_kernel_text_rw(void)
> >  {
> > +       if (!kernel_set_to_readonly)
> > +               return;
> > +
> >         set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
> >                                 current->active_mm);
> >  }
> >
> >  void set_kernel_text_ro(void)
> >  {
> > +       if (!kernel_set_to_readonly)
> > +               return;
> > +
> >         set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
> >                                 current->active_mm);
> >  }  
> 
> Does arm64 suffer from a similar condition? (It looks like no, as text
> patching is done with a fixmap poke.)
> 
> -Kees
>