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

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Date:   Fri, 29 Jun 2018 19:29:40 +0200
From:   Matthias Reichl <hias@...us.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Eric Anholt <eric@...olt.net>,
        Phil Elwell <phil@...pberrypi.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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 Fri, Jun 29, 2018 at 11:16:58AM -0400, Steven Rostedt wrote:
> On Fri, 29 Jun 2018 16:47:14 +0200
> Matthias Reichl <hias@...us.com> wrote:
> 
> > On Tue, Dec 05, 2017 at 12:14:46PM -0800, Kees Cook wrote:
> > > 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.  
> > 
> > Gentle ping, arm is still oopsing when the function tracer is
> > enabled at boot time.
> > 
> 
> I take it that my patch never got applied:
> 
>  http://lkml.kernel.org/r/20180621124710.453ee0ae@gandalf.local.home

Yes, sorry, forgot to include this info in my mail.

Your patch no longer applies cleanly - a8e53c151fe7a added a
debug_checkwx() to mark_rodata_ro() - but when applying it manually
it still fixes the oops.

so long,

Hias
> 
> -- Steve
> 
> 
> > Tested on bcm2835 (RPiB+) with current mainline tree
> > (githash 90368a37fbbe) and bcm2835_defconfig.
> > 
> > arm64 seems to be fine, tested on bcm2837 (RPi3) with same tree and
> > arm64 defconfig plus function tracer enabled.
>