Re: [PATCH 2/2] ARM: mm: keep rodata non-executable

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Feb 18, 2014 at 10:10:03AM -0800, Kees Cook wrote:
> On Mon, Feb 17, 2014 at 4:34 AM, Dave Martin <Dave.Martin@arm.com> wrote:
> > On Fri, Feb 14, 2014 at 11:11:07AM -0800, Kees Cook wrote:
> >> On Fri, Feb 14, 2014 at 8:22 AM, Dave Martin <Dave.Martin@arm.com> wrote:
> >> > On Thu, Feb 13, 2014 at 05:04:10PM -0800, Kees Cook wrote:
> >> >> Introduce "CONFIG_DEBUG_RODATA" to mostly match the x86 config, though
> >> >> the behavior is different: it depends on STRICT_KERNMEM_PERMS, which
> >> >> sets rodata read-only (but executable), where as this option additionally
> >> >> splits rodata from the kernel text (resulting in potentially more memory
> >> >> lost to padding) and sets it non-executable as well. The end result is
> >> >> that on builds with CONFIG_DEBUG_RODATA=y (like x86) the rodata with be
> >> >> marked purely read-only.
> >> >
> >> > This triggers an Oops in kexec, because we have a block of code in .text
> >> > which is a template for generating baremetal code to relocate the new
> >> > kernel, and some literal words are written into it before copying.
> >>
> >> You're writing into the text area? I would imagine that
> >> CONFIG_ARM_KERNMEM_PERMS would break that. However, that's not the
> >> right place to be building code -- shouldn't the module area be used
> >> for that?
> >>
> >> > Possibly this should be in .rodata, not .text.
> >>
> >> Well, rodata should be neither writable nor executable.
> >
> > We're not writing into code exactly.
> >
> > This code is never executed in-place in vmlinux.  It gets copied, and
> > only copies are ever executed.
> >
> > Some pointers and offsets get poked into the code to configure it.
> >
> > I think it would be better simply to put the code in .rodata, and
> > poke paramaters into the copy, not the original -- but that's a bit
> > more awkward to code up, since the values can't be poked simply by
> > writing global variables.
> 
> Okay, interesting. I'll be curious to see what the patch for this looks like.
> 
> >> > There may be a few other instances of this kind of thing.
> >>
> >> This config will certainly find them! :) But, that's why it's behind a config.
> >
> > I haven't tested exhaustively, but it think this is sufficient for a
> > Tested-by.  The patch does seem to be doing what it is intended to
> > do, and doesn't seem to be triggering false positives all over the
> > place.
> 
> Great, thanks for taking the time to check on it!
> 
> Should I send this to the patch tracker, or wait for more feedback?

It would be good if someone who's more familiar with the parms and
vmlinux.lds stuff could take a look at it, though I don't see any
obvious problem yet.

If you don't receive further comments, you could try reposting once
to alert people to the fact that you're still waiting.

Cheers
---Dave