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

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

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.

> 
> > 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.

> 
> > Are you aware of similar situations on other arches?
> 
> I think there were some problems a long time ago on x86 for rodata too.

It would be good to get this kexec case fixed -- I'll try to hack up
a separate patch.

Cheers
---Dave