Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

Mathias Krause
2015-11-25T09:14:02Z

On 24 November 2015 at 22:38, Kees Cook <keescook@chromium.org> wrote:
> Many things are written to only during __init, and never changed
> again. These cannot be made "const" since the compiler will do the wrong
> thing (we do actually need to write to them). Instead, move these items
> into a memory region that will be made read-only during mark_rodata_ro()
> which happens after all kernel __init code has finished.
>
> This introduces __read_only as a way to mark such memory, and uses it on
> the x86 vDSO to kill an extant kernel exploitation method.

...just some random notes on the experience with kernels implementing
such a feature for quite a lot of locations, not just the vDSO.

While having that annotation makes perfect sense, not only from a
security perspective but also from a micro-optimization point of view
(much like the already existing __read_mostly annotation), it has its
drawbacks. Violating the "r/o after init" rule by writing to such
annotated variables from non-init code goes unnoticed as far as it
concerns the toolchain. Neither the compiler nor the linker will flag
that incorrect use. It'll just trap at runtime and that's bad.

I myself had some educating experience seeing my machine triple fault
when resuming from a S3 sleep. The root cause was a variable that was
annotated __read_only but that was (unnecessarily) modified during CPU
bring-up phase. Debugging that kind of problems is sort of a PITA, you
could imagine.

So, prior extending the usage of the __read_only annotation some
toolchain support is needed. Maybe a gcc plugin that'll warn/error on
code that writes to such a variable but is not __init itself. The
initify and checker plugins from the PaX patch might be worth to look
at for that purpose, as they're doing similar things already. Adding
such a check to sparse might be worth it, too.
A modpost check probably won't work as it's unable to tell if it's a
legitimate access (r/o) or a violation (/w access). So the gcc plugin
is the way to go, IMHO.

Regards,
Mathias