Re: [PATCH v3 3/3] module: Make __tracepoints_ptrs as read-only

Joel Fernandes
2019-04-10T20:29:07Z

On Wed, Apr 10, 2019 at 04:11:12PM -0400, Steven Rostedt wrote:
> On Wed, 10 Apr 2019 15:57:08 -0400
> "Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:
>
> > This series hardens the tracepoints in modules by making the array of
> > pointers referring to the tracepoints as read-only. This array is needed
> > during module unloading to verify that the tracepoint is quiescent.
> > There is no reason for the array to be to be writable after init, and
> > can cause security or other hidden bugs. Mark these as ro_after_init.
> >
> > Suggested-by: paulmck@linux.vnet.ibm.com
> > Suggested-by: keescook@chromium.org
> > Suggested-by: mathieu.desnoyers@efficios.com
> > Cc: rostedt@goodmis.org
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> > kernel/module.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/kernel/module.c b/kernel/module.c
> > index 8b9631e789f0..be980aaa8804 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -3320,6 +3320,12 @@ static const char * const ro_after_init_sections[] = {
> > * by the SRCU notifiers
> > */
> > "___srcu_struct_ptrs",
> > +
> > + /*
> > + * Array of tracepoint pointers used for checking if tracepoints are
> > + * quiescent during unloading.
> > + */
> > + "__tracepoints_ptrs",
>
> Do we ever modify the __tracepoint_ptrs section? I know the jump_label
> sections are sorted on load, which means they need to be writable
> during init, but if __tracepoint_ptrs is not sorted or touched during
> load, why not just put them in the rodata section to begin with?
>
> -- Steve

The srcu structure pointer array is modified at module load time because the
array is fixed up by the module loader at load-time with the final locations
of the tracepoints right? Basically relocation fixups. At compile time, I
believe it is not know what the values in the ptr array are. I believe same
is true for the tracepoint ptrs array.

Also it needs to be in a separate __tracepoint_ptrs so that this code works:

#ifdef CONFIG_TRACEPOINTS
mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
sizeof(*mod->tracepoints_ptrs),
&mod->num_tracepoints);
#endif

Did I miss some point? Thanks,

- Joel