Re: [PATCH] module: Make srcu_struct ptr array as read-only

[Paul E. McKenney]

On Thu, Apr 11, 2019 at 10:14:22PM -0400, Joel Fernandes wrote:
> On Thu, Apr 11, 2019 at 02:31:55PM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 11, 2019 at 04:24:21PM -0400, Joel Fernandes (Google) wrote:
> > > Since commit title ("srcu: Allocate per-CPU data for DEFINE_SRCU() in
> > > modules"), modules that call DEFINE_{STATIC,}SRCU will have a new array
> > > of srcu_struct pointers, which is used by srcu code to initialize and
> > > clean up these structures and save valuable per-cpu reserved space.
> > > 
> > > There is no reason for this array of pointers to be writable, and can
> > > cause security or other hidden bugs. Mark these are read-only after the
> > > module init has completed.
> > > 
> > > Tested with the following diff to ensure array not writable:
> > > 
> > > (diff is a bit reduced to avoid patch command getting confused)
> > >  a/kernel/module.c
> > >  b/kernel/module.c
> > >   -3506,6 +3506,14  static noinline int do_init_module [snip]
> > >  	rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
> > >  #endif
> > >  	module_enable_ro(mod, true);
> > > +
> > > +	if (mod->srcu_struct_ptrs) {
> > > +		// Check if srcu_struct_ptrs access is possible
> > > +		char x = *(char *)mod->srcu_struct_ptrs;
> > > +		*(char *)mod->srcu_struct_ptrs = 0;
> > > +		*(char *)mod->srcu_struct_ptrs = x;
> > > +	}
> > > +
> > >  	mod_tree_remove_init(mod);
> > >  	disable_ro_nx(&mod->init_layout);
> > >  	module_arch_freeing_init(mod);
> > > 
> > > Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> > > Cc: paulmck@xxxxxxxxxxxxxxxxxx
> > > Cc: rostedt@xxxxxxxxxxx
> > > Cc: mathieu.desnoyers@xxxxxxxxxxxx
> > > Cc: rcu@xxxxxxxxxxxxxxx
> > > Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
> > > Cc: kernel-team@xxxxxxxxxxx
> > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > 
> > Queued for testing and further review, thank you, Joel!
> 
> Thanks a lot! I also just saw you added the rcutorture module to be built as
> a part kselftests which is really cool ;-)

Just a smoke test, really, but it will be interesting to see how
it goes.  ;-)

							Thanx, Paul