Re: [PATCH v2] init: Fix false positives in W+X checking

[Jeffrey Hugo]

On 4/28/2018 12:14 AM, Ingo Molnar wrote:
> * Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx> wrote:
>
> > load_module() creates W+X mappings via __vmalloc_node_range() (from
> > layout_and_allocate()->move_module()->module_alloc()) by using
> > PAGE_KERNEL_EXEC.  These mappings are later cleaned up via
> > "call_rcu_sched(&freeinit->rcu, do_free_init)" from do_init_module().
> >
> > This is a problem because call_rcu_sched() queues work, which can be run
> > after debug_checkwx() is run, resulting in a race condition.  If hit, the
> > race results in a nasty splat about insecure W+X mappings, which results
> > in a poor user experience as these are not the mappings that
> > debug_checkwx() is intended to catch.
> >
> > This issue is observed on multiple arm64 platforms, and has been
> > artificially triggered on an x86 platform.
> >
> > Address the race by flushing the queued work before running the
> > arch-defined mark_rodata_ro() which then calls debug_checkwx().
> >
> > Reported-by: Timur Tabi <timur@xxxxxxxxxxxxxx>
> > Reported-by: Jan Glauber <jan.glauber@xxxxxxxxxxxxxxxxxx>
> > Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
> > Signed-off-by: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
> > ---
> >
> > v1:
> > -was "arm64: mm: Fix false positives in W+X checking" (see [1])
> > -moved to common code based on review and confirmation of issue on x86
> >
> > [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-April/573776.html
> >
> >   init/main.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/init/main.c b/init/main.c
> > index b795aa3..499d957 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -1034,6 +1034,13 @@ static int __init set_debug_rodata(char *str)
> >   static void mark_readonly(void)
> >   {
> >   	if (rodata_enabled) {
> > +		/*
> > +		 * load_module() results in W+X mappings, which are cleaned up
> > +		 * with call_rcu_sched().  Let's make sure that queued work is
> > +		 * flushed so that we don't hit false positives looking for
> > +		 * insecure pages which are W+X.
> > +		 */
> > +		rcu_barrier_sched();
>
> I'd suggest adding a matching comment to the module loading portion as well,
> to make sure this connection does not get lost. With that:
>
> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>

Sure.  Will add that in a v3.

--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.