Re: [PATCH] x86/suspend: fix false positive KASAN warning on suspend/resume

[Josh Poimboeuf]

On Thu, Dec 01, 2016 at 08:58:21AM -0600, Josh Poimboeuf wrote:
> On Thu, Dec 01, 2016 at 12:05:34PM +0300, Andrey Ryabinin wrote:
> > 
> > 
> > On 12/01/2016 02:10 AM, Josh Poimboeuf wrote:
> > > Resuming from a suspend operation is showing a KASAN false positive
> > > warning:
> > > 
> > 
> > > KASAN instrumentation poisons the stack when entering a function and
> > > unpoisons it when exiting the function.  However, in the suspend path,
> > > some functions never return, so their stack never gets unpoisoned,
> > > resulting in stale KASAN shadow data which can cause false positive
> > > warnings like the one above.
> > > 
> > > Reported-by: Scott Bauer <scott.bauer@xxxxxxxxx>
> > > Tested-by: Scott Bauer <scott.bauer@xxxxxxxxx>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kernel/acpi/sleep.c | 3 +++
> > >  include/linux/kasan.h        | 7 +++++++
> > >  2 files changed, 10 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/acpi/sleep.c b/arch/x86/kernel/acpi/sleep.c
> > > index 4858733..62bd046 100644
> > > --- a/arch/x86/kernel/acpi/sleep.c
> > > +++ b/arch/x86/kernel/acpi/sleep.c
> > > @@ -115,6 +115,9 @@ int x86_acpi_suspend_lowlevel(void)
> > >  	pause_graph_tracing();
> > >  	do_suspend_lowlevel();
> > >  	unpause_graph_tracing();
> > > +
> > > +	kasan_unpoison_stack_below_sp();
> > > +
> > 
> > I think this might be too late. We may hit stale poison in the first C function called
> > after resume (restore_processor_state()). Thus the shadow must be unpoisoned prior such call,
> > i.e. somewhere in do_suspend_lowlevel() after .Lresume_point.
> 
> Yeah, I think you're right.  Will spin a v2.

So I tried calling kasan_unpoison_task_stack_below() from
do_suspend_lowlevel(), but it hung on the resume.  Presumably because
restore_processor_state() does some important setup which would be
needed before calling into kasan_unpoison_task_stack_below().  For
example, setting up the gs register.  So it's a bit of a catch-22.

It could probably be fixed properly by rewriting do_suspend_lowlevel()
to call restore_processor_state() with the temporary stack before
switching to the original stack and doing the unpoison.

(And there are some other issues with do_suspend_lowlevel() and I'd love
to try taking a scalpel to it.  But I have too many knives in the air
already to want to try to attempt that right now...)

Unless somebody else wants to take a stab at it, my original patch is
probably good enough for now, since restore_processor_state() doesn't
seem to be triggering any KASAN warnings.

-- 
Josh