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

[Josh Poimboeuf]

On Thu, Dec 01, 2016 at 05:51:52PM +0100, Dmitry Vyukov wrote:
> On Thu, Dec 1, 2016 at 5:45 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > 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.
> 
> restore_processor_state/__restore_processor_state does not seem to
> have any local variables, so KASAN does not do any stack checks there.

Actually, looking at the object code, it uses a lot of stack space and
has several calls to __asan_report_load*() functions.  Probably due to
inlining of other functions which have stack variables.

> We could disable KASAN instrumentation of the file, or of particular
> functions.

I don't think that would be sufficient unless it were disabled for
__restore_processor_state() and all the functions it calls (and the
functions they call, etc), which wouldn't necessarily be
straightforward.

> Or we could call kasan_unpoison_shadow() on the stack range
> before switching to it.

I tried that already, but it hung because restore_processor_state()
hadn't been called yet (the catch-22 I mentioned aboved).

-- 
Josh