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

[Josh Poimboeuf]

On Thu, Dec 01, 2016 at 06:47:07PM +0100, Dmitry Vyukov wrote:
> On Thu, Dec 1, 2016 at 6:34 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.
> >>
> >> That can be loads of heap variables (or other non-stack data). KASAN
> >> will emit these checks for lots of loads, but they don't necessary go
> >> to stack.
> >
> > I also see the stack poisoning instructions:
> >
> >  54f:   49 c1 ee 03             shr    $0x3,%r14
> >  553:   4c 01 f0                add    %r14,%rax
> >  556:   c7 00 f1 f1 f1 f1       movl   $0xf1f1f1f1,(%rax)
> >  55c:   c7 40 04 00 00 f4 f4    movl   $0xf4f40000,0x4(%rax)
> >  563:   c7 40 08 f3 f3 f3 f3    movl   $0xf3f3f3f3,0x8(%rax)
> 
> OK, then we are in trouble potentially.
> It may work as long as as the stack region that is used for local vars
> in restore_processor_state() does not contain any stale poisoning. But
> it can break at any moment.
> 
> Have you tried kasan_unpoison_task_stack_below() or kasan_unpoison_shadow()?
> I can see how kasan_unpoison_task_stack_below() can hang (it at least
> uses current). But kasan_unpoison_shadow() is quite trivial, it
> computes shadow address with simple math and writes zeroes there.

Good idea, I'll give kasan_unpoison_shadow() a shot.

-- 
Josh