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

Dmitry Vyukov
2016-12-01T17:48:58Z

On Thu, Dec 1, 2016 at 6:34 PM, Josh Poimboeuf <jpoimboe@redhat.com> 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@intel.com>
>> >> >> > > Tested-by: Scott Bauer <scott.bauer@intel.com>
>> >> >> > > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
>> >> >> > > ---
>> >> >> > > 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.

>> >> 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).
>>
>> Ah, I see, we just can't execute normal C code at that point...
>
> Right.
>
> --
> Josh
>
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