Re: [PATCH v4 1/6] timer: kasan: record timer stack

[Walter Wu]

On Thu, 2020-09-24 at 23:41 +0200, Thomas Gleixner wrote:
> On Thu, Sep 24 2020 at 12:03, Walter Wu wrote:
> > When analyze use-after-free or double-free issue, recording the timer
> > stacks is helpful to preserve usage history which potentially gives
> > a hint about the affected code.
> >
> > Record the most recent two timer init calls in KASAN which are printed
> > on failure in the KASAN report.
> >
> > For timers it has turned out to be useful to record the stack trace
> > of the timer init call.
> 
> In which way? And what kind of bug does it catch which cannot be catched
> by existing debug mechanisms already?
> 

We only provide another debug mechanisms to debug use-after-free or
double-free, it can be displayed together in KASAN report and have a
chance to debug, and it doesn't need to enable existing debug mechanisms
at the same time. then it has a chance to resolve issue.

> > Because if the UAF root cause is in timer init, then user can see
> > KASAN report to get where it is registered and find out the root
> > cause.
> 
> What? If the UAF root cause is in timer init, then registering it after
> using it in that very same function is pretty pointless.
> 

See [1], the call stack shows UAF happen at dummy_timer(), it is the
callback function and set by timer_setup(), if KASAN report shows the
timer call stack, it should be useful for programmer.

[1]
https://syzkaller.appspot.com/bug?id=34e69b7c8c0165658cbc987da0b61dadec644b6b


> > It don't need to enable DEBUG_OBJECTS_TIMERS, but they have a chance
> > to find out the root cause.
> 
> There is a lot of handwaving how useful this is, but TBH I don't see the
> value at all.
> 
> DEBUG_OBJECTS_TIMERS does a lot more than crashing on UAF. If KASAN
> provides additional value over DEBUG_OBJECTS_TIMERS then spell it out,
> but just saying that you don't need to enable DEBUG_OBJECTS_TIMERS is
> not making an argument for that change.
> 

We don't want to replace DEBUG_OBJECTS_TIMERS with this patches, only
hope to use low overhead(compare with DEBUG_OBJECTS_TIMERS) to debug
use-after-free/double-free issue. If you have some concerns, we can add
those message into commit log.

Thanks.

Walter