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

[Thomas Gleixner]

Walter,

On Fri, Sep 25 2020 at 15:18, Walter Wu wrote:
> On Thu, 2020-09-24 at 23:41 +0200, Thomas Gleixner wrote:
>> > 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.

Again. KASAN can only cover UAF, but there are a dozen other ways to
wreck the system with wrong usage of timers which can't be caught by
KASAN.

>> > 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.

The report you linked to has absolutely nothing to do with a timer
related UAF. The timer callback calls kfree_skb() on something which is
already freed. So the root cause of this is NOT in timer init as you
claimed above. The timer callback is just exposing a problem in the URB
management of this driver. IOW the recording of the timer init stack is
completely useless for decoding this problem.

>> 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

KASAN has lower overhead than DEBUG_OBJECTS_TIMERS? Maybe in a different
universe.

That said, I'm not opposed to the change per se, but without a sensible
justification this is just pointless.

Sprinkling kasan_foo() all over the place and claiming it's useful
without a valid example does not provide any value.

Quite the contrary it gives the completely wrong sense what KASAN can do
and what not.

Thanks,

        tglx