[OpenWrt-Devel] [PATCH] runqueue: Fix the callbacks order in runqueue_task_kill()

[Alban]

On Mon, 1 Jul 2019 15:48:47 +0200
John Crispin <john at phrozen.org> wrote:

> On 21/06/2019 17:38, Alban Bedel wrote:
> > Since commit 11e8afea (runqueue should cal the complete handler from
> > more places) the call to the complete() callback has been moved to
> > runqueue_task_complete().  However in runqueue_task_kill()
> > runqueue_task_complete() is called before the kill() callback.
> > This will result in a use after free if the complete() callback free
> > the task struct.
> >
> > Furthermore runqueue_start_next() is already called at the end of
> > runqueue_task_complete(), so there is no need to call it again in
> > runqueue_task_kill().
> >
> > Signed-off-by: Alban Bedel <albeu at free.fr>
> > ---
> >   runqueue.c | 4 +---
> >   1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/runqueue.c b/runqueue.c
> > index a1d0133..4c621aa 100644
> > --- a/runqueue.c
> > +++ b/runqueue.c
> > @@ -196,11 +196,9 @@ void runqueue_task_kill(struct runqueue_task
> > *t) if (!t->queued)
> >   		return;
> >   
> > -	runqueue_task_complete(t);
> >   	if (running && t->type->kill)
> >   		t->type->kill(q, t);
> > -
> > -	runqueue_start_next(q);
> > +	runqueue_task_complete(t);
> >   }
> >   
> >   void runqueue_stop(struct runqueue *q)  
> 
> Hi,
> 
> runqueue_task_complete() will decrement running which, if called
> after the kill clause might not even trigger the kill() call. I am
> assuming you are running a custom runqueue_task_type ?

No, TBH I haven't tested this, but as there is no documentation I had
to go through the code and noticed this suspicious construct. I then
saw commit 6a7fb7d8d (runqueue: fix use-after-free bug) which confirmed
that complete() is expected to free the task struct, which with the
above code will clearly lead to an access after free.

But I don't really see what you mean, 'running' is a boolean so it
can't be decremented. Did you mean 'running_tasks'?

Alban
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