Re: [PATCH 3/5] padata: grab parallel_data refcnt for reorder

[Daniel Jordan]

On Wed, Nov 09, 2022 at 02:03:15PM +0100, Nicolai Stange wrote:
> Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:
> 
> > On Wed, Oct 19, 2022 at 10:37:06AM +0200, Nicolai Stange wrote:
> >> On entry of padata_do_serial(), the in-flight padata_priv owns a reference
> >> to the associated parallel_data instance.
> >> 
> >> However, as soon as the padata_priv got enqueued on the reorder list, it
> >> can be completed from a different context, causing the reference to get
> >> released in the course.
> >> 
> >> This would potentially cause UAFs from the subsequent padata_reorder()
> >> operations invoked from the enqueueing padata_do_serial() or from the
> >> reorder work.
> >> 
> 
> <snip>
> 
> >> ---
> >>  kernel/padata.c | 26 +++++++++++++++++++++-----
> >>  1 file changed, 21 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/kernel/padata.c b/kernel/padata.c
> >> index 0bf8c80dad5a..b79226727ef7 100644
> >> --- a/kernel/padata.c
> >> +++ b/kernel/padata.c
> >> @@ -275,7 +275,7 @@ static struct padata_priv *padata_find_next(struct parallel_data *pd,
> >>  	return padata;
> >>  }
> >>  
> >> -static void padata_reorder(struct parallel_data *pd)
> >> +static bool padata_reorder(struct parallel_data *pd)
> >>  {
> >>  	struct padata_instance *pinst = pd->ps->pinst;
> >>  	int cb_cpu;
> >> @@ -294,7 +294,7 @@ static void padata_reorder(struct parallel_data *pd)
> >>  	 * care for all the objects enqueued during the holdtime of the lock.
> >>  	 */
> >>  	if (!spin_trylock_bh(&pd->lock))
> >> -		return;
> >> +		return false;
> >>  
> >>  	while (1) {
> >>  		padata = padata_find_next(pd, true);
> >> @@ -331,17 +331,23 @@ static void padata_reorder(struct parallel_data *pd)
> >>  
> >>  	reorder = per_cpu_ptr(pd->reorder_list, pd->cpu);
> >>  	if (!list_empty(&reorder->list) && padata_find_next(pd, false))
> >> -		queue_work(pinst->serial_wq, &pd->reorder_work);
> >> +		return queue_work(pinst->serial_wq, &pd->reorder_work);
> >> +
> >> +	return false;
> >>  }
> >>  
> >>  static void invoke_padata_reorder(struct work_struct *work)
> >>  {
> >>  	struct parallel_data *pd;
> >> +	bool keep_refcnt;
> >>  
> >>  	local_bh_disable();
> >>  	pd = container_of(work, struct parallel_data, reorder_work);
> >> -	padata_reorder(pd);
> >> +	keep_refcnt = padata_reorder(pd);
> >>  	local_bh_enable();
> >> +
> >> +	if (!keep_refcnt)
> >> +		padata_put_pd(pd);
> >>  }
> >>  
> >>  static void padata_serial_worker(struct work_struct *serial_work)
> >> @@ -392,6 +398,15 @@ void padata_do_serial(struct padata_priv *padata)
> >>  	struct padata_list *reorder = per_cpu_ptr(pd->reorder_list, hashed_cpu);
> >>  	struct padata_priv *cur;
> >>  
> >> +	/*
> >> +	 * The in-flight padata owns a reference on pd. However, as
> >> +	 * soon as it's been enqueued on the reorder list, another
> >> +	 * task can dequeue and complete it, thereby dropping the
> >> +	 * reference. Grab another reference here, it will eventually
> >> +	 * be released from a reorder work, if any, or below.
> >> +	 */
> >> +	padata_get_pd(pd);
> >> +
> >>  	spin_lock(&reorder->lock);
> >>  	/* Sort in ascending order of sequence number. */
> >>  	list_for_each_entry_reverse(cur, &reorder->list, list)
> >> @@ -407,7 +422,8 @@ void padata_do_serial(struct padata_priv *padata)
> >>  	 */
> >>  	smp_mb();
> >>  
> >> -	padata_reorder(pd);
> >> +	if (!padata_reorder(pd))
> >> +		padata_put_pd(pd);
> >
> > do_serial is supposed to be called with BHs disabled and will be in
> > every case after a fix for a separate issue that I plan to send this
> > cycle.  Given that it (will soon...) always happen under RCU protection,
> > part of this issue could be addressed like this, which puts the expense
> > of dealing with this rare problem in the slow path:
> >
> > diff --git a/kernel/padata.c b/kernel/padata.c
> > index 0bf8c80dad5a..cd6740ae6629 100644
> > --- a/kernel/padata.c
> > +++ b/kernel/padata.c
> > @@ -1110,6 +1110,12 @@ void padata_free_shell(struct padata_shell *ps)
> >  	if (!ps)
> >  		return;
> >  
> > +	/*
> > +	 * Wait for all _do_serial calls to finish to avoid touching freed pd's
> > +	 * and ps's.
> > +	 */
> > +	synchronize_rcu();
> > +
> >  	mutex_lock(&ps->pinst->lock);
> >  	list_del(&ps->list);
> >  	padata_put_pd(rcu_dereference_protected(ps->pd, 1));
> >
> > pcrypt calls padata_free_shell() when all outstanding transforms (and
> > thus requests, I think) have been freed/completed, so no new task can
> > come into padata_reorder.  synchronize_rcu() then flushes out any
> > remaining _reorder calls.
> >
> > This doesn't deal with pending reorder_work items, but we can fix it
> > later in the series.
> >
> > What do you think?
> 
> Yes, I think that would work. Will you handle it alongside that fix for
> a separate issue you mentioned above? Or shall I once this fix has
> landed?

Please go ahead and do it yourself.  I'll send mine soon, I think it
should be ok for them to go in in the same cycle.