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

[Nicolai Stange]

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.

Note that this is a purely theroretical concern, the problem has never been
actually observed -- it would require multiple pcrypt request submissions
racing against each other, ultimately a pcrypt instance destruction
(DELALG) short after request completions as well as unfortunate timing.

However, for the sake of correctness, it is still worth fixing.

Make padata_do_serial() grab a reference count on the parallel_data for
the subsequent reorder operation(s). As long as the padata_priv has not
been enqueued, this is safe, because as mentioned above, in-flight
pdata_privs own a reference already.

Note that padata_reorder() might schedule another padata_reorder() work
and thus, care must be taken to not prematurely release that "reorder
refcount" from padata_do_serial() again in case that has happened.
Make padata_reorder() return a bool for indicating whether or not a
reorder work has been scheduled. Let padata_do_serial() drop its refcount
only if this is not the case. Accordingly, make the reorder work handler,
invoke_padata_reorder(), drop it then as appropriate.

A remark on the commit chosen for the Fixes tag reference below: before
commit bbefa1dd6a6d ("crypto: pcrypt - Avoid deadlock by using per-instance
padata queues"), the padata_parallel lifetime had been tied to the
padata_instance. The padata_free() resp. padata_stop() issued a
synchronize_rcu() before padata_free_pd() from the instance destruction
path, rendering UAFs from the padata_do_serial()=>padata_reorder()
invocations with BHs disabled impossible AFAICS. With that, the
padata_reorder() work remains to be considered. Before
commit b128a3040935 ("padata: allocate workqueue internally"), the
workqueue got destroyed (from pcrypt), hence drained, before the padata
instance destruction, but this change moved that to after the
padata_free_pd() invocation from __padata_free(). So, while the Fixes
reference from below is most likely technically correct, I would still like
to reiterate that this problem is probably hard to trigger in practice,
even more so before commit bbefa1dd6a6d ("crypto: pcrypt - Avoid deadlock
by using per-instance padata queues").

Fixes: b128a3040935 ("padata: allocate workqueue internally")
Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
---
 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);
 }
 EXPORT_SYMBOL(padata_do_serial);
 
-- 
2.37.3