[PATCH -next v2] locking/percpu-rwsem: fix a task_struct refcount

[Qian Cai]

The commit 7f26482a872c ("locking/percpu-rwsem: Remove the embedded
rwsem") introduced some task_struct memory leaks due to messing up with
a task_struct refcount. At the beginning of
percpu_rwsem_wake_function(), it calls get_task_struct(), but if the
trylock failed, it will remain in the waitqueue. However, it will run
percpu_rwsem_wake_function() again with get_task_struct() to increase
the refcount but then only call put_task_struct() once the trylock
succeeded.

Fix it by adjusting percpu_rwsem_wake_function() a bit to guard against
when percpu_rwsem_wait() observing !private, terminating the wait and
doing a quick exit() while percpu_rwsem_wake_function() then doing
wake_up_process(p) as a use-after-free.

Fixes: 7f26482a872c ("locking/percpu-rwsem: Remove the embedded rwsem")
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Qian Cai <cai@xxxxxx>
---
 kernel/locking/percpu-rwsem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index a008a1ba21a7..8bbafe3e5203 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -118,14 +118,15 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
 				      unsigned int mode, int wake_flags,
 				      void *key)
 {
-	struct task_struct *p = get_task_struct(wq_entry->private);
 	bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
 	struct percpu_rw_semaphore *sem = key;
+	struct task_struct *p;
 
 	/* concurrent against percpu_down_write(), can get stolen */
 	if (!__percpu_rwsem_trylock(sem, reader))
 		return 1;
 
+	p = get_task_struct(wq_entry->private);
 	list_del_init(&wq_entry->entry);
 	smp_store_release(&wq_entry->private, NULL);
 
-- 
2.21.0 (Apple Git-122.2)