[PATCH] locking/rwsem: Don't call owner_on_cpu() on read-owner

[Waiman Long <longman@redhat.com>]

For writer, the owner value is cleared on unlock. For reader, it is
left intact on unlock for providing better debugging aid on crash dump
and the unlock of one reader may not mean the lock is free.

As a result, the owner_on_cpu() shouldn't be used on read-owner as
the task pointer value may not be valid and the task structure might
have been freed. That is the case in rwsem_can_spin_on_owner() where
the following use-after-free error from KASAN may appear under certain
circumstances:

  BUG: KASAN: use-after-free in rwsem_down_write_slowpath
  (/home/miguel/kernel/linux/kernel/locking/rwsem.c:669
  /home/miguel/kernel/linux/kernel/locking/rwsem.c:1125)

Fix this by checking for RWSEM_READER_OWNED flag before calling
owner_on_cpu().

Fixes: 94a9717b3c40e ("locking/rwsem: Make rwsem->owner an atomic_long_t")
Signed-off-by: Waiman Long <longman@redhat.com>
Tested-by: Luis Henriques <lhenriques@suse.com>
---
 kernel/locking/rwsem.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 37524a47f002..bc91aacaab58 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -666,7 +666,11 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
 	preempt_disable();
 	rcu_read_lock();
 	owner = rwsem_owner_flags(sem, &flags);
-	if ((flags & nonspinnable) || (owner && !owner_on_cpu(owner)))
+	/*
+	 * Don't check the read-owner as the entry may be stale.
+	 */
+	if ((flags & nonspinnable) ||
+	    (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
 		ret = false;
 	rcu_read_unlock();
 	preempt_enable();
-- 
2.18.1