[PATCH] ipc: fix to protect IPCS lookups using RCU instead of semaphore

[Jeongjun Park <aha310510@gmail.com>]

In shm_destroy_orphaned(), we are not performing updates to the IPCS and are
only calling idr_for_each(), which can be protected by the RCU read-critical
section.

And if idr_for_each() is not protected by the RCU read-critical section,
then when radix_tree_node_free() is called to free the struct radix_tree_node
through call_rcu(), the node will be freed immediately, and when reading the
next node in radix_tree_for_each_slot(), the memory that has already been
freed may be read. 

Therefore, I think it is appropriate to use RCU instead of semaphore to
protect it.

Fixes: d9a605e40b13 ("ipc: rename ids->rw_mutex")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
 ipc/shm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 99564c870084..8fd87c4e5dc8 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -430,10 +430,10 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data)
 
 void shm_destroy_orphaned(struct ipc_namespace *ns)
 {
-	down_write(&shm_ids(ns).rwsem);
+	rcu_read_lock();
 	if (shm_ids(ns).in_use)
 		idr_for_each(&shm_ids(ns).ipcs_idr, &shm_try_destroy_orphaned, ns);
-	up_write(&shm_ids(ns).rwsem);
+	rcu_read_unlock();
 }
 
 /* Locking assumes this will only be called with task == current */
--