ipc/shm: do not use ipc_rcu_free()

Avoid using ipc_rcu_free, since it just re-finds the original structure
pointer.  For the pre-list-init failure path, there is no RCU needed,
since it was just allocated.  It can be directly freed.

Link: http://lkml.kernel.org/r/20170525185107.12869-7-manfred@colorfullife.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -172,6 +172,11 @@ static inline void shm_lock_by_ptr(struct shmid_kernel *ipcp)
 	ipc_lock_object(&ipcp->shm_perm);
 }
 
+static void __shm_free(struct shmid_kernel *shp)
+{
+	kvfree(shp);
+}
+
 static void shm_rcu_free(struct rcu_head *head)
 {
 	struct kern_ipc_perm *ptr = container_of(head, struct kern_ipc_perm,
@@ -179,7 +184,7 @@ static void shm_rcu_free(struct rcu_head *head)
 	struct shmid_kernel *shp = container_of(ptr, struct shmid_kernel,
 							shm_perm);
 	security_shm_free(shp);
-	ipc_rcu_free(head);
+	__shm_free(shp);
 }
 
 static inline void shm_rmid(struct ipc_namespace *ns, struct shmid_kernel *s)
@@ -557,7 +562,7 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	shp->shm_perm.security = NULL;
 	error = security_shm_alloc(shp);
 	if (error) {
-		ipc_rcu_putref(&shp->shm_perm, ipc_rcu_free);
+		__shm_free(shp);
 		return error;
 	}