ipc/shm.c: avoid ipc_rcu_putref for failed ipc_addid()

Loosely based on a patch from Kees Cook <keescook@chromium.org>:
 - id and error can be merged
 - if operations before ipc_addid() fail, then use call_rcu() directly.

The difference is that call_rcu is used for failures after
security_shm_alloc(), to continue to guaranteed an rcu delay for
security_sem_free().

Link: http://lkml.kernel.org/r/20170525185107.12869-15-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Kees Cook <keescook@chromium.org>
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

@@ -548,7 +548,6 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	size_t numpages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	struct file *file;
 	char name[13];
-	int id;
 	vm_flags_t acctflag = 0;
 
 	if (size < SHMMIN || size > ns->shm_ctlmax)
@@ -617,11 +616,9 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	shp->shm_file = file;
 	shp->shm_creator = current;
 
-	id = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
-	if (id < 0) {
-		error = id;
+	error = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
+	if (error < 0)
 		goto no_id;
-	}
 
 	list_add(&shp->shm_clist, &current->sysvshm.shm_clist);
 
@@ -643,7 +640,7 @@ no_id:
 		user_shm_unlock(size, shp->mlock_user);
 	fput(file);
 no_file:
-	ipc_rcu_putref(&shp->shm_perm, shm_rcu_free);
+	call_rcu(&shp->shm_perm.rcu, shm_rcu_free);
 	return error;
 }