ipc,shm: correct error return value in shmctl (SHM_UNLOCK)

Commit 2caacaa82a51 ("ipc,shm: shorten critical region for shmctl")
restructured the ipc shm to shorten critical region, but introduced a
path where the return value could be -EPERM, even if the operation
actually was performed.

Before the commit, the err return value was reset by the return value
from security_shm_shmctl() after the if (!ns_capable(...)) statement.

Now, we still exit the if statement with err set to -EPERM, and in the
case of SHM_UNLOCK, it is not reset at all, and used as the return value
from shmctl.

To fix this, we only set err when errors occur, leaving the fallthrough
case alone.

Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>	[3.12.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -977,12 +977,15 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
 		ipc_lock_object(&shp->shm_perm);
 		if (!ns_capable(ns->user_ns, CAP_IPC_LOCK)) {
 			kuid_t euid = current_euid();
-			err = -EPERM;
 			if (!uid_eq(euid, shp->shm_perm.uid) &&
-			    !uid_eq(euid, shp->shm_perm.cuid))
+			    !uid_eq(euid, shp->shm_perm.cuid)) {
+				err = -EPERM;
 				goto out_unlock0;
-			if (cmd == SHM_LOCK && !rlimit(RLIMIT_MEMLOCK))
+			}
+			if (cmd == SHM_LOCK && !rlimit(RLIMIT_MEMLOCK)) {
+				err = -EPERM;
 				goto out_unlock0;
+			}
 		}
 
 		shm_file = shp->shm_file;