ipc/msg: introduce msgctl(MSG_STAT_ANY)

There is a permission discrepancy when consulting msq ipc object
metadata between /proc/sysvipc/msg (0444) and the MSG_STAT shmctl
command. The later does permission checks for the object vs S_IRUGO.
As such there can be cases where EACCESS is returned via syscall but the
info is displayed anyways in the procfs files.

While this might have security implications via info leaking (albeit no
writing to the msq metadata), this behavior goes way back and showing
all the objects regardless of the permissions was most likely an
overlook - so we are stuck with it. Furthermore, modifying either the
syscall or the procfs file can cause userspace programs to break (ie
ipcs). Some applications require getting the procfs info (without root
privileges) and can be rather slow in comparison with a syscall -- up to
500x in some reported cases for shm.

This patch introduces a new MSG_STAT_ANY command such that the msq ipc
object permissions are ignored, and only audited instead. In addition,
I've left the lsm security hook checks in place, as if some policy can
block the call, then the user has no other choice than just parsing the
procfs file.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Davidlohr Bueso <[e-mail]>
Reported-by: Robert Kettler <[e-mail]>
Cc: Eric W. Biederman <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Manfred Spraul <[e-mail]>
Cc: Michael Kerrisk <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/uapi/linux/msg.h

···
 /* ipcs ctl commands */
 #define MSG_STAT 11
 #define MSG_INFO 12
+#define MSG_STAT_ANY 13
 
 /* msgrcv options */
 #define MSG_NOERROR     010000  /* no error if message is too big */

ipc/msg.c

···
 	memset(p, 0, sizeof(*p));
 
 	rcu_read_lock();
-	if (cmd == MSG_STAT) {
+	if (cmd == MSG_STAT || cmd == MSG_STAT_ANY) {
 		msq = msq_obtain_object(ns, msqid);
 		if (IS_ERR(msq)) {
 			err = PTR_ERR(msq);
 			goto out_unlock;
 		}
 		id = msq->q_perm.id;
-	} else {
+	} else { /* IPC_STAT */
 		msq = msq_obtain_object_check(ns, msqid);
 		if (IS_ERR(msq)) {
 			err = PTR_ERR(msq);
···
 		}
 	}
 
-	err = -EACCES;
-	if (ipcperms(ns, &msq->q_perm, S_IRUGO))
-		goto out_unlock;
+	/* see comment for SHM_STAT_ANY */
+	if (cmd == MSG_STAT_ANY)
+		audit_ipc_obj(&msq->q_perm);
+	else {
+		err = -EACCES;
+		if (ipcperms(ns, &msq->q_perm, S_IRUGO))
+			goto out_unlock;
+	}
 
 	err = security_msg_queue_msgctl(&msq->q_perm, cmd);
 	if (err)
···
 		return err;
 	}
 	case MSG_STAT:	/* msqid is an index rather than a msg queue id */
+	case MSG_STAT_ANY:
 	case IPC_STAT:
 		err = msgctl_stat(ns, msqid, cmd, &msqid64);
 		if (err < 0)
···
 	}
 	case IPC_STAT:
 	case MSG_STAT:
+	case MSG_STAT_ANY:
 		err = msgctl_stat(ns, msqid, cmd, &msqid64);
 		if (err < 0)
 			return err;

security/selinux/hooks.c

···
 				    SECCLASS_SYSTEM, SYSTEM__IPC_INFO, NULL);
 	case IPC_STAT:
 	case MSG_STAT:
+	case MSG_STAT_ANY:
 		perms = MSGQ__GETATTR | MSGQ__ASSOCIATE;
 		break;
 	case IPC_SET:

security/smack/smack_lsm.c

···
 	switch (cmd) {
 	case IPC_STAT:
 	case MSG_STAT:
+	case MSG_STAT_ANY:
 		may = MAY_READ;
 		break;
 	case IPC_SET: