ipc/sem: introduce semctl(SEM_STAT_ANY)

There is a permission discrepancy when consulting shm ipc object
metadata between /proc/sysvipc/sem (0444) and the SEM_STAT semctl
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 sma 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 SEM_STAT_ANY command such that the sem 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/20180215162458.10059-3-dave@stgolabs.netSigned-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Robert Kettler <robert.kettler@outlook.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/uapi/linux/sem.h

@@ -19,6 +19,7 @@
 /* ipcs ctl cmds */
 #define SEM_STAT 18
 #define SEM_INFO 19
+#define SEM_STAT_ANY 20
 
 /* Obsolete, used only for backwards compatibility and libc5 compiles */
 struct semid_ds {

ipc/sem.c

@@ -1220,14 +1220,14 @@ static int semctl_stat(struct ipc_namespace *ns, int semid,
 	memset(semid64, 0, sizeof(*semid64));
 
 	rcu_read_lock();
-	if (cmd == SEM_STAT) {
+	if (cmd == SEM_STAT || cmd == SEM_STAT_ANY) {
 		sma = sem_obtain_object(ns, semid);
 		if (IS_ERR(sma)) {
 			err = PTR_ERR(sma);
 			goto out_unlock;
 		}
 		id = sma->sem_perm.id;
-	} else {
+	} else { /* IPC_STAT */
 		sma = sem_obtain_object_check(ns, semid);
 		if (IS_ERR(sma)) {
 			err = PTR_ERR(sma);
@@ -1235,9 +1235,14 @@ static int semctl_stat(struct ipc_namespace *ns, int semid,
 		}
 	}
 
-	err = -EACCES;
-	if (ipcperms(ns, &sma->sem_perm, S_IRUGO))
-		goto out_unlock;
+	/* see comment for SHM_STAT_ANY */
+	if (cmd == SEM_STAT_ANY)
+		audit_ipc_obj(&sma->sem_perm);
+	else {
+		err = -EACCES;
+		if (ipcperms(ns, &sma->sem_perm, S_IRUGO))
+			goto out_unlock;
+	}
 
 	err = security_sem_semctl(&sma->sem_perm, cmd);
 	if (err)
@@ -1626,6 +1631,7 @@ long ksys_semctl(int semid, int semnum, int cmd, unsigned long arg)
 		return semctl_info(ns, semid, cmd, p);
 	case IPC_STAT:
 	case SEM_STAT:
+	case SEM_STAT_ANY:
 		err = semctl_stat(ns, semid, cmd, &semid64);
 		if (err < 0)
 			return err;
@@ -1732,6 +1738,7 @@ long compat_ksys_semctl(int semid, int semnum, int cmd, int arg)
 		return semctl_info(ns, semid, cmd, p);
 	case IPC_STAT:
 	case SEM_STAT:
+	case SEM_STAT_ANY:
 		err = semctl_stat(ns, semid, cmd, &semid64);
 		if (err < 0)
 			return err;

security/selinux/hooks.c

@@ -6273,6 +6273,7 @@ static int selinux_sem_semctl(struct kern_ipc_perm *sma, int cmd)
 		break;
 	case IPC_STAT:
 	case SEM_STAT:
+	case SEM_STAT_ANY:
 		perms = SEM__GETATTR | SEM__ASSOCIATE;
 		break;
 	default:

security/smack/smack_lsm.c

@@ -3140,6 +3140,7 @@ static int smack_sem_semctl(struct kern_ipc_perm *isp, int cmd)
 	case GETALL:
 	case IPC_STAT:
 	case SEM_STAT:
+	case SEM_STAT_ANY:
 		may = MAY_READ;
 		break;
 	case SETVAL: