smack: do not dereference NULL ipc object

In the SYSV ipc msgctl(),semctl(),shmctl() family, if the user passed *_INFO
as the desired operation, no specific object is meant to be controlled and
only system-wide information is returned.  This leads to a NULL IPC object in
the LSM hooks if the _INFO flag is given.

Avoid dereferencing this NULL pointer in Smack ipc *ctl() methods.
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

security/smack/smack_lsm.c

@@ -1508,7 +1508,7 @@ static int smack_shm_associate(struct shmid_kernel *shp, int shmflg)
  */
 static int smack_shm_shmctl(struct shmid_kernel *shp, int cmd)
 {
-	char *ssp = smack_of_shm(shp);
+	char *ssp;
 	int may;
 
 	switch (cmd) {
@@ -1532,6 +1532,7 @@ static int smack_shm_shmctl(struct shmid_kernel *shp, int cmd)
 		return -EINVAL;
 	}
 
+	ssp = smack_of_shm(shp);
 	return smk_curacc(ssp, may);
 }
 
@@ -1616,7 +1617,7 @@ static int smack_sem_associate(struct sem_array *sma, int semflg)
  */
 static int smack_sem_semctl(struct sem_array *sma, int cmd)
 {
-	char *ssp = smack_of_sem(sma);
+	char *ssp;
 	int may;
 
 	switch (cmd) {
@@ -1645,6 +1646,7 @@ static int smack_sem_semctl(struct sem_array *sma, int cmd)
 		return -EINVAL;
 	}
 
+	ssp = smack_of_sem(sma);
 	return smk_curacc(ssp, may);
 }
 
@@ -1730,7 +1732,7 @@ static int smack_msg_queue_associate(struct msg_queue *msq, int msqflg)
  */
 static int smack_msg_queue_msgctl(struct msg_queue *msq, int cmd)
 {
-	char *msp = smack_of_msq(msq);
+	char *msp;
 	int may;
 
 	switch (cmd) {
@@ -1752,6 +1754,7 @@ static int smack_msg_queue_msgctl(struct msg_queue *msq, int cmd)
 		return -EINVAL;
 	}
 
+	msp = smack_of_msq(msq);
 	return smk_curacc(msp, may);
 }