ipc: compute kern_ipc_perm.id under the ipc lock

ipc_addid() initializes kern_ipc_perm.id after having called
ipc_idr_alloc().

Thus a parallel semctl() or msgctl() that uses e.g.  MSG_STAT may use this
unitialized value as the return code.

The patch moves all accesses to kern_ipc_perm.id under the spin_lock().

The issues is related to the finding of
syzbot+2827ef6b3385deb07eaf@syzkaller.appspotmail.com: syzbot found an
issue with kern_ipc_perm.seq

Link: http://lkml.kernel.org/r/20180712185241.4017-2-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/msg.c

@@ -492,7 +492,6 @@ static int msgctl_stat(struct ipc_namespace *ns, int msqid,
 			 int cmd, struct msqid64_ds *p)
 {
 	struct msg_queue *msq;
-	int id = 0;
 	int err;
 
 	memset(p, 0, sizeof(*p));
@@ -504,7 +503,6 @@ static int msgctl_stat(struct ipc_namespace *ns, int msqid,
 			err = PTR_ERR(msq);
 			goto out_unlock;
 		}
-		id = msq->q_perm.id;
 	} else { /* IPC_STAT */
 		msq = msq_obtain_object_check(ns, msqid);
 		if (IS_ERR(msq)) {
@@ -549,10 +547,21 @@ static int msgctl_stat(struct ipc_namespace *ns, int msqid,
 	p->msg_lspid  = pid_vnr(msq->q_lspid);
 	p->msg_lrpid  = pid_vnr(msq->q_lrpid);
 
-	ipc_unlock_object(&msq->q_perm);
-	rcu_read_unlock();
-	return id;
+	if (cmd == IPC_STAT) {
+		/*
+		 * As defined in SUS:
+		 * Return 0 on success
+		 */
+		err = 0;
+	} else {
+		/*
+		 * MSG_STAT and MSG_STAT_ANY (both Linux specific)
+		 * Return the full id, including the sequence number
+		 */
+		err = msq->q_perm.id;
+	}
 
+	ipc_unlock_object(&msq->q_perm);
 out_unlock:
 	rcu_read_unlock();
 	return err;

ipc/sem.c

@@ -1223,7 +1223,6 @@ static int semctl_stat(struct ipc_namespace *ns, int semid,
 {
 	struct sem_array *sma;
 	time64_t semotime;
-	int id = 0;
 	int err;
 
 	memset(semid64, 0, sizeof(*semid64));
@@ -1235,7 +1234,6 @@ static int semctl_stat(struct ipc_namespace *ns, int semid,
 			err = PTR_ERR(sma);
 			goto out_unlock;
 		}
-		id = sma->sem_perm.id;
 	} else { /* IPC_STAT */
 		sma = sem_obtain_object_check(ns, semid);
 		if (IS_ERR(sma)) {
@@ -1275,10 +1273,20 @@ static int semctl_stat(struct ipc_namespace *ns, int semid,
 #endif
 	semid64->sem_nsems = sma->sem_nsems;
 
+	if (cmd == IPC_STAT) {
+		/*
+		 * As defined in SUS:
+		 * Return 0 on success
+		 */
+		err = 0;
+	} else {
+		/*
+		 * SEM_STAT and SEM_STAT_ANY (both Linux specific)
+		 * Return the full id, including the sequence number
+		 */
+		err = sma->sem_perm.id;
+	}
 	ipc_unlock_object(&sma->sem_perm);
-	rcu_read_unlock();
-	return id;
-
 out_unlock:
 	rcu_read_unlock();
 	return err;

ipc/shm.c

@@ -962,7 +962,6 @@ static int shmctl_stat(struct ipc_namespace *ns, int shmid,
 			int cmd, struct shmid64_ds *tbuf)
 {
 	struct shmid_kernel *shp;
-	int id = 0;
 	int err;
 
 	memset(tbuf, 0, sizeof(*tbuf));
@@ -974,7 +973,6 @@ static int shmctl_stat(struct ipc_namespace *ns, int shmid,
 			err = PTR_ERR(shp);
 			goto out_unlock;
 		}
-		id = shp->shm_perm.id;
 	} else { /* IPC_STAT */
 		shp = shm_obtain_object_check(ns, shmid);
 		if (IS_ERR(shp)) {
@@ -1024,10 +1022,21 @@ static int shmctl_stat(struct ipc_namespace *ns, int shmid,
 	tbuf->shm_lpid	= pid_vnr(shp->shm_lprid);
 	tbuf->shm_nattch = shp->shm_nattch;
 
-	ipc_unlock_object(&shp->shm_perm);
-	rcu_read_unlock();
-	return id;
+	if (cmd == IPC_STAT) {
+		/*
+		 * As defined in SUS:
+		 * Return 0 on success
+		 */
+		err = 0;
+	} else {
+		/*
+		 * SHM_STAT and SHM_STAT_ANY (both Linux specific)
+		 * Return the full id, including the sequence number
+		 */
+		err = shp->shm_perm.id;
+	}
 
+	ipc_unlock_object(&shp->shm_perm);
 out_unlock:
 	rcu_read_unlock();
 	return err;