[PATCH v3] ipc: introduce ipc_valid_object() helper to sort out IPC_RMID races

[Rafael Aquini <aquini@redhat.com>]

After the locking semantics for the SysV IPC API got improved, a couple of
IPC_RMID race windows were opened because we ended up dropping the
'kern_ipc_perm.deleted' check performed way down in ipc_lock().
The spotted races got sorted out by re-introducing the old test within
the racy critical sections.

This patch introduces ipc_valid_object() to consolidate the way we cope with
IPC_RMID races by using the same abstraction across the API implementation.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Greg Thelen <gthelen@google.com>
---
Changelog:
* v3:
 - code commentary changes as requested by reviewers

* v2:
 - drop assert_spin_locked() from ipc_valid_object() for less overhead
 - extend ipc_valid_object() usage in sem.c (not spotted checkpoints)
 - keep the received ACKs

 ipc/msg.c  |  7 ++++---
 ipc/sem.c  | 24 ++++++++++++++++--------
 ipc/shm.c  | 16 ++++++++--------
 ipc/util.h | 13 +++++++++++++
 4 files changed, 41 insertions(+), 19 deletions(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index 558aa91..8983ea5 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -696,7 +696,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 			goto out_unlock0;
 
 		/* raced with RMID? */
-		if (msq->q_perm.deleted) {
+		if (!ipc_valid_object(&msq->q_perm)) {
 			err = -EIDRM;
 			goto out_unlock0;
 		}
@@ -731,7 +731,8 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 		ipc_lock_object(&msq->q_perm);
 
 		ipc_rcu_putref(msq, ipc_rcu_free);
-		if (msq->q_perm.deleted) {
+		/* raced with RMID? */
+		if (!ipc_valid_object(&msq->q_perm)) {
 			err = -EIDRM;
 			goto out_unlock0;
 		}
@@ -909,7 +910,7 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 		ipc_lock_object(&msq->q_perm);
 
 		/* raced with RMID? */
-		if (msq->q_perm.deleted) {
+		if (!ipc_valid_object(&msq->q_perm)) {
 			msg = ERR_PTR(-EIDRM);
 			goto out_unlock0;
 		}
diff --git a/ipc/sem.c b/ipc/sem.c
index db9d241..5972e60 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1282,7 +1282,7 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
 
 	sem_lock(sma, NULL, -1);
 
-	if (sma->sem_perm.deleted) {
+	if (!ipc_valid_object(&sma->sem_perm)) {
 		sem_unlock(sma, -1);
 		rcu_read_unlock();
 		return -EIDRM;
@@ -1342,7 +1342,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		int i;
 
 		sem_lock(sma, NULL, -1);
-		if (sma->sem_perm.deleted) {
+		if (!ipc_valid_object(&sma->sem_perm)) {
 			err = -EIDRM;
 			goto out_unlock;
 		}
@@ -1361,7 +1361,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 
 			rcu_read_lock();
 			sem_lock_and_putref(sma);
-			if (sma->sem_perm.deleted) {
+			if (!ipc_valid_object(&sma->sem_perm)) {
 				err = -EIDRM;
 				goto out_unlock;
 			}
@@ -1409,7 +1409,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		}
 		rcu_read_lock();
 		sem_lock_and_putref(sma);
-		if (sma->sem_perm.deleted) {
+		if (!ipc_valid_object(&sma->sem_perm)) {
 			err = -EIDRM;
 			goto out_unlock;
 		}
@@ -1435,7 +1435,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		goto out_rcu_wakeup;
 
 	sem_lock(sma, NULL, -1);
-	if (sma->sem_perm.deleted) {
+	if (!ipc_valid_object(&sma->sem_perm)) {
 		err = -EIDRM;
 		goto out_unlock;
 	}
@@ -1699,7 +1699,7 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
 	/* step 3: Acquire the lock on semaphore array */
 	rcu_read_lock();
 	sem_lock_and_putref(sma);
-	if (sma->sem_perm.deleted) {
+	if (!ipc_valid_object(&sma->sem_perm)) {
 		sem_unlock(sma, -1);
 		rcu_read_unlock();
 		kfree(new);
@@ -1846,7 +1846,15 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 
 	error = -EIDRM;
 	locknum = sem_lock(sma, sops, nsops);
-	if (sma->sem_perm.deleted)
+	/*
+	 * We eventually might perform the following check in a lockless
+	 * fashion, considering ipc_valid_object() locking constraints.
+	 * If nsops == 1 and there is no contention for sem_perm.lock, then
+	 * only a per-semaphore lock is held and it's OK to proceed with the
+	 * check below. More details on the fine grained locking scheme
+	 * entangled here and why it's RMID race safe on comments at sem_lock()
+	 */
+	if (!ipc_valid_object(&sma->sem_perm))
 		goto out_unlock_free;
 	/*
 	 * semid identifiers are not unique - find_alloc_undo may have
@@ -2068,7 +2076,7 @@ void exit_sem(struct task_struct *tsk)
 
 		sem_lock(sma, NULL, -1);
 		/* exit_sem raced with IPC_RMID, nothing to do */
-		if (sma->sem_perm.deleted) {
+		if (!ipc_valid_object(&sma->sem_perm)) {
 			sem_unlock(sma, -1);
 			rcu_read_unlock();
 			continue;
diff --git a/ipc/shm.c b/ipc/shm.c
index 7a51443..1bc68f1 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -975,6 +975,13 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
 			goto out_unlock1;
 
 		ipc_lock_object(&shp->shm_perm);
+
+		/* check if shm_destroy() is tearing down shp */
+		if (!ipc_valid_object(&shp->shm_perm)) {
+			err = -EIDRM;
+			goto out_unlock0;
+		}
+
 		if (!ns_capable(ns->user_ns, CAP_IPC_LOCK)) {
 			kuid_t euid = current_euid();
 			if (!uid_eq(euid, shp->shm_perm.uid) &&
@@ -989,13 +996,6 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
 		}
 
 		shm_file = shp->shm_file;
-
-		/* check if shm_destroy() is tearing down shp */
-		if (shm_file == NULL) {
-			err = -EIDRM;
-			goto out_unlock0;
-		}
-
 		if (is_file_hugepages(shm_file))
 			goto out_unlock0;
 
@@ -1116,7 +1116,7 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
 	ipc_lock_object(&shp->shm_perm);
 
 	/* check if shm_destroy() is tearing down shp */
-	if (shp->shm_file == NULL) {
+	if (!ipc_valid_object(&shp->shm_perm)) {
 		ipc_unlock_object(&shp->shm_perm);
 		err = -EIDRM;
 		goto out_unlock;
diff --git a/ipc/util.h b/ipc/util.h
index 59d78aa..d05b708 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -185,6 +185,19 @@ static inline void ipc_unlock(struct kern_ipc_perm *perm)
 	rcu_read_unlock();
 }
 
+/*
+ * ipc_valid_object() - helper to sort out IPC_RMID races for codepaths
+ * where the respective ipc_ids.rwsem is not being held down.
+ * Checks whether the ipc object is still around or if it's gone already, as
+ * ipc_rmid() may have already freed the ID while the ipc lock was spinning.
+ * Needs to be called with kern_ipc_perm.lock held -- exception made for one
+ * checkpoint case at sys_semtimedop() as noted in code commentary.
+ */
+static inline bool ipc_valid_object(struct kern_ipc_perm *perm)
+{
+	return perm->deleted == 0;
+}
+
 struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id);
 int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
 			struct ipc_ops *ops, struct ipc_params *params);
-- 
1.8.3.1