[PATCH 3/6] ipc/util.c: Use ipc_rcu_putref() for failues in ipc_addid()

[Manfred Spraul]

ipc_addid() is impossible to use:
- for certain failures, the caller must not use ipc_rcu_putref(),
  because the reference counter is not yet initialized.
- for other failures, the caller must use ipc_rcu_putref(),
  because parallel operations could be ongoing already.

The patch cleans that up, by initializing the refcount early,
and by modifying all callers.

The issues is related to the finding of
syzbot+2827ef6b3385deb07eaf@xxxxxxxxxxxxxxxxxxxxxxxxx:
syzbot found an issue with reading kern_ipc_perm.seq,
here both read and write to already released memory could happen.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
---
 ipc/msg.c  |  2 +-
 ipc/sem.c  |  2 +-
 ipc/shm.c  |  2 ++
 ipc/util.c | 12 ++++++++++--
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index 829c2062ded4..5bf5cb8017ea 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -162,7 +162,7 @@ static int newque(struct ipc_namespace *ns, struct ipc_params *params)
 	/* ipc_addid() locks msq upon success. */
 	retval = ipc_addid(&msg_ids(ns), &msq->q_perm, ns->msg_ctlmni);
 	if (retval < 0) {
-		call_rcu(&msq->q_perm.rcu, msg_rcu_free);
+		ipc_rcu_putref(&msq->q_perm, msg_rcu_free);
 		return retval;
 	}
 
diff --git a/ipc/sem.c b/ipc/sem.c
index e8971fa1d847..9d49efeac2e5 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -556,7 +556,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
 	/* ipc_addid() locks sma upon success. */
 	retval = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni);
 	if (retval < 0) {
-		call_rcu(&sma->sem_perm.rcu, sem_rcu_free);
+		ipc_rcu_putref(&sma->sem_perm, sem_rcu_free);
 		return retval;
 	}
 	ns->used_sems += nsems;
diff --git a/ipc/shm.c b/ipc/shm.c
index 59fe8b3b3794..06b7bf11a011 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -671,6 +671,8 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	if (is_file_hugepages(file) && shp->mlock_user)
 		user_shm_unlock(size, shp->mlock_user);
 	fput(file);
+	ipc_rcu_putref(&shp->shm_perm, shm_rcu_free);
+	return error;
 no_file:
 	call_rcu(&shp->shm_perm.rcu, shm_rcu_free);
 	return error;
diff --git a/ipc/util.c b/ipc/util.c
index 662c28c6c9fa..8b09496ed720 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -248,7 +248,9 @@ static inline void ipc_set_seq(struct ipc_ids *ids,
  * Add an entry 'new' to the ipc ids idr. The permissions object is
  * initialised and the first free entry is set up and the id assigned
  * is returned. The 'new' entry is returned in a locked state on success.
+ *
  * On failure the entry is not locked and a negative err-code is returned.
+ * The caller must use ipc_rcu_putref() to free the identifier.
  *
  * Called with writer ipc_ids.rwsem held.
  */
@@ -258,6 +260,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	kgid_t egid;
 	int id, err;
 
+	/* 1) Initialize the refcount so that ipc_rcu_putref works */
+	refcount_set(&new->refcount, 1);
+
 	if (limit > IPCMNI)
 		limit = IPCMNI;
 
@@ -266,9 +271,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 
 	idr_preload(GFP_KERNEL);
 
-	refcount_set(&new->refcount, 1);
 	spin_lock_init(&new->lock);
-	new->deleted = false;
 	rcu_read_lock();
 	spin_lock(&new->lock);
 
@@ -277,6 +280,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	new->gid = new->cgid = egid;
 
 	ipc_set_seq(ids, new);
+	new->deleted = false;
 
 	/*
 	 * As soon as a new object is inserted into the idr,
@@ -288,6 +292,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	 * Thus the object must be fully initialized, and if something fails,
 	 * then the full tear-down sequence must be followed.
 	 * (i.e.: set new->deleted, reduce refcount, call_rcu())
+	 *
+	 * This function sets new->deleted, the caller must use ipc_rcu_putef()
+	 * for the remaining steps.
 	 */
 	id = ipc_idr_alloc(ids, new);
 	idr_preload_end();
@@ -301,6 +308,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 		}
 	}
 	if (id < 0) {
+		new->deleted = true;
 		spin_unlock(&new->lock);
 		rcu_read_unlock();
 		return id;
-- 
2.17.1