linux-kernel - [PATCH 03/12] ipc/util.c: Use ipc_rcu_putref() for failues in ipc

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Date:   Thu, 12 Jul 2018 20:52:32 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Kees Cook <keescook@...omium.org>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 03/12] ipc/util.c: Use ipc_rcu_putref() for failues in ipc_addid()

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@...kaller.appspotmail.com:
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@...orfullife.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Davidlohr Bueso <dave@...olabs.net>
---
 ipc/msg.c  |  2 +-
 ipc/sem.c  |  2 +-
 ipc/shm.c  |  2 ++
 ipc/util.c | 10 ++++++++--
 4 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index 49358f474fc9..38119c1f0da3 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 d89ce69b2613..8a0a1eb05765 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 f3bae59bed08..92d71abe9e8f 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 4998f8fa8ce0..f3447911c81e 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -250,7 +250,9 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
  * 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.
  */
@@ -260,6 +262,9 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	kgid_t egid;
 	int idx, err;
 
+	/* 1) Initialize the refcount so that ipc_rcu_putref works */
+	refcount_set(&new->refcount, 1);
+
 	if (limit > IPCMNI)
 		limit = IPCMNI;
 
@@ -268,9 +273,7 @@ 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);
 
@@ -278,6 +281,8 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	new->cuid = new->uid = euid;
 	new->gid = new->cgid = egid;
 
+	new->deleted = false;
+
 	idx = ipc_idr_alloc(ids, new);
 	idr_preload_end();
 
@@ -290,6 +295,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 		}
 	}
 	if (idx < 0) {
+		new->deleted = true;
 		spin_unlock(&new->lock);
 		rcu_read_unlock();
 		return idx;
-- 
2.17.1