[PATCH 3/5] fs: convert mnt_namespace.count from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Signed-off-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: David Windsor <dwindsor@xxxxxxxxx>
---
 fs/mount.h     | 5 +++--
 fs/namespace.c | 4 ++--
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/mount.h b/fs/mount.h
index 2826543..106240d 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -3,9 +3,10 @@
 #include <linux/poll.h>
 #include <linux/ns_common.h>
 #include <linux/fs_pin.h>
+#include <linux/refcount.h>
 
 struct mnt_namespace {
-	atomic_t		count;
+	refcount_t		count;
 	struct ns_common	ns;
 	struct mount *	root;
 	struct list_head	list;
@@ -110,7 +111,7 @@ static inline void detach_mounts(struct dentry *dentry)
 
 static inline void get_mnt_ns(struct mnt_namespace *ns)
 {
-	atomic_inc(&ns->count);
+	refcount_inc(&ns->count);
 }
 
 extern seqlock_t mount_lock;
diff --git a/fs/namespace.c b/fs/namespace.c
index 8bfad42..85cc8ca 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2888,7 +2888,7 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
 	}
 	new_ns->ns.ops = &mntns_operations;
 	new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
-	atomic_set(&new_ns->count, 1);
+	refcount_set(&new_ns->count, 1);
 	new_ns->root = NULL;
 	INIT_LIST_HEAD(&new_ns->list);
 	init_waitqueue_head(&new_ns->poll);
@@ -3278,7 +3278,7 @@ void __init mnt_init(void)
 
 void put_mnt_ns(struct mnt_namespace *ns)
 {
-	if (!atomic_dec_and_test(&ns->count))
+	if (!refcount_dec_and_test(&ns->count))
 		return;
 	drop_collected_mounts(&ns->root->mnt);
 	free_mnt_ns(ns);
-- 
2.7.4