Re: [PATCH] vfs: fsmount: add missing mntget()

[Miklos Szeredi]

On Wed, Jun 12, 2019 at 11:43:13AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> 
> sys_fsmount() needs to take a reference to the new mount when adding it
> to the anonymous mount namespace.  Otherwise the filesystem can be
> unmounted while it's still in use, as found by syzkaller.

So it needs one count for the file (which dentry_open() obtains) and one for the
attachment into the anonymous namespace.  The latter one is dropped at cleanup
time, so your patch appears to be correct at getting that ref.

I wonder why such a blatant use-after-free was missed in normal testing.  RCU
delayed freeing, I guess?

How about this additional sanity checking patch?

Thanks,
Miklos


diff --git a/fs/namespace.c b/fs/namespace.c
index b26778bdc236..c638f220805a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -153,10 +153,10 @@ static inline void mnt_add_count(struct mount *mnt, int n)
 /*
  * vfsmount lock must be held for write
  */
-unsigned int mnt_get_count(struct mount *mnt)
+int mnt_get_count(struct mount *mnt)
 {
 #ifdef CONFIG_SMP
-	unsigned int count = 0;
+	int count = 0;
 	int cpu;
 
 	for_each_possible_cpu(cpu) {
@@ -1140,6 +1140,8 @@ static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);
 
 static void mntput_no_expire(struct mount *mnt)
 {
+	int count;
+
 	rcu_read_lock();
 	if (likely(READ_ONCE(mnt->mnt_ns))) {
 		/*
@@ -1162,11 +1164,13 @@ static void mntput_no_expire(struct mount *mnt)
 	 */
 	smp_mb();
 	mnt_add_count(mnt, -1);
-	if (mnt_get_count(mnt)) {
+	count = mnt_get_count(mnt);
+	if (count > 0) {
 		rcu_read_unlock();
 		unlock_mount_hash();
 		return;
 	}
+	WARN_ON(count < 0);
 	if (unlikely(mnt->mnt.mnt_flags & MNT_DOOMED)) {
 		rcu_read_unlock();
 		unlock_mount_hash();
diff --git a/fs/pnode.h b/fs/pnode.h
index 49a058c73e4c..26f74e092bd9 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -44,7 +44,7 @@ int propagate_mount_busy(struct mount *, int);
 void propagate_mount_unlock(struct mount *);
 void mnt_release_group_id(struct mount *);
 int get_dominating_id(struct mount *mnt, const struct path *root);
-unsigned int mnt_get_count(struct mount *mnt);
+int mnt_get_count(struct mount *mnt);
 void mnt_set_mountpoint(struct mount *, struct mountpoint *,
 			struct mount *);
 void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp,