[PATCH] namespace.c: fix race in mark_mounts_for_expiry()

This patch fixes a race found by Ram in mark_mounts_for_expiry() in
fs/namespace.c.

The bug can only be triggered with simultaneous exiting of a process having
a private namespace, and expiry of a mount from within that namespace.
It's practically impossible to trigger, and I haven't even tried. But
still, a bug is a bug.

The race happens when put_namespace() is called by another task, while
mark_mounts_for_expiry() is between atomic_read() and get_namespace(). In
that case get_namespace() will be called on an already dead namespace with
unforeseeable results.

The solution was suggested by Al Viro, with his own words:

Instead of screwing with atomic_read() in there, why don't we
simply do the following:
a) atomic_dec_and_lock() in put_namespace()
b) __put_namespace() called without dropping lock
c) the first thing done by __put_namespace would be
struct vfsmount *root = namespace->root;
namespace->root = NULL;
spin_unlock(...);
....
umount_tree(root);
...
d) check in mark_... would be simply namespace && namespace->root.

And we are all set; no screwing around with atomic_read(), no magic
at all. Dying namespace gets NULL ->root.
All changes of ->root happen under spinlock.
If under a spinlock we see non-NULL ->mnt_namespace, it won't be
freed until we drop the lock (we will set ->mnt_namespace to NULL
under that lock before we get to freeing namespace).
If under a spinlock we see non-NULL ->mnt_namespace and
->mnt_namespace->root, we can grab a reference to namespace and be
sure that it won't go away.

Signed-off-by: Miklos Szeredi <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Acked-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/namespace.c

···
 		/* don't do anything if the namespace is dead - all the
 		 * vfsmounts from it are going away anyway */
 		namespace = mnt->mnt_namespace;
-		if (!namespace || atomic_read(&namespace->count) <= 0)
+		if (!namespace || !namespace->root)
 			continue;
 		get_namespace(namespace);
 
···
 
 void __put_namespace(struct namespace *namespace)
 {
+	struct vfsmount *root = namespace->root;
+	namespace->root = NULL;
+	spin_unlock(&vfsmount_lock);
 	down_write(&namespace->sem);
 	spin_lock(&vfsmount_lock);
-	umount_tree(namespace->root);
+	umount_tree(root);
 	spin_unlock(&vfsmount_lock);
 	up_write(&namespace->sem);
 	kfree(namespace);

include/linux/namespace.h

···
 
 static inline void put_namespace(struct namespace *namespace)
 {
-	if (atomic_dec_and_test(&namespace->count))
+	if (atomic_dec_and_lock(&namespace->count, &vfsmount_lock))
+		/* releases vfsmount_lock */
 		__put_namespace(namespace);
 }