fs/fhandle.c: fix a race in call of has_locked_children()

may_decode_fh() is calling has_locked_children() while holding no locks.
That's an oopsable race...

The rest of the callers are safe since they are holding namespace_sem and
are guaranteed a positive refcount on the mount in question.

Rename the current has_locked_children() to __has_locked_children(), make
it static and switch the fs/namespace.c users to it.

Make has_locked_children() a wrapper for __has_locked_children(), calling
the latter under read_seqlock_excl(&mount_lock).

Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Fixes: 620c266f3949 ("fhandle: relax open_by_handle_at() permission checks")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 7c0ebc4f4ef259e3c9f6ffdf2c6636ced244d01e..a33553bc12d0681d9db07b0183735fc277d978b7 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2425,7 +2425,7 @@ void drop_collected_mounts(struct vfsmount *mnt)
        namespace_unlock();
 }
 
-bool has_locked_children(struct mount *mnt, struct dentry *dentry)
+static bool __has_locked_children(struct mount *mnt, struct dentry *dentry)
 {
        struct mount *child;
 
@@ -2439,6 +2439,16 @@ bool has_locked_children(struct mount *mnt, struct dentry *dentry)
        return false;
 }
 
+bool has_locked_children(struct mount *mnt, struct dentry *dentry)
+{
+       bool res;
+
+       read_seqlock_excl(&mount_lock);
+       res = __has_locked_children(mnt, dentry);
+       read_sequnlock_excl(&mount_lock);
+       return res;
+}
+
 /*
  * Check that there aren't references to earlier/same mount namespaces in the
  * specified subtree.  Such references can act as pins for mount namespaces
@@ -2499,7 +2509,7 @@ struct vfsmount *clone_private_mount(const struct path *path)
                        return ERR_PTR(-EINVAL);
        }
 
-       if (has_locked_children(old_mnt, path->dentry))
+       if (__has_locked_children(old_mnt, path->dentry))
                return ERR_PTR(-EINVAL);
 
        new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
@@ -3036,7 +3046,7 @@ static struct mount *__do_loopback(struct path *old_path, int recurse)
        if (!may_copy_tree(old_path))
                return mnt;
 
-       if (!recurse && has_locked_children(old, old_path->dentry))
+       if (!recurse && __has_locked_children(old, old_path->dentry))
                return mnt;
 
        if (recurse)
@@ -3429,7 +3439,7 @@ static int do_set_group(struct path *from_path, struct path *to_path)
                goto out;
 
        /* From mount should not have locked children in place of To's root */
-       if (has_locked_children(from, to->mnt.mnt_root))
+       if (__has_locked_children(from, to->mnt.mnt_root))
                goto out;
 
        /* Setting sharing groups is only allowed on private mounts */