Re: [PATCH 1/2] VFS: change kern_path_locked() and user_path_locked_at() to never return negative dentry

[Christian Brauner <brauner@kernel.org>]

> diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
> index 7f358740e958..367eaf2c78b7 100644
> --- a/kernel/audit_watch.c
> +++ b/kernel/audit_watch.c
> @@ -350,11 +350,10 @@ static int audit_get_nd(struct audit_watch *watch, struct path *parent)
>  	struct dentry *d = kern_path_locked(watch->path, parent);
>  	if (IS_ERR(d))
>  		return PTR_ERR(d);
> -	if (d_is_positive(d)) {
> -		/* update watch filter fields */
> -		watch->dev = d->d_sb->s_dev;
> -		watch->ino = d_backing_inode(d)->i_ino;
> -	}
> +	/* update watch filter fields */
> +	watch->dev = d->d_sb->s_dev;
> +	watch->ino = d_backing_inode(d)->i_ino;
> +
>  	inode_unlock(d_backing_inode(parent->dentry));
>  	dput(d);
>  	return 0;
> @@ -419,10 +418,11 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
>  	/* caller expects mutex locked */
>  	mutex_lock(&audit_filter_mutex);
>  
> -	if (ret) {
> +	if (ret && ret != -ENOENT) {
>  		audit_put_watch(watch);
>  		return ret;
>  	}
> +	ret = 0;

So this is broken.

If kern_path_locked() fails due to a negative dentry and returns ENOENT
it will have already called path_put() and @parent_path is invalid.

But right after this audit does:

>  
>  	/* either find an old parent or attach a new one */
>  	parent = audit_find_parent(d_backing_inode(parent_path.dentry));

and then later on calls path_put() again. So this is a UAF. We need to
fix this.

This used to work before because kern_path_locked() return a path with a
negative dentry.