debugfs: fix use-after-free on symlink traversal

symlink body shouldn't be freed without an RCU delay.  Switch debugfs to
->destroy_inode() and use of call_rcu(); free both the inode and symlink
body in the callback.  Similar to solution for bpf, only here it's even
more obvious that ->evict_inode() can be dropped.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 95b5e78c22b1e98811d3aca9c64c2c5deb54c6fe..f25daa207421c50cf38b1e4771ef5ab3332e9be1 100644 (file)
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -163,19 +163,24 @@ static int debugfs_show_options(struct seq_file *m, struct dentry *root)
        return 0;
 }
 
-static void debugfs_evict_inode(struct inode *inode)
+static void debugfs_i_callback(struct rcu_head *head)
 {
-       truncate_inode_pages_final(&inode->i_data);
-       clear_inode(inode);
+       struct inode *inode = container_of(head, struct inode, i_rcu);
        if (S_ISLNK(inode->i_mode))
                kfree(inode->i_link);
+       free_inode_nonrcu(inode);
+}
+
+static void debugfs_destroy_inode(struct inode *inode)
+{
+       call_rcu(&inode->i_rcu, debugfs_i_callback);
 }
 
 static const struct super_operations debugfs_super_operations = {
        .statfs         = simple_statfs,
        .remount_fs     = debugfs_remount,
        .show_options   = debugfs_show_options,
-       .evict_inode    = debugfs_evict_inode,
+       .destroy_inode  = debugfs_destroy_inode,
 };
 
 static void debugfs_release_dentry(struct dentry *dentry)