ecryptfs: do not mount on top of idmapped mounts

Prevent ecryptfs from being mounted on top of idmapped mounts.
Stacking filesystems need to be prevented from being mounted on top of
idmapped mounts until they have have been converted to handle this.

Link: https://lore.kernel.org/r/20210121131959.646623-28-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/fs/ecryptfs/main.c b/fs/ecryptfs/main.c
index e63259f..cdf40a5 100644 (file)
--- a/fs/ecryptfs/main.c
+++ b/fs/ecryptfs/main.c
@@ -531,6 +531,12 @@ static struct dentry *ecryptfs_mount(struct file_system_type *fs_type, int flags
                goto out_free;
        }
 
+       if (mnt_user_ns(path.mnt) != &init_user_ns) {
+               rc = -EINVAL;
+               printk(KERN_ERR "Mounting on idmapped mounts currently disallowed\n");
+               goto out_free;
+       }
+
        if (check_ruid && !uid_eq(d_inode(path.dentry)->i_uid, current_uid())) {
                rc = -EPERM;
                printk(KERN_ERR "Mount of device (uid: %d) not owned by "