fs: Allow superblock owner to access do_remount_sb()

Superblock level remounts are currently restricted to global
CAP_SYS_ADMIN, as is the path for changing the root mount to
read only on umount. Loosen both of these permission checks to
also allow CAP_SYS_ADMIN in any namespace which is privileged
towards the userns which originally mounted the filesystem.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/fs/namespace.c b/fs/namespace.c
index 5f75969..8ddd148 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1590,7 +1590,7 @@ static int do_umount(struct mount *mnt, int flags)
                 * Special case for "unmounting" root ...
                 * we just try to remount it readonly.
                 */
-               if (!capable(CAP_SYS_ADMIN))
+               if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
                        return -EPERM;
                down_write(&sb->s_umount);
                if (!sb_rdonly(sb))
@@ -2333,7 +2333,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
        down_write(&sb->s_umount);
        if (ms_flags & MS_BIND)
                err = change_mount_flags(path->mnt, ms_flags);
-       else if (!capable(CAP_SYS_ADMIN))
+       else if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
                err = -EPERM;
        else
                err = do_remount_sb(sb, sb_flags, data, 0);