mnt: Implicitly add MNT_NODEV on remount when it was implicitly added by mount

Now that remount is properly enforcing the rule that you can't remove
nodev at least sandstorm.io is breaking when performing a remount.

It turns out that there is an easy intuitive solution implicitly
add nodev on remount when nodev was implicitly added on mount.

Tested-by: Cedric Bosdonnat <cbosdonnat@suse.com>
Tested-by: Richard Weinberger <richard@nod.at>
Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 5b66b2b3624d..3a1a87dc33df 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2098,7 +2098,13 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
         }
         if ((mnt->mnt.mnt_flags & MNT_LOCK_NODEV) &&
             !(mnt_flags & MNT_NODEV)) {
-                return -EPERM;
+                /* Was the nodev implicitly added in mount? */
+                if ((mnt->mnt_ns->user_ns != &init_user_ns) &&
+                    !(sb->s_type->fs_flags & FS_USERNS_DEV_MOUNT)) {
+                        mnt_flags |= MNT_NODEV;
+                } else {
+                        return -EPERM;
+                }
         }
         if ((mnt->mnt.mnt_flags & MNT_LOCK_NOSUID) &&
             !(mnt_flags & MNT_NOSUID)) {