mnt: Only change user settable mount flags in remount

commit a6138db815df5ee542d848318e5dae681590fccd upstream.

Kenton Varda <kenton@sandstorm.io> discovered that by remounting a
read-only bind mount read-only in a user namespace the
MNT_LOCK_READONLY bit would be cleared, allowing an unprivileged user
to the remount a read-only mount read-write.

Correct this by replacing the mask of mount flags to preserve
with a mask of mount flags that may be changed, and preserve
all others.   This ensures that any future bugs with this mask and
remount will fail in an easy to detect way where new mount flags
simply won't change.

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Francis Moreau <francis.moro@gmail.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>

diff --git a/fs/namespace.c b/fs/namespace.c
index 2985879371ebc1dd7d25e4946b37ff40404aa826..f0f2e067c5df17ed38f49aeb31c504b1151b9c79 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1681,7 +1681,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
                err = do_remount_sb(sb, flags, data, 0);
        if (!err) {
                br_write_lock(vfsmount_lock);
-               mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
+               mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
                mnt->mnt.mnt_flags = mnt_flags;
                br_write_unlock(vfsmount_lock);
        }

diff --git a/include/linux/mount.h b/include/linux/mount.h
index d7029f4a191a093ef9294104c75756b6d3b7faee..2044aacc0d2b2e1511332f671d248332bb756fac 100644 (file)
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -42,7 +42,9 @@ struct mnt_namespace;
  * flag, consider how it interacts with shared mounts.
  */
 #define MNT_SHARED_MASK        (MNT_UNBINDABLE)
-#define MNT_PROPAGATION_MASK   (MNT_SHARED | MNT_UNBINDABLE)
+#define MNT_USER_SETTABLE_MASK  (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \
+                                | MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \
+                                | MNT_READONLY)
 
 
 #define MNT_INTERNAL   0x4000