Linux kernel: locked mount namespace file lets a local user unmask files

In the Linux kernel, the following vulnerability has been resolved:

namespace: a mount that copy_tree() leaves out can't take over a lock

Since commit c62a4766937e ("move_mount: transfer MNT_LOCKED") a mount can be moved beneath a locked mount. The lock moves from the old cover to the new mount which takes over the job of hiding what's under it and the old cover can be unmounted. That works as long as the new mount stays. But the caller picks the new mount and copy_tree() leaves a mount of a mount namespace file out of every copy made without CL_COPY_MNT_NS_FILE, together with everything mounted on it, and never looks at MNT_LOCKED. unshare(CLONE_NEWNS) and mount propagation make such copies.

So a task that is root in its own user namespace gets rid of a locked cover on a file without a race and without touching the filesystem below it. It makes a child with a mount namespace of its own, binds the child's /proc/<pid>/ns/mnt beneath the cover with MOVE_MOUNT_BENEATH, which hands the lock to that bind, and calls unshare(CLONE_NEWNS). The new mount namespace has nothing on the file. That's the usual way to mask a single file, a bind of /dev/null or of an empty file. With a bind of an ordinary file the file stays covered.

copy_tree() already refuses to leave out a locked unbindable mount with -EPERM since commit df7342b24018 ("mount: Don't allow copying MNT_UNBINDABLE|MNT_LOCKED mounts"). Do the same for a locked mount of a mount namespace file. That also covers a tree copied with CL_COPY_MNT_NS_FILE and attached into a mount namespace of another user namespace where lock_mnt_tree() locks it. Note that unshare(CLONE_NEWNS) and clone(CLONE_NEWNS) fail with EPERM from such a mount namespace from then on. The mount can't be left out and it can't be unmounted.

And refuse to move a mount of a mount namespace file beneath a locked mount in the first place. Otherwise the task ends up with a mount it can't unmount and an unshare() that fails for good.

Chris also found a second mount that goes away by itself, a clone of an automounted mount that shrink_submounts() took along with its parent. Commit f78ddcc87224 ("namespace: never expire a locked mount") took care of that one.

Chris's reproducer is appended. Mode nsfile reads SECRET after the unshare() on 7.3-rc7, with this patch the move_mount() fails with EINVAL and the file stays covered. nsfile-control uses an ordinary file and reads MASK both ways.

// SPDX-License-Identifier: GPL-2.0 /*

• Get rid of a locked cover with MOVE_MOUNT_BENEATH.
• The "admin" (root) makes a tmpfs W:

• W/D         file, holds SECRET, covered by a bind of a file with MASK
  

• W/Dd/inner  file, holds SECRET, Dd is covered by a bind of a directory
  

•             whose "inner" holds MASK
  

• W/own       directory of uid 1000 with the file "mine" and the
  

•             directory "minedir", whose "inner" holds MINE
  

• W/dbg       debugfs, mode 755. dbg/tracing is an automount point
  

• The "attacker" is uid 1000. It enters a new user namespace with a new mount
• namespace, so W and both covers are locked there.

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