pnode: terminate at peers of source

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

pnode: terminate at peers of source

The propagate_mnt() function handles mount propagation when creating
mounts and propagates the source mount tree @source_mnt to all
applicable nodes of the destination propagation mount tree headed by
@dest_mnt.

Unfortunately it contains a bug where it fails to terminate at peers of
@source_mnt when looking up copies of the source mount that become
masters for copies of the source mount tree mounted on top of slaves in
the destination propagation tree causing a NULL dereference.

Once the mechanics of the bug are understood it's easy to trigger.
Because of unprivileged user namespaces it is available to unprivileged
users.

While fixing this bug we've gotten confused multiple times due to
unclear terminology or missing concepts. So let's start this with some
clarifications:

* The terms "master" or "peer" denote a shared mount. A shared mount
belongs to a peer group.

* A peer group is a set of shared mounts that propagate to each other.
They are identified by a peer group id. The peer group id is available
in @shared_mnt->mnt_group_id.
Shared mounts within the same peer group have the same peer group id.
The peers in a peer group can be reached via @shared_mnt->mnt_share.

* The terms "slave mount" or "dependent mount" denote a mount that
receives propagation from a peer in a peer group. IOW, shared mounts
may have slave mounts and slave mounts have shared mounts as their
master. Slave mounts of a given peer in a peer group are listed on
that peers slave list available at @shared_mnt->mnt_slave_list.

* The term "master mount" denotes a mount in a peer group. IOW, it
denotes a shared mount or a peer mount in a peer group. The term
"master mount" - or "master" for short - is mostly used when talking
in the context of slave mounts that receive propagation from a master
mount. A master mount of a slave identifies the closest peer group a
slave mount receives propagation from. The master mount of a slave can
be identified via @slave_mount->mnt_master. Different slaves may point
to different masters in the same peer group.

* Multiple peers in a peer group can have non-empty ->mnt_slave_lists.
Non-empty ->mnt_slave_lists of peers don't intersect. Consequently, to
ensure all slave mounts of a peer group are visited the
->mnt_slave_lists of all peers in a peer group have to be walked.

* Slave mounts point to a peer in the closest peer group they receive
propagation from via @slave_mnt->mnt_master (see above). Together with
these peers they form a propagation group (see below). The closest
peer group can thus be identified through the peer group id
@slave_mnt->mnt_master->mnt_group_id of the peer/master that a slave
mount receives propagation from.

* A shared-slave mount is a slave mount to a peer group pg1 while also
a peer in another peer group pg2. IOW, a peer group may receive
propagation from another peer group.

If a peer group pg1 is a slave to another peer group pg2 then all
peers in peer group pg1 point to the same peer in peer group pg2 via
->mnt_master. IOW, all peers in peer group pg1 appear on the same
->mnt_slave_list. IOW, they cannot be slaves to different peer groups.

* A pure slave mount is a slave mount that is a slave to a peer group
but is not a peer in another peer group.

* A propagation group denotes the set of mounts consisting of a single
peer group pg1 and all slave mounts and shared-slave mounts that point
to a peer in that peer group via ->mnt_master. IOW, all slave mounts
such that @slave_mnt->mnt_master->mnt_group_id is equal to
@shared_mnt->mnt_group_id.

The concept of a propagation group makes it easier to talk about a
single propagation level in a propagation tree.

For example, in propagate_mnt() the immediate peers of @dest_mnt and
all slaves of @dest_mnt's peer group form a propagation group pr
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