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 propg1.
  So a shared-slave mount that is a slave in propg1 and that is a peer
  in another peer group pg2 forms another propagation group propg2
  together with all slaves that point to that shared-slave mount in
  their ->mnt_master.

* A propagation tree refers to all mounts that receive propagation
  starting from a specific shared mount.

  For example, for propagate_mnt() @dest_mnt is the start of a
  propagation tree. The propagation tree ecompasses all mounts that
  receive propagation from @dest_mnt's peer group down to the leafs.

With that out of the way let's get to the actual algorithm.

We know that @dest_mnt is guaranteed to be a pure shared mount or a
shared-slave mount. This is guaranteed by a check in
attach_recursive_mnt(). So propagate_mnt() will first propagate the
source mount tree to all peers in @dest_mnt's peer group:

for (n = next_peer(dest_mnt); n != dest_mnt; n = next_peer(n)) {
        ret = propagate_one(n);
        if (ret)
               goto out;
}

Notice, that the peer propagation loop of propagate_mnt() doesn't
propagate @dest_mnt itself. @dest_mnt is mounted directly in
attach_recursive_mnt() after we propagated to the destination
propagation tree.

The mount that will be mounted on top of @dest_mnt is @source_mnt. This
copy was created earlier even before we entered attach_recursive_mnt()
and doesn't concern us a lot here.

It's just important to notice that when propagate_mnt() is called
@source_mnt will not yet have been mounted on top of @dest_mnt. Thus,
@source_mnt->mnt_parent will either still point to @source_mnt or - in
the case @source_mnt is moved and thus already attached - still to its
former parent.

For each peer @M in @dest_mnt's peer group propagate_one() will create a
new copy of the source mount tree and mount that copy @child on @M such
that @child->mnt_parent points to @M after propagate_one() returns.

propagate_one() will stash the last destination propagation node @M in
@last_dest and the last copy it created for the source mount tree in
@last_source.

Hence, if we call into propagate_one() again for the next destination
propagation node @M, @last_dest will point to the previous destination
propagation node and @last_source will point to the previous copy of the
source mount tree and mounted on @last_dest.

Each new copy of the source mount tree is created from the previous copy
of the source mount tree. This will become important later.

The peer loop in propagate_mnt() is straightforward. We iterate through
the peers copying and updating @last_source and @last_dest as we go
through them and mount each copy of the source mount tree @child on a
peer @M in @dest_mnt's peer group.

After propagate_mnt() handled the peers in @dest_mnt's peer group
propagate_mnt() will propagate the source mount tree down the
propagation tree that @dest_mnt's peer group propagates to:

for (m = next_group(dest_mnt, dest_mnt); m;
                m = next_group(m, dest_mnt)) {
        /* everything in that slave group */
        n = m;
        do {
                ret = propagate_one(n);
                if (ret)
                        goto out;
                n = next_peer(n);
        } while (n != m);
}

The next_group() helper will recursively walk the destination
propagation tree, descending into each propagation group of the
propagation tree.

The important part is that it takes care to propagate the source mount
tree to all peers in the peer group of a propagation group before it
propagates to the slaves to those peers in the propagation group. IOW,
it creates and mounts copies of the source mount tree that become
masters before it creates and mounts copies of the source mount tree
that become slaves to these masters.

It is important to remember that propagating the source mount tree to
each mount @M in the destination propagation tree simply means that we
create and mount new copies @child of the source mount tree on @M such
that @child->mnt_parent points to @M.

Since we know that each node @M in the destination propagation tree
headed by @dest_mnt's peer group will be overmounted with a copy of the
source mount tree and since we know that the propagation properties of
each copy of the source mount tree we create and mount at @M will mostly
mirror the propagation properties of @M. We can use that information to
create and mount the copies of the source mount tree that become masters
before their slaves.

The easy case is always when @M and @last_dest are peers in a peer group
of a given propagation group. In that case we know that we can simply
copy @last_source without having to figure out what the master for the
new copy @child of the source mount tree needs to be as we've done that
in a previous call to propagate_one().

The hard case is when we're dealing with a slave mount or a shared-slave
mount @M in a destination propagation group that we need to create and
mount a copy of the source mount tree on.

For each propagation group in the destination propagation tree we
propagate the source mount tree to we want to make sure that the copies
@child of the source mount tree we create and mount on slaves @M pick an
ealier copy of the source mount tree that we mounted on a master @M of
the destination propagation group as their master. This is a mouthful
but as far as we can tell that's the core of it all.

But, if we keep track of the masters in the destination propagation tree
@M we can use the information to find the correct master for each copy
of the source mount tree we create and mount at the slaves in the
destination propagation tree @M.

Let's walk through the base case as that's still fairly easy to grasp.

If we're dealing with the first slave in the propagation group that
@dest_mnt is in then we don't yet have marked any masters in the
destination propagation tree.

We know the master for the first slave to @dest_mnt's peer group is
simple @dest_mnt. So we expect this algorithm to yield a copy of the
source mount tree that was mounted on a peer in @dest_mnt's peer group
as the master for the copy of the source mount tree we want to mount at
the first slave @M:

for (n = m; ; n = p) {
        p = n->mnt_master;
        if (p == dest_master || IS_MNT_MARKED(p))
                break;
}

For the first slave we walk the destination propagation tree all the way
up to a peer in @dest_mnt's peer group. IOW, the propagation hierarchy
can be walked by walking up the @mnt->mnt_master hierarchy of the
destination propagation tree @M. We will ultimately find a peer in
@dest_mnt's peer group and thus ultimately @dest_mnt->mnt_master.

Btw, here the assumption we listed at the beginning becomes important.
Namely, that peers in a peer group pg1 that are slaves in another peer
group pg2 appear on the same ->mnt_slave_list. IOW, all slaves who are
peers in peer group pg1 point to the same peer in peer group pg2 via
their ->mnt_master. Otherwise the termination condition in the code
above would be wrong and next_group() would be broken too.

So the first iteration sets:

n = m;
p = n->mnt_master;

such that @p now points to a peer or @dest_mnt itself. We walk up one
more level since we don't have any marked mounts. So we end up with:

n = dest_mnt;
p = dest_mnt->mnt_master;

If @dest_mnt's peer group is not slave to another peer group then @p is
now NULL. If @dest_mnt's peer group is a slave to another peer group
then @p now points to @dest_mnt->mnt_master points which is a master
outside the propagation tree we're dealing with.

Now we need to figure out the master for the copy of the source mount
tree we're about to create and mount on the first slave of @dest_mnt's
peer group:

do {
        struct mount *parent = last_source->mnt_parent;
        if (last_source == first_source)
                break;
        done = parent->mnt_master == p;
        if (done && peers(n, parent))
                break;
        last_source = last_source->mnt_master;
} while (!done);

We know that @last_source->mnt_parent points to @last_dest and
@last_dest is the last peer in @dest_mnt's peer group we propagated to
in the peer loop in propagate_mnt().

Consequently, @last_source is the last copy we created and mount on that
last peer in @dest_mnt's peer group. So @last_source is the master we
want to pick.

We know that @last_source->mnt_parent->mnt_master points to
@last_dest->mnt_master. We also know that @last_dest->mnt_master is
either NULL or points to a master outside of the destination propagation
tree and so does @p. Hence:

done = parent->mnt_master == p;

is trivially true in the base condition.

We also know that for the first slave mount of @dest_mnt's peer group
that @last_dest either points @dest_mnt itself because it was
initialized to:

last_dest = dest_mnt;

at the beginning of propagate_mnt() or it will point to a peer of
@dest_mnt in its peer group. In both cases it is guaranteed that on the
first iteration @n and @parent are peers (Please note the check for
peers here as that's important.):

if (done && peers(n, parent))
        break;

So, as we expected, we select @last_source, which referes to the last
copy of the source mount tree we mounted on the last peer in @dest_mnt's
peer group, as the master of the first slave in @dest_mnt's peer group.
The rest is taken care of by clone_mnt(last_source, ...). We'll skip
over that part otherwise this becomes a blogpost.

At the end of propagate_mnt() we now mark @m->mnt_master as the first
master in the destination propagation tree that is distinct from
@dest_mnt->mnt_master. IOW, we mark @dest_mnt itself as a master.

By marking @dest_mnt or one of it's peers we are able to easily find it
again when we later lookup masters for other copies of the source mount
tree we mount copies of the source mount tree on slaves @M to
@dest_mnt's peer group. This, in turn allows us to find the master we
selected for the copies of the source mount tree we mounted on master in
the destination propagation tree again.

The important part is to realize that the code makes use of the fact
that the last copy of the source mount tree stashed in @last_source was
mounted on top of the previous destination propagation node @last_dest.
What this means is that @last_source allows us to walk the destination
propagation hierarchy the same way each destination propagation node @M
does.

If we take @last_source, which is the copy of @source_mnt we have
mounted on @last_dest in the previous iteration of propagate_one(), then
we know @last_source->mnt_parent points to @last_dest but we also know
that as we walk through the destination propagation tree that
@last_source->mnt_master will point to an earlier copy of the source
mount tree we mounted one an earlier destination propagation node @M.

IOW, @last_source->mnt_parent will be our hook into the destination
propagation tree and each consecutive @last_source->mnt_master will lead
us to an earlier propagation node @M via
@last_source->mnt_master->mnt_parent.

Hence, by walking up @last_source->mnt_master, each of which is mounted
on a node that is a master @M in the destination propagation tree we can
also walk up the destination propagation hierarchy.

So, for each new destination propagation node @M we use the previous
copy of @last_source and the fact it's mounted on the previous
propagation node @last_dest via @last_source->mnt_master->mnt_parent to
determine what the master of the new copy of @last_source needs to be.

The goal is to find the _closest_ master that the new copy of the source
mount tree we are about to create and mount on a slave @M in the
destination propagation tree needs to pick. IOW, we want to find a
suitable master in the propagation group.

As the propagation structure of the source mount propagation tree we
create mirrors the propagation structure of the destination propagation
tree we can find @M's closest master - i.e., a marked master - which is
a peer in the closest peer group that @M receives propagation from. We
store that closest master of @M in @p as before and record the slave to
that master in @n

We then search for this master @p via @last_source by walking up the
master hierarchy starting from the last copy of the source mount tree
stored in @last_source that we created and mounted on the previous
destination propagation node @M.

We will try to find the master by walking @last_source->mnt_master and
by comparing @last_source->mnt_master->mnt_parent->mnt_master to @p. If
we find @p then we can figure out what earlier copy of the source mount
tree needs to be the master for the new copy of the source mount tree
we're about to create and mount at the current destination propagation
node @M.

If @last_source->mnt_master->mnt_parent and @n are peers then we know
that the closest master they receive propagation from is
@last_source->mnt_master->mnt_parent->mnt_master. If not then the
closest immediate peer group that they receive propagation from must be
one level higher up.

This builds on the earlier clarification at the beginning that all peers
in a peer group which are slaves of other peer groups all point to the
same ->mnt_master, i.e., appear on the same ->mnt_slave_list, of the
closest peer group that they receive propagation from.

However, terminating the walk has corner cases.

If the closest marked master for a given destination node @M cannot be
found by walking up the master hierarchy via @last_source->mnt_master
then we need to terminate the walk when we encounter @source_mnt again.

This isn't an arbitrary termination. It simply means that the new copy
of the source mount tree we're about to create has a copy of the source
mount tree we created and mounted on a peer in @dest_mnt's peer group as
its master. IOW, @source_mnt is the peer in the closest peer group that
the new copy of the source mount tree receives propagation from.

We absolutely have to stop @source_mnt because @last_source->mnt_master
either points outside the propagation hierarchy we're dealing with or it
is NULL because @source_mnt isn't a shared-slave.

So continuing the walk past @source_mnt would cause a NULL dereference
via @last_source->mnt_master->mnt_parent. And so we have to stop the
walk when we encounter @source_mnt again.

One scenario where this can happen is when we first handled a series of
slaves of @dest_mnt's peer group and then encounter peers in a new peer
group that is a slave to @dest_mnt's peer group. We handle them and then
we encounter another slave mount to @dest_mnt that is a pure slave to
@dest_mnt's peer group. That pure slave will have a peer in @dest_mnt's
peer group as its master. Consequently, the new copy of the source mount
tree will need to have @source_mnt as it's master. So we walk the
propagation hierarchy all the way up to @source_mnt based on
@last_source->mnt_master.

So terminate on @source_mnt, easy peasy. Except, that the check misses
something that the rest of the algorithm already handles.

If @dest_mnt has peers in it's peer group the peer loop in
propagate_mnt():

for (n = next_peer(dest_mnt); n != dest_mnt; n = next_peer(n)) {
        ret = propagate_one(n);
        if (ret)
                goto out;
}

will consecutively update @last_source with each previous copy of the
source mount tree we created and mounted at the previous peer in
@dest_mnt's peer group. So after that loop terminates @last_source will
point to whatever copy of the source mount tree was created and mounted
on the last peer in @dest_mnt's peer group.

Furthermore, if there is even a single additional peer in @dest_mnt's
peer group then @last_source will __not__ point to @source_mnt anymore.
Because, as we mentioned above, @dest_mnt isn't even handled in this
loop but directly in attach_recursive_mnt(). So it can't even accidently
come last in that peer loop.

So the first time we handle a slave mount @M of @dest_mnt's peer group
the copy of the source mount tree we create will make the __last copy of
the source mount tree we created and mounted on the last peer in
@dest_mnt's peer group the master of the new copy of the source mount
tree we create and mount on the first slave of @dest_mnt's peer group__.

But this means that the termination condition that checks for
@source_mnt is wrong. The @source_mnt cannot be found anymore by
propagate_one(). Instead it will find the last copy of the source mount
tree we created and mounted for the last peer of @dest_mnt's peer group
again. And that is a peer of @source_mnt not @source_mnt itself.

IOW, we fail to terminate the loop correctly and ultimately dereference
@last_source->mnt_master->mnt_parent. When @source_mnt's peer group
isn't slave to another peer group then @last_source->mnt_master is NULL
causing the splat below.

For example, assume @dest_mnt is a pure shared mount and has three peers
in its peer group:

===================================================================================
                                         mount-id   mount-parent-id   peer-group-id
===================================================================================
(@dest_mnt) mnt_master[216]              309        297               shared:216
    \
     (@source_mnt) mnt_master[218]:      609        609               shared:218

(1) mnt_master[216]:                     607        605               shared:216
    \
     (P1) mnt_master[218]:               624        607               shared:218

(2) mnt_master[216]:                     576        574               shared:216
    \
     (P2) mnt_master[218]:               625        576               shared:218

(3) mnt_master[216]:                     545        543               shared:216
    \
     (P3) mnt_master[218]:               626        545               shared:218

After this sequence has been processed @last_source will point to (P3),
the copy generated for the third peer in @dest_mnt's peer group we
handled. So the copy of the source mount tree (P4) we create and mount
on the first slave of @dest_mnt's peer group:

===================================================================================
                                         mount-id   mount-parent-id   peer-group-id
===================================================================================
    mnt_master[216]                      309        297               shared:216
   /
  /
(S0) mnt_slave                           483        481               master:216
  \
   \    (P3) mnt_master[218]             626        545               shared:218
    \  /
     \/
    (P4) mnt_slave                       627        483               master:218

will pick the last copy of the source mount tree (P3) as master, not (S0).

When walking the propagation hierarchy via @last_source's master
hierarchy we encounter (P3) but not (S0), i.e., @source_mnt.

We can fix this in multiple ways:

(1) By setting @last_source to @source_mnt after we processed the peers
    in @dest_mnt's peer group right after the peer loop in
    propagate_mnt().

(2) By changing the termination condition that relies on finding exactly
    @source_mnt to finding a peer of @source_mnt.

(3) By only moving @last_source when we actually venture into a new peer
    group or some clever variant thereof.

The first two options are minimally invasive and what we want as a fix.
The third option is more intrusive but something we'd like to explore in
the near future.

This passes all LTP tests and specifically the mount propagation
testsuite part of it. It also holds up against all known reproducers of
this issues.

Final words.
First, this is a clever but __worringly__ underdocumented algorithm.
There isn't a single detailed comment to be found in next_group(),
propagate_one() or anywhere else in that file for that matter. This has
been a giant pain to understand and work through and a bug like this is
insanely difficult to fix without a detailed understanding of what's
happening. Let's not talk about the amount of time that was sunk into
fixing this.

Second, all the cool kids with access to
unshare --mount --user --map-root --propagation=unchanged
are going to have a lot of fun. IOW, triggerable by unprivileged users
while namespace_lock() lock is held.

[  115.848393] BUG: kernel NULL pointer dereference, address: 0000000000000010
[  115.848967] #PF: supervisor read access in kernel mode
[  115.849386] #PF: error_code(0x0000) - not-present page
[  115.849803] PGD 0 P4D 0
[  115.850012] Oops: 0000 [#1] PREEMPT SMP PTI
[  115.850354] CPU: 0 PID: 15591 Comm: mount Not tainted 6.1.0-rc7 #3
[  115.850851] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS
VirtualBox 12/01/2006
[  115.851510] RIP: 0010:propagate_one.part.0+0x7f/0x1a0
[  115.851924] Code: 75 eb 4c 8b 05 c2 25 37 02 4c 89 ca 48 8b 4a 10
49 39 d0 74 1e 48 3b 81 e0 00 00 00 74 26 48 8b 92 e0 00 00 00 be 01
00 00 00 <48> 8b 4a 10 49 39 d0 75 e2 40 84 f6 74 38 4c 89 05 84 25 37
02 4d
[  115.853441] RSP: 0018:ffffb8d5443d7d50 EFLAGS: 00010282
[  115.853865] RAX: ffff8e4d87c41c80 RBX: ffff8e4d88ded780 RCX: ffff8e4da4333a00
[  115.854458] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8e4d88ded780
[  115.855044] RBP: ffff8e4d88ded780 R08: ffff8e4da4338000 R09: ffff8e4da43388c0
[  115.855693] R10: 0000000000000002 R11: ffffb8d540158000 R12: ffffb8d5443d7da8
[  115.856304] R13: ffff8e4d88ded780 R14: 0000000000000000 R15: 0000000000000000
[  115.856859] FS:  00007f92c90c9800(0000) GS:ffff8e4dfdc00000(0000)
knlGS:0000000000000000
[  115.857531] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  115.858006] CR2: 0000000000000010 CR3: 0000000022f4c002 CR4: 00000000000706f0
[  115.858598] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  115.859393] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  115.860099] Call Trace:
[  115.860358]  <TASK>
[  115.860535]  propagate_mnt+0x14d/0x190
[  115.860848]  attach_recursive_mnt+0x274/0x3e0
[  115.861212]  path_mount+0x8c8/0xa60
[  115.861503]  __x64_sys_mount+0xf6/0x140
[  115.861819]  do_syscall_64+0x5b/0x80
[  115.862117]  ? do_faccessat+0x123/0x250
[  115.862435]  ? syscall_exit_to_user_mode+0x17/0x40
[  115.862826]  ? do_syscall_64+0x67/0x80
[  115.863133]  ? syscall_exit_to_user_mode+0x17/0x40
[  115.863527]  ? do_syscall_64+0x67/0x80
[  115.863835]  ? do_syscall_64+0x67/0x80
[  115.864144]  ? do_syscall_64+0x67/0x80
[  115.864452]  ? exc_page_fault+0x70/0x170
[  115.864775]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[  115.865187] RIP: 0033:0x7f92c92b0ebe
[  115.865480] Code: 48 8b 0d 75 4f 0c 00 f7 d8 64 89 01 48 83 c8 ff
c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 49 89 ca b8 a5 00 00
00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 42 4f 0c 00 f7 d8 64 89
01 48
[  115.866984] RSP: 002b:00007fff000aa728 EFLAGS: 00000246 ORIG_RAX:
00000000000000a5
[  115.867607] RAX: ffffffffffffffda RBX: 000055a77888d6b0 RCX: 00007f92c92b0ebe
[  115.868240] RDX: 000055a77888d8e0 RSI: 000055a77888e6e0 RDI: 000055a77888e620
[  115.868823] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
[  115.869403] R10: 0000000000001000 R11: 0000000000000246 R12: 000055a77888e620
[  115.869994] R13: 000055a77888d8e0 R14: 00000000ffffffff R15: 00007f92c93e4076
[  115.870581]  </TASK>
[  115.870763] Modules linked in: nft_fib_inet nft_fib_ipv4
nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6
nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6
nf_defrag_ipv4 ip_set rfkill nf_tables nfnetlink qrtr snd_intel8x0
sunrpc snd_ac97_codec ac97_bus snd_pcm snd_timer intel_rapl_msr
intel_rapl_common snd vboxguest intel_powerclamp video rapl joydev
soundcore i2c_piix4 wmi fuse zram xfs vmwgfx crct10dif_pclmul
crc32_pclmul crc32c_intel polyval_clmulni polyval_generic
drm_ttm_helper ttm e1000 ghash_clmulni_intel serio_raw ata_generic
pata_acpi scsi_dh_rdac scsi_dh_emc scsi_dh_alua dm_multipath
[  115.875288] CR2: 0000000000000010
[  115.875641] ---[ end trace 0000000000000000 ]---
[  115.876135] RIP: 0010:propagate_one.part.0+0x7f/0x1a0
[  115.876551] Code: 75 eb 4c 8b 05 c2 25 37 02 4c 89 ca 48 8b 4a 10
49 39 d0 74 1e 48 3b 81 e0 00 00 00 74 26 48 8b 92 e0 00 00 00 be 01
00 00 00 <48> 8b 4a 10 49 39 d0 75 e2 40 84 f6 74 38 4c 89 05 84 25 37
02 4d
[  115.878086] RSP: 0018:ffffb8d5443d7d50 EFLAGS: 00010282
[  115.878511] RAX: ffff8e4d87c41c80 RBX: ffff8e4d88ded780 RCX: ffff8e4da4333a00
[  115.879128] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8e4d88ded780
[  115.879715] RBP: ffff8e4d88ded780 R08: ffff8e4da4338000 R09: ffff8e4da43388c0
[  115.880359] R10: 0000000000000002 R11: ffffb8d540158000 R12: ffffb8d5443d7da8
[  115.880962] R13: ffff8e4d88ded780 R14: 0000000000000000 R15: 0000000000000000
[  115.881548] FS:  00007f92c90c9800(0000) GS:ffff8e4dfdc00000(0000)
knlGS:0000000000000000
[  115.882234] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  115.882713] CR2: 0000000000000010 CR3: 0000000022f4c002 CR4: 00000000000706f0
[  115.883314] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  115.883966] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Fixes: f2ebb3a9 ("smarter propagate_mnt()")
Fixes: 5ec0811d ("propogate_mnt: Handle the first propogated copy being a slave")
Cc: <stable@vger.kernel.org>
Reported-by: Ditang Chen <ditang.c@gmail.com>
Signed-off-by: Seth Forshee (Digital Ocean) <sforshee@kernel.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
---
If there are no big objections I'll get this to Linus rather sooner than later.