Subject: CVE-2024-26837: net: bridge: switchdev: Skip MDB replays of deferred events on offload

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

net: bridge: switchdev: Skip MDB replays of deferred events on offload

Before this change, generation of the list of MDB events to replay
would race against the creation of new group memberships, either from
the IGMP/MLD snooping logic or from user configuration.

While new memberships are immediately visible to walkers of
br->mdb_list, the notification of their existence to switchdev event
subscribers is deferred until a later point in time. So if a replay
list was generated during a time that overlapped with such a window,
it would also contain a replay of the not-yet-delivered event.

The driver would thus receive two copies of what the bridge internally
considered to be one single event. On destruction of the bridge, only
a single membership deletion event was therefore sent. As a
consequence of this, drivers which reference count memberships (at
least DSA), would be left with orphan groups in their hardware
database when the bridge was destroyed.

This is only an issue when replaying additions. While deletion events
may still be pending on the deferred queue, they will already have
been removed from br->mdb_list, so no duplicates can be generated in
that scenario.

To a user this meant that old group memberships, from a bridge in
which a port was previously attached, could be reanimated (in
hardware) when the port joined a new bridge, without the new bridge's
knowledge.

For example, on an mv88e6xxx system, create a snooping bridge and
immediately add a port to it:

    root@infix-06-0b-00:~$ ip link add dev br0 up type bridge mcast_snooping 1 && \
    > ip link set dev x3 up master br0

And then destroy the bridge:

    root@infix-06-0b-00:~$ ip link del dev br0
    root@infix-06-0b-00:~$ mvls atu
    ADDRESS             FID  STATE      Q  F  0  1  2  3  4  5  6  7  8  9  a
    DEV:0 Marvell 88E6393X
    33:33:00:00:00:6a     1  static     -  -  0  .  .  .  .  .  .  .  .  .  .
    33:33:ff:87:e4:3f     1  static     -  -  0  .  .  .  .  .  .  .  .  .  .
    ff:ff:ff:ff:ff:ff     1  static     -  -  0  1  2  3  4  5  6  7  8  9  a
    root@infix-06-0b-00:~$

The two IPv6 groups remain in the hardware database because the
port (x3) is notified of the host's membership twice: once via the
original event and once via a replay. Since only a single delete
notification is sent, the count remains at 1 when the bridge is
destroyed.

Then add the same port (or another port belonging to the same hardware
domain) to a new bridge, this time with snooping disabled:

    root@infix-06-0b-00:~$ ip link add dev br1 up type bridge mcast_snooping 0 && \
    > ip link set dev x3 up master br1

All multicast, including the two IPv6 groups from br0, should now be
flooded, according to the policy of br1. But instead the old
memberships are still active in the hardware database, causing the
switch to only forward traffic to those groups towards the CPU (port
0).

Eliminate the race in two steps:

1. Grab the write-side lock of the MDB while generating the replay
   list.

This prevents new memberships from showing up while we are generating
the replay list. But it leaves the scenario in which a deferred event
was already generated, but not delivered, before we grabbed the
lock. Therefore:

2. Make sure that no deferred version of a replay event is already
   enqueued to the switchdev deferred queue, before adding it to the
   replay list, when replaying additions.

The Linux kernel CVE team has assigned CVE-2024-26837 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.13 with commit 4f2673b3a2b6 and fixed in 6.1.80 with commit 2d5b4b3376fa
	Issue introduced in 5.13 with commit 4f2673b3a2b6 and fixed in 6.6.19 with commit 603be95437e7
	Issue introduced in 5.13 with commit 4f2673b3a2b6 and fixed in 6.7.7 with commit e0b4c5b1d760
	Issue introduced in 5.13 with commit 4f2673b3a2b6 and fixed in 6.8 with commit dc489f86257c

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26837
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	include/net/switchdev.h
	net/bridge/br_switchdev.c
	net/switchdev/switchdev.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/2d5b4b3376fa146a23917b8577064906d643925f
	https://git.kernel.org/stable/c/603be95437e7fd85ba694e75918067fb9e7754db
	https://git.kernel.org/stable/c/e0b4c5b1d760008f1dd18c07c35af0442e54f9c8
	https://git.kernel.org/stable/c/dc489f86257cab5056e747344f17a164f63bff4b