Subject: CVE-2024-27415: netfilter: bridge: confirm multicast packets before passing them up the stack

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

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

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

netfilter: bridge: confirm multicast packets before passing them up the stack

conntrack nf_confirm logic cannot handle cloned skbs referencing
the same nf_conn entry, which will happen for multicast (broadcast)
frames on bridges.

 Example:
    macvlan0
       |
      br0
     /  \
  ethX    ethY

 ethX (or Y) receives a L2 multicast or broadcast packet containing
 an IP packet, flow is not yet in conntrack table.

 1. skb passes through bridge and fake-ip (br_netfilter)Prerouting.
    -> skb->_nfct now references a unconfirmed entry
 2. skb is broad/mcast packet. bridge now passes clones out on each bridge
    interface.
 3. skb gets passed up the stack.
 4. In macvlan case, macvlan driver retains clone(s) of the mcast skb
    and schedules a work queue to send them out on the lower devices.

    The clone skb->_nfct is not a copy, it is the same entry as the
    original skb.  The macvlan rx handler then returns RX_HANDLER_PASS.
 5. Normal conntrack hooks (in NF_INET_LOCAL_IN) confirm the orig skb.

The Macvlan broadcast worker and normal confirm path will race.

This race will not happen if step 2 already confirmed a clone. In that
case later steps perform skb_clone() with skb->_nfct already confirmed (in
hash table).  This works fine.

But such confirmation won't happen when eb/ip/nftables rules dropped the
packets before they reached the nf_confirm step in postrouting.

Pablo points out that nf_conntrack_bridge doesn't allow use of stateful
nat, so we can safely discard the nf_conn entry and let inet call
conntrack again.

This doesn't work for bridge netfilter: skb could have a nat
transformation. Also bridge nf prevents re-invocation of inet prerouting
via 'sabotage_in' hook.

Work around this problem by explicit confirmation of the entry at LOCAL_IN
time, before upper layer has a chance to clone the unconfirmed entry.

The downside is that this disables NAT and conntrack helpers.

Alternative fix would be to add locking to all code parts that deal with
unconfirmed packets, but even if that could be done in a sane way this
opens up other problems, for example:

-m physdev --physdev-out eth0 -j SNAT --snat-to 1.2.3.4
-m physdev --physdev-out eth1 -j SNAT --snat-to 1.2.3.5

For multicast case, only one of such conflicting mappings will be
created, conntrack only handles 1:1 NAT mappings.

Users should set create a setup that explicitly marks such traffic
NOTRACK (conntrack bypass) to avoid this, but we cannot auto-bypass
them, ruleset might have accept rules for untracked traffic already,
so user-visible behaviour would change.

The Linux kernel CVE team has assigned CVE-2024-27415 to this issue.

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

	Issue introduced in 2.6.12 with commit 1da177e4c3f4 and fixed in 5.15.151 with commit 7c3f28599652
	Issue introduced in 2.6.12 with commit 1da177e4c3f4 and fixed in 6.1.81 with commit 2b1414d5e94e
	Issue introduced in 2.6.12 with commit 1da177e4c3f4 and fixed in 6.6.21 with commit 80cd0487f630
	Issue introduced in 2.6.12 with commit 1da177e4c3f4 and fixed in 6.7.9 with commit cb734975b0ff
	Issue introduced in 2.6.12 with commit 1da177e4c3f4 and fixed in 6.8 with commit 62e7151ae3eb

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-2024-27415
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/linux/netfilter.h
	net/bridge/br_netfilter_hooks.c
	net/bridge/netfilter/nf_conntrack_bridge.c
	net/netfilter/nf_conntrack_core.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/7c3f28599652acf431a2211168de4a583f30b6d5
	https://git.kernel.org/stable/c/2b1414d5e94e477edff1d2c79030f1d742625ea0
	https://git.kernel.org/stable/c/80cd0487f630b5382734997c3e5e3003a77db315
	https://git.kernel.org/stable/c/cb734975b0ffa688ff6cc0eed463865bf07b6c01
	https://git.kernel.org/stable/c/62e7151ae3eb465e0ab52a20c941ff33bb6332e9