linux-cve-announce - CVE-2025-37864: net: dsa: clean up FDB, MDB, VLAN entries on unbind

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Message-ID: <2025050957-CVE-2025-37864-ffd5@gregkh>
Date: Fri,  9 May 2025 08:43:58 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-37864: net: dsa: clean up FDB, MDB, VLAN entries on unbind

From: Greg Kroah-Hartman <gregkh@...nel.org>

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

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

net: dsa: clean up FDB, MDB, VLAN entries on unbind

As explained in many places such as commit b117e1e8a86d ("net: dsa:
delete dsa_legacy_fdb_add and dsa_legacy_fdb_del"), DSA is written given
the assumption that higher layers have balanced additions/deletions.
As such, it only makes sense to be extremely vocal when those
assumptions are violated and the driver unbinds with entries still
present.

But Ido Schimmel points out a very simple situation where that is wrong:
https://lore.kernel.org/netdev/ZDazSM5UsPPjQuKr@shredder/
(also briefly discussed by me in the aforementioned commit).

Basically, while the bridge bypass operations are not something that DSA
explicitly documents, and for the majority of DSA drivers this API
simply causes them to go to promiscuous mode, that isn't the case for
all drivers. Some have the necessary requirements for bridge bypass
operations to do something useful - see dsa_switch_supports_uc_filtering().

Although in tools/testing/selftests/net/forwarding/local_termination.sh,
we made an effort to popularize better mechanisms to manage address
filters on DSA interfaces from user space - namely macvlan for unicast,
and setsockopt(IP_ADD_MEMBERSHIP) - through mtools - for multicast, the
fact is that 'bridge fdb add ... self static local' also exists as
kernel UAPI, and might be useful to someone, even if only for a quick
hack.

It seems counter-productive to block that path by implementing shim
.ndo_fdb_add and .ndo_fdb_del operations which just return -EOPNOTSUPP
in order to prevent the ndo_dflt_fdb_add() and ndo_dflt_fdb_del() from
running, although we could do that.

Accepting that cleanup is necessary seems to be the only option.
Especially since we appear to be coming back at this from a different
angle as well. Russell King is noticing that the WARN_ON() triggers even
for VLANs:
https://lore.kernel.org/netdev/Z_li8Bj8bD4-BYKQ@shell.armlinux.org.uk/

What happens in the bug report above is that dsa_port_do_vlan_del() fails,
then the VLAN entry lingers on, and then we warn on unbind and leak it.

This is not a straight revert of the blamed commit, but we now add an
informational print to the kernel log (to still have a way to see
that bugs exist), and some extra comments gathered from past years'
experience, to justify the logic.

The Linux kernel CVE team has assigned CVE-2025-37864 to this issue.

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

	Issue introduced in 5.18 with commit 0832cd9f1f023226527e95002d537123061ddac4 and fixed in 6.6.88 with commit 86c6613a69bca815f1865ed8cedfd4b9142621ab
	Issue introduced in 5.18 with commit 0832cd9f1f023226527e95002d537123061ddac4 and fixed in 6.12.25 with commit 8fcc1e6f808912977caf17366c625b95dc29ba4f
	Issue introduced in 5.18 with commit 0832cd9f1f023226527e95002d537123061ddac4 and fixed in 6.14.4 with commit 99c50c98803425378e08a7394dc885506dc85f06
	Issue introduced in 5.18 with commit 0832cd9f1f023226527e95002d537123061ddac4 and fixed in 6.15-rc3 with commit 7afb5fb42d4950f33af2732b8147c552659f79b7

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-2025-37864
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:
	net/dsa/dsa.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/86c6613a69bca815f1865ed8cedfd4b9142621ab
	https://git.kernel.org/stable/c/8fcc1e6f808912977caf17366c625b95dc29ba4f
	https://git.kernel.org/stable/c/99c50c98803425378e08a7394dc885506dc85f06
	https://git.kernel.org/stable/c/7afb5fb42d4950f33af2732b8147c552659f79b7