CVE-2026-74659: net: bridge: mrp: fix uninitialised bytes on the wire

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

net: bridge: mrp: fix uninitialised bytes on the wire

br_mrp_alloc_test_skb() builds MRP test frames on an skb from
dev_alloc_skb(), which does not clear the linear data area.  On the MRA
ring-role branch the sub-option TLV header is appended with

	sub_tlv = skb_put(skb, sizeof(*sub_tlv));
	sub_tlv->type = BR_MRP_SUB_TLV_HEADER_TEST_AUTO_MGR;

so sub_tlv->length is never written, and the two trailing alignment bytes
are appended with a bare skb_put() that does not clear them either.  The
neighbouring oui and sub_opt regions are explicitly zeroed, so three
uninitialised bytes are left in every MRA MRP_Test frame that goes out.

Put the sub-option TLV header and the alignment padding in a single
skb_put_zero(), which clears both.  The AUTO_MGR sub-TLV carries no
payload, so the zeroed length field is already the value it should have.

The Linux kernel CVE team has assigned CVE-2026-74659 to this issue.


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

	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 5.15.216 with commit 014c062d23c63ec77ef2cf17a0d9363c7441cc94
	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 6.1.183 with commit 7ebc23ff03668042e0b0e4034bb1518d36198d9e
	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 6.6.152 with commit 06d58b8d2f053ced82e01efaeb6e7c82891eed58
	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 6.12.104 with commit a5e385eeb2d6dbbbdebfa050e67c34734ae12693
	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 6.18.45 with commit 5912cf1822fbe53ae275c147868740eb384a5d3e
	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 7.1.9 with commit e08665218040f8e312abe40f74543186f3c2c941
	Issue introduced in 5.14 with commit f7458934b0791c39a001e4d902fc3bf697b439b5 and fixed in 7.2 with commit 63488dba65ef91373ef616575b32eb0eb21459f4

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-2026-74659
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/bridge/br_mrp.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/014c062d23c63ec77ef2cf17a0d9363c7441cc94
	https://git.kernel.org/stable/c/7ebc23ff03668042e0b0e4034bb1518d36198d9e
	https://git.kernel.org/stable/c/06d58b8d2f053ced82e01efaeb6e7c82891eed58
	https://git.kernel.org/stable/c/a5e385eeb2d6dbbbdebfa050e67c34734ae12693
	https://git.kernel.org/stable/c/5912cf1822fbe53ae275c147868740eb384a5d3e
	https://git.kernel.org/stable/c/e08665218040f8e312abe40f74543186f3c2c941
	https://git.kernel.org/stable/c/63488dba65ef91373ef616575b32eb0eb21459f4