CVE-2026-93203: batman-adv: bla: avoid CRC corruption due to parallel claim add

CVE-2026-93203: batman-adv: bla: avoid CRC corruption due to parallel claim add

[Greg Kroah-Hartman]

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

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

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

batman-adv: bla: avoid CRC corruption due to parallel claim add

batadv_bla_add_claim() is used to add claims and modify the backbone of
claims for CLAIM frames from remote backbones and local packets. When it
handles a claim, it needs to either

* add the new claim's CRC to the backbone CRC
* remove the already existing claim's CRC from the old backbone and add it
  to the new backbone

But when the "new" claim code was running in parallel to the "change
backbone" code, it can happen that the CRC was invalid because the
backbone_gw of the claim was changed twice in the "new" claim code path:

* CPU0 creates the claim for gateway A and publishes it in the claim
  hash. The crc16 of the address has not yet been added to A's crc at
  this point.

* CPU1 processes a claim frame of gateway B for the same client, finds
  the just published claim, and performs the ownership change: it
  switches the pointer to B, removes the crc16 from A's crc - which
  never contained it - and adds it to B's crc.

* CPU0 continues behind the creation branch, unconditionally switches
  the pointer back to A without compensating B's crc (its remove_crc
  is false for the creation path), and finally adds the crc16 to A's
  crc

The CRC is then wrong for both:

* claim belongs to A: but CRC is not part of backbone A's CRC
* claim doesn't belong to B: CRC is still part of backbone B's CRC

This wrong CRC is never recomputated from the stored claims. For local
backbone claims, this can also not recovered using syncs.

To avoid this, split the functionality in clear separate parts:

* new claim which always adds claim CRC to the backbone CRC (but never
  changes the already set backbone_gw of the claim back)

* update of existing claim which automatically changes the backbone_gw
  entry and only updates both backbone CRCs when there was an actual change

The Linux kernel CVE team has assigned CVE-2026-93203 to this issue.


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

	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 5.10.270 with commit 3d4a9ae2c69c0fe4c3c69587288241243bb7a39b
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 5.15.221 with commit ad982d925f00d860d2467811256ffadcccd9e4a7
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.1.188 with commit d5b0e7574da5a485f4dbec32a15911900a4487c2
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.6.157 with commit a3e07db74f22a85aeeb51c600da5c98b9cbca11f
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.12.110 with commit 1d390db003cbb52cdd86837608bff3794501a5ea
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.18.52 with commit 8bcfa58c4277bff1882a23547c37ab08aa1de12f
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 7.2.6 with commit e54602d9e38db8573f2097d60c7ea8be0a5af1fa
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 7.3-rc1 with commit 08645ab95768b88e2ff85a89211994651710465b

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-2026-93203
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/batman-adv/bridge_loop_avoidance.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/3d4a9ae2c69c0fe4c3c69587288241243bb7a39b
	https://git.kernel.org/stable/c/ad982d925f00d860d2467811256ffadcccd9e4a7
	https://git.kernel.org/stable/c/d5b0e7574da5a485f4dbec32a15911900a4487c2
	https://git.kernel.org/stable/c/a3e07db74f22a85aeeb51c600da5c98b9cbca11f
	https://git.kernel.org/stable/c/1d390db003cbb52cdd86837608bff3794501a5ea
	https://git.kernel.org/stable/c/8bcfa58c4277bff1882a23547c37ab08aa1de12f
	https://git.kernel.org/stable/c/e54602d9e38db8573f2097d60c7ea8be0a5af1fa
	https://git.kernel.org/stable/c/08645ab95768b88e2ff85a89211994651710465b