CVE-2026-80839: batman-adv: reject unrepresentable multicast TVLV offsets

CVE-2026-80839: batman-adv: reject unrepresentable multicast TVLV offsets

[Greg Kroah-Hartman]

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

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

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

batman-adv: reject unrepresentable multicast TVLV offsets

The network and transport header fields in struct sk_buff are 16-bit
offsets from skb->head, and U16_MAX is reserved as the unset transport
header value. batadv_tvlv_call_handler() sets both fields from a received
multicast TVLV without checking whether the TVLV end is representable.

If the end offset exceeds the field's range, skb_set_transport_header()
truncates it so that the transport header precedes the network header.
The negative difference is then returned by skb_network_header_len() as
a large u32. batadv_mcast_forw_packet() consequently accepts an oversized
multicast tracker and accesses memory beyond the skb data.

Add skb_set_transport_header_careful(), an offset-aware counterpart to
skb_reset_transport_header_careful(), which validates the final
head-relative offset before assigning it. Use the new helper in
batadv_tvlv_call_handler() and reject unrepresentable TVLVs before
setting the network header.

The Linux kernel CVE team has assigned CVE-2026-80839 to this issue.


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

	Issue introduced in 6.8 with commit 07afe1ba288c04280622fa002ed385f1ac0b6fe6 and fixed in 6.12.108 with commit da1f5aa7ec93f2cc17f5cd30efc54f62af433cf2
	Issue introduced in 6.8 with commit 07afe1ba288c04280622fa002ed385f1ac0b6fe6 and fixed in 6.18.49 with commit 916ec741e65af072b98e475feaad98c063da1b7c
	Issue introduced in 6.8 with commit 07afe1ba288c04280622fa002ed385f1ac0b6fe6 and fixed in 7.1.13 with commit 1b466746fe109127fd983100a228cdd1f1f6ece2
	Issue introduced in 6.8 with commit 07afe1ba288c04280622fa002ed385f1ac0b6fe6 and fixed in 7.2.3 with commit 2b46baa591d0a7c16b62f150917187e70d053be6
	Issue introduced in 6.8 with commit 07afe1ba288c04280622fa002ed385f1ac0b6fe6 and fixed in 7.3-rc1 with commit f12c2de4f542e3220e17e0606f492110064f04cb

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-2026-80839
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/skbuff.h
	net/batman-adv/tvlv.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/da1f5aa7ec93f2cc17f5cd30efc54f62af433cf2
	https://git.kernel.org/stable/c/916ec741e65af072b98e475feaad98c063da1b7c
	https://git.kernel.org/stable/c/1b466746fe109127fd983100a228cdd1f1f6ece2
	https://git.kernel.org/stable/c/2b46baa591d0a7c16b62f150917187e70d053be6
	https://git.kernel.org/stable/c/f12c2de4f542e3220e17e0606f492110064f04cb