CVE-2026-64095: batman-adv: bla: avoid double decrement of bla.num_requests

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

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

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

batman-adv: bla: avoid double decrement of bla.num_requests

The bla.num_requests is increased when no request_sent was in progress. And
it is decremented in various places (announcement was received, backbone is
purged, periodic work). But the check if the request_sent is actually set
to a specific state and the atomic_dec/_inc are not safe because they are
not atomic (TOCTOU) and multiple such code portions can run concurrently.

At the same time, it is necessary to modify request_sent (state) and
bla.num_requests atomically. Otherwise batadv_bla_send_request() might set
request_sent to 1 and is interrupted.  batadv_handle_announce() can then
set request_sent back to 0 and decrement num_requests before
batadv_bla_send_request() incremented it.

The two operations must therefore be locked. And since state (request_sent)
and wait_periods are only accessed inside this lock, they can be converted
to simpler datatypes. And to avoid that the bla.num_requests is touched by
a parallel running context with a valid backbone_gw reference after
batadv_bla_purge_backbone_gw() ran, a third state "stopped" is required to
correctly signal that a backbone_gw is in the state of being cleaned up.

The Linux kernel CVE team has assigned CVE-2026-64095 to this issue.


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

	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 5.10.259 with commit 1f013bc94154f2e78e97d0296175664224c796e0
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 5.15.210 with commit 5328b95960774f2e189f22485616bc7b8eb2f7e3
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.1.176 with commit 8ff9c59d1b7b48c2596878341a5310f32895d52b
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.6.143 with commit a9393751ecf7e9096f93cb6eed02db4f79125765
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.12.93 with commit 461f1e3dfb888701895b766446c55db2b10db705
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 6.18.34 with commit 45384612f29692fbf0c770200361a7acff90125c
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 7.0.11 with commit 65497ad155a3246df177b5ef662cd6e5a32cb470
	Issue introduced in 3.5 with commit 23721387c409087fd3b97e274f34d3ddc0970b74 and fixed in 7.1 with commit 83ab69bd12b80f6ea169c8bea6977701b53a043d

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-2026-64095
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
	net/batman-adv/mesh-interface.c
	net/batman-adv/types.h


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/1f013bc94154f2e78e97d0296175664224c796e0
	https://git.kernel.org/stable/c/5328b95960774f2e189f22485616bc7b8eb2f7e3
	https://git.kernel.org/stable/c/8ff9c59d1b7b48c2596878341a5310f32895d52b
	https://git.kernel.org/stable/c/a9393751ecf7e9096f93cb6eed02db4f79125765
	https://git.kernel.org/stable/c/461f1e3dfb888701895b766446c55db2b10db705
	https://git.kernel.org/stable/c/45384612f29692fbf0c770200361a7acff90125c
	https://git.kernel.org/stable/c/65497ad155a3246df177b5ef662cd6e5a32cb470
	https://git.kernel.org/stable/c/83ab69bd12b80f6ea169c8bea6977701b53a043d