linux-cve-announce - CVE-2025-71080: ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT

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Message-ID: <2026011338-CVE-2025-71080-f9ae@gregkh>
Date: Tue, 13 Jan 2026 16:35:39 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-71080: ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT

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

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

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

ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT

On PREEMPT_RT kernels, after rt6_get_pcpu_route() returns NULL, the
current task can be preempted. Another task running on the same CPU
may then execute rt6_make_pcpu_route() and successfully install a
pcpu_rt entry. When the first task resumes execution, its cmpxchg()
in rt6_make_pcpu_route() will fail because rt6i_pcpu is no longer
NULL, triggering the BUG_ON(prev). It's easy to reproduce it by adding
mdelay() after rt6_get_pcpu_route().

Using preempt_disable/enable is not appropriate here because
ip6_rt_pcpu_alloc() may sleep.

Fix this by handling the cmpxchg() failure gracefully on PREEMPT_RT:
free our allocation and return the existing pcpu_rt installed by
another task. The BUG_ON is replaced by WARN_ON_ONCE for non-PREEMPT_RT
kernels where such races should not occur.

The Linux kernel CVE team has assigned CVE-2025-71080 to this issue.

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

	Issue introduced in 6.12 with commit d2d6422f8bd17c6bb205133e290625a564194496 and fixed in 6.12.64 with commit 1dc33ad0867325f8d2c6d7b2a6f542d4f3121f66
	Issue introduced in 6.12 with commit d2d6422f8bd17c6bb205133e290625a564194496 and fixed in 6.18.4 with commit 787515ccb2292f82eb0876993129154629a49651
	Issue introduced in 6.12 with commit d2d6422f8bd17c6bb205133e290625a564194496 and fixed in 6.19-rc4 with commit 1adaea51c61b52e24e7ab38f7d3eba023b2d050d

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-2025-71080
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/ipv6/route.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/1dc33ad0867325f8d2c6d7b2a6f542d4f3121f66
	https://git.kernel.org/stable/c/787515ccb2292f82eb0876993129154629a49651
	https://git.kernel.org/stable/c/1adaea51c61b52e24e7ab38f7d3eba023b2d050d