CVE-2026-63829: net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink

CVE-2026-63829: net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink

[Greg Kroah-Hartman]

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

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

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

net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink

A tunnel changelink() operates on at most two netns, dev_net(dev) and
the tunnel link netns t->net. They differ once the device is created in
or moved to a netns other than the one the request runs in. The rtnl
changelink path checks CAP_NET_ADMIN only against dev_net(dev), so a
caller privileged there but not in t->net can rewrite a tunnel that
lives in t->net.

Add rtnl_dev_link_net_capable() next to rtnl_get_net_ns_capable() in
net/core/rtnetlink.c. It requires CAP_NET_ADMIN in the link netns and is
skipped when the link netns is dev_net(dev), where the rtnl path already
checked it. The other patches in this series use the same helper.

Gate ipgre_changelink() and erspan_changelink() with it, at the top of
the op before any attribute is parsed, because the parsers update live
tunnel fields first. ipgre_netlink_parms() sets t->collect_md before
ip_tunnel_changelink() runs.

Commit 8b484efd5cb4 ("ip6: vti: Use ip6_tnl.net in
vti6_siocdevprivate().") added the same check on the ioctl path. This
adds it on RTM_NEWLINK.

The Linux kernel CVE team has assigned CVE-2026-63829 to this issue.


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

	Issue introduced in 3.16 with commit b57708add31494175be741ed3fd24023b50c3423 and fixed in 6.12.95 with commit 9831bc9ecb402957810c2045c663fbfe9b09e296
	Issue introduced in 3.16 with commit b57708add31494175be741ed3fd24023b50c3423 and fixed in 6.18.38 with commit 1697957eb0971d420dde42862b88eb43506a1105
	Issue introduced in 3.16 with commit b57708add31494175be741ed3fd24023b50c3423 and fixed in 7.1.3 with commit 47b5d3d506609b08b2e1f7c14f0b681a1953d572
	Issue introduced in 3.16 with commit b57708add31494175be741ed3fd24023b50c3423 and fixed in 7.2-rc1 with commit 8165f7ff57d9667d2bb477ef6af83ede7fed4ad7

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-2026-63829
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/net/rtnetlink.h
	net/core/rtnetlink.c
	net/ipv4/ip_gre.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/9831bc9ecb402957810c2045c663fbfe9b09e296
	https://git.kernel.org/stable/c/1697957eb0971d420dde42862b88eb43506a1105
	https://git.kernel.org/stable/c/47b5d3d506609b08b2e1f7c14f0b681a1953d572
	https://git.kernel.org/stable/c/8165f7ff57d9667d2bb477ef6af83ede7fed4ad7