CVE-2026-89563: ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

CVE-2026-89563: ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

[Greg Kroah-Hartman]

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

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

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

ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

ip6_tnl_xmit() may need to expand headroom before it can push the
outer IPv6 and optional encap headers. It currently does that with
skb_realloc_headroom(), copies skb->sk ownership, consumes the original
skb, and then continues processing with the replacement skb kept only in
its local variable.

That is safe only if the helper cannot fail afterwards. But this helper
still has post-reallocation error exits. collect_md tunnels reject
non-NONE encap after the replacement, and ip6_tnl_encap() can also fail
later. In those cases the helper returns an error to its callers while
the caller still only has the original skb pointer.

Both ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on
error, so they can end up freeing an skb that ip6_tnl_xmit() already
consumed.

Use skb_cow_head() instead. It provides the required headroom and
writability without privately replacing the caller-owned skb, so later
error returns cannot leave callers with a stale pointer.

The Ethernet users, ip6gretap and ip6erspan, clear IFF_TX_SKB_SHARING
and already call skb_cow_head() before entering ip6_tnl_xmit(). They do
not rely on the removed skb_shared() reallocation. This also makes the
IPv6 tunnel path consistent with ip_tunnel_xmit().

The Linux kernel CVE team has assigned CVE-2026-89563 to this issue.


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

	Issue introduced in 4.7 with commit 058214a4d1dfefed9f01a277fadd3590acb5f990 and fixed in 6.12.109 with commit 37df5bc6b5ccad88c2faa0f5466f150178882b74
	Issue introduced in 4.7 with commit 058214a4d1dfefed9f01a277fadd3590acb5f990 and fixed in 6.18.50 with commit b36dfd6e8cff00cea81c11cc0af88d1564763461
	Issue introduced in 4.7 with commit 058214a4d1dfefed9f01a277fadd3590acb5f990 and fixed in 7.2.4 with commit 8fc56ca49fc0ee1725b55c67bb43ab0dd01f3f52
	Issue introduced in 4.7 with commit 058214a4d1dfefed9f01a277fadd3590acb5f990 and fixed in 7.3-rc1 with commit 87f21b59ddc618eff9670c174842964ad65fdade

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-2026-89563
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/ip6_tunnel.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/37df5bc6b5ccad88c2faa0f5466f150178882b74
	https://git.kernel.org/stable/c/b36dfd6e8cff00cea81c11cc0af88d1564763461
	https://git.kernel.org/stable/c/8fc56ca49fc0ee1725b55c67bb43ab0dd01f3f52
	https://git.kernel.org/stable/c/87f21b59ddc618eff9670c174842964ad65fdade