CVE-2026-63994: tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]()

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

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

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

tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]()

Sashiko found that iptunnel_pmtud_build_icmp() and
iptunnel_pmtud_build_icmpv6() were caching ip_hdr() and ipv6_hdr()
before an skb_cow() call which can reallocate skb->head.

Fix this possible UAF by initializing the local variables
after the skb_cow() call.

Remove skb_reset_network_header() calls which were not needed.

The Linux kernel CVE team has assigned CVE-2026-63994 to this issue.


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

	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 5.10.259 with commit 95b6d772bfe788331d9742d73eaa12e113b2adc4
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 5.15.210 with commit 7254aef4d1a7e18e887af9010e2f2dc34806789b
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 6.1.176 with commit bf8b3f34c37c162357138e7c0942723b8b94fed1
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 6.6.143 with commit 76cd9398a0470257ab765bdf5f358a2af2e17934
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 6.12.93 with commit 50750d86a2e5266aba0c295483b3397843198b11
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 6.18.35 with commit 6dff77899b9e9fe5d854abda3a98ad04e7229ef7
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 7.0.12 with commit f3f204541f280a6ecb04503a0d6794d93990ca43
	Issue introduced in 5.9 with commit 4cb47a8644cc9eb8ec81190a50e79e6530d0297f and fixed in 7.1 with commit b4bc94353050b1fa7b702bd4c6600710dd926cff

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-2026-63994
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/ipv4/ip_tunnel_core.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/95b6d772bfe788331d9742d73eaa12e113b2adc4
	https://git.kernel.org/stable/c/7254aef4d1a7e18e887af9010e2f2dc34806789b
	https://git.kernel.org/stable/c/bf8b3f34c37c162357138e7c0942723b8b94fed1
	https://git.kernel.org/stable/c/76cd9398a0470257ab765bdf5f358a2af2e17934
	https://git.kernel.org/stable/c/50750d86a2e5266aba0c295483b3397843198b11
	https://git.kernel.org/stable/c/6dff77899b9e9fe5d854abda3a98ad04e7229ef7
	https://git.kernel.org/stable/c/f3f204541f280a6ecb04503a0d6794d93990ca43
	https://git.kernel.org/stable/c/b4bc94353050b1fa7b702bd4c6600710dd926cff