CVE-2026-68302: amt: re-read skb header pointers after every pull

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

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

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

amt: re-read skb header pointers after every pull

Several AMT receive and transmit paths cache a pointer into the skb head
(ip_hdr(), ipv6_hdr(), eth_hdr() or the AMT message header) and then call
a helper that can reallocate that head before the cached pointer is used
again.  pskb_may_pull(), ip_mc_may_pull(), ipv6_mc_may_pull(),
iptunnel_pull_header(), ip_mc_check_igmp() and ipv6_mc_check_mld() can all
free the old head and move the data, so a pointer taken before the call
dangles afterwards and the later access is a use-after-free of the freed
head.

The affected sites are:

  amt_rcv() caches ip_hdr() before amt_parse_type() pulls, then reads
  iph->saddr.

  amt_dev_xmit() caches ip_hdr()/ipv6_hdr() before ip_mc_check_igmp()/
  ipv6_mc_check_mld() and pskb_may_pull(), then reads the group address.

  amt_multicast_data_handler() caches eth_hdr() before pskb_may_pull(),
  then writes the L2 header.

  amt_membership_query_handler() caches the AMT header, the outer and
  inner eth_hdr() and ip_hdr() before iptunnel_pull_header() and several
  pulls, then reads and writes them.

  amt_igmpv3_report_handler() and amt_mldv2_report_handler() cache
  ip_hdr()/ipv6_hdr() and the current group record and read the record
  count from the report header inside the record loop, across the
  *_mc_may_pull() calls.

  amt_update_handler() caches ip_hdr() and the AMT membership-update
  header before pskb_may_pull(), iptunnel_pull_header(),
  ip_mc_check_igmp() and the report handler, then reads iph->daddr and
  amtmu->nonce / amtmu->response_mac.

Fix each site by either snapshotting the scalar that is used after the
pull before the first pull runs, or re-deriving the header pointer from
the skb after the last pull that can move the head.  Values that are
stable across the pull (source and group address, the response MAC and
nonce, the record count, the outer source MAC) are snapshotted; pointers
that are written through or read repeatedly are re-derived.

The Linux kernel CVE team has assigned CVE-2026-68302 to this issue.


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

	Issue introduced in 5.16 with commit cbc21dc1cfe949e37b2a54c71511579f1899e8d4 and fixed in 6.6.148 with commit 9005b221cb1f9c3c1a2ef656fb0e8fa80c0a187e
	Issue introduced in 5.16 with commit cbc21dc1cfe949e37b2a54c71511579f1899e8d4 and fixed in 6.12.101 with commit 7746d588d42a4ac0117b68ed8e9b22a9da53dfb7
	Issue introduced in 5.16 with commit cbc21dc1cfe949e37b2a54c71511579f1899e8d4 and fixed in 6.18.42 with commit ca0e8b661957f777591efe874cd9d9a63619cd99
	Issue introduced in 5.16 with commit cbc21dc1cfe949e37b2a54c71511579f1899e8d4 and fixed in 7.1.6 with commit 7f48e3ddad8e97545b25788b8203b3a539df1621
	Issue introduced in 5.16 with commit cbc21dc1cfe949e37b2a54c71511579f1899e8d4 and fixed in 7.2-rc5 with commit 3656a79f94c471827a08f2cacce5f94ad5e52c24

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-2026-68302
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:
	drivers/net/amt.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/9005b221cb1f9c3c1a2ef656fb0e8fa80c0a187e
	https://git.kernel.org/stable/c/7746d588d42a4ac0117b68ed8e9b22a9da53dfb7
	https://git.kernel.org/stable/c/ca0e8b661957f777591efe874cd9d9a63619cd99
	https://git.kernel.org/stable/c/7f48e3ddad8e97545b25788b8203b3a539df1621
	https://git.kernel.org/stable/c/3656a79f94c471827a08f2cacce5f94ad5e52c24