CVE-2026-68426: xfrm: fix stale skb->prev after async crypto steals a GSO segment

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

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

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

xfrm: fix stale skb->prev after async crypto steals a GSO segment

skb_gso_segment() leaves the segment list head with ->prev pointing at
the last segment, an invariant validate_xmit_skb_list() relies on when
it sets its tail pointer (tail = skb->prev).

When validate_xmit_xfrm() walks a GSO list and some segments are stolen
by async crypto (->xmit() returns -EINPROGRESS), those segments are
unlinked from the list but the head ->prev is never updated.  If the
last segment is the one stolen, the returned head still has ->prev
pointing at it, even though it is now owned by the crypto engine and may
be freed.  validate_xmit_skb_list() later does tail->next = skb, writing
through that stale pointer -- a use-after-free.

Repoint skb->prev at the last retained segment before returning.

The Linux kernel CVE team has assigned CVE-2026-68426 to this issue.


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

	Issue introduced in 4.16 with commit f53c723902d1ac5f0b0a11d7c9dcbff748dde74e and fixed in 6.18.42 with commit 33e1b0d25ca0d2818c635ff80e6aa0d295e08a98
	Issue introduced in 4.16 with commit f53c723902d1ac5f0b0a11d7c9dcbff748dde74e and fixed in 7.1.6 with commit bbca7cc3b2b4b10afbfee99b81d9ee78f5423046
	Issue introduced in 4.16 with commit f53c723902d1ac5f0b0a11d7c9dcbff748dde74e and fixed in 7.2-rc4 with commit 3f4c3919baf0944ad96580467c302bc6c7758b00

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-2026-68426
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/xfrm/xfrm_device.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/33e1b0d25ca0d2818c635ff80e6aa0d295e08a98
	https://git.kernel.org/stable/c/bbca7cc3b2b4b10afbfee99b81d9ee78f5423046
	https://git.kernel.org/stable/c/3f4c3919baf0944ad96580467c302bc6c7758b00