CVE-2026-74398: ipv6: addrconf: bail out of dad_failure when state is no longer POSTDAD

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

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

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

ipv6: addrconf: bail out of dad_failure when state is no longer POSTDAD

addrconf_dad_failure() transitions ifp->state from DAD to POSTDAD
via addrconf_dad_end(), which drops ifp->lock on return.  The lock
is re-acquired after net_info_ratelimited().  A concurrent
ipv6_del_addr() can take the lock in that window, set ifp->state
to DEAD and run list_del_rcu(&ifp->if_list).

addrconf_dad_failure() then overwrites DEAD with ERRDAD at errdad:
and schedules a new dad_work.  The work calls ipv6_del_addr()
again, hitting the already-poisoned list entry:

  general protection fault: 0000 [#1] SMP NOPTI
  CPU: 4 PID: 217 Comm: kworker/4:1
  Workqueue: ipv6_addrconf addrconf_dad_work
  RIP: 0010:ipv6_del_addr+0xe9/0x280
  RAX: dead000000000122
  Call Trace:
   addrconf_dad_stop+0x113/0x140
   addrconf_dad_work+0x28c/0x430
   process_one_work+0x1eb/0x3b0
   worker_thread+0x4d/0x400
   kthread+0x104/0x140
   ret_from_fork+0x35/0x40

Fold the addrconf_dad_end() logic into addrconf_dad_failure() under
a single ifp->lock critical section.  The STABLE_PRIVACY branch
temporarily drops ifp->lock around address regeneration, so at
lock_errdad: verify the state is still POSTDAD before transitioning
to ERRDAD; bail out otherwise to avoid overwriting a state set by
another path while the lock was released.

The Linux kernel CVE team has assigned CVE-2026-74398 to this issue.


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

	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 5.10.261 with commit 47b05836705b63dab93d9ac7c69a3a507375ef80
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 5.15.212 with commit d21be7d051012c6b572fa4e3334443c250216f7b
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 6.1.178 with commit 875c284c0f98b042bb97abad460f63a24c977f88
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 6.6.145 with commit 8ed0ce9ea58d677d1bac92614ee5f60f8ea57363
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 6.12.97 with commit 3bdc86d89fd6c6523753fa6f42fcfaf30ee699cb
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 6.18.40 with commit b61af0268e3d1308c466bf0be5dced844eafc1ef
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 7.1.5 with commit e889aa99ad3ed48bb0ddcff6475b17542532d18b
	Issue introduced in 3.14 with commit c15b1ccadb323ea50023e8f1cca2954129a62b51 and fixed in 7.2-rc1 with commit 627ac78f2741e2ebd2225e2e953b6964a8a9182f
	Issue introduced in 3.10.105 with commit 835b474b8f70fa68d68abffad37378e92f661802

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-2026-74398
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/addrconf.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/47b05836705b63dab93d9ac7c69a3a507375ef80
	https://git.kernel.org/stable/c/d21be7d051012c6b572fa4e3334443c250216f7b
	https://git.kernel.org/stable/c/875c284c0f98b042bb97abad460f63a24c977f88
	https://git.kernel.org/stable/c/8ed0ce9ea58d677d1bac92614ee5f60f8ea57363
	https://git.kernel.org/stable/c/3bdc86d89fd6c6523753fa6f42fcfaf30ee699cb
	https://git.kernel.org/stable/c/b61af0268e3d1308c466bf0be5dced844eafc1ef
	https://git.kernel.org/stable/c/e889aa99ad3ed48bb0ddcff6475b17542532d18b
	https://git.kernel.org/stable/c/627ac78f2741e2ebd2225e2e953b6964a8a9182f