CVE-2026-74609: tipc: read le->link under the node lock in tipc_node_link_down()

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

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

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

tipc: read le->link under the node lock in tipc_node_link_down()

tipc_node_link_down() caches the link pointer before taking n->lock:

	struct tipc_link *l = le->link;		/* unlocked */

	if (!l)
		return;
	tipc_node_write_lock(n);
	if (!tipc_link_is_establishing(l)) {	/* deref l */
	...
		tipc_link_reset(l);		/* write into l */
	if (delete) {
		kfree(l);
		le->link = NULL;

The delete=true caller frees that very object under n->lock, so the lock
does not protect the cached pointer against it:

 - CPU A, delete=false: tipc_rcv() on TIPC_LINK_DOWN_EVT, or the link
   supervision timer via tipc_node_timeout(), reads l unlocked and then
   dereferences it under n->lock;
 - CPU B, delete=true: netlink TIPC_NL_BEARER_DISABLE -> bearer_disable()
   -> tipc_node_delete_links() -> tipc_node_link_down(n, bearer_id, true)
   -> kfree(l).

The link is freed with plain kfree(), not kfree_rcu(), and for UDP bearers
disable_media() only schedules the asynchronous cleanup_bearer() work, so
its synchronize_net() runs after the links are already gone.  An in-flight
CPU A that has read l therefore dereferences freed memory once B frees it:
a use-after-free read in tipc_link_is_establishing(), and a use-after-free
write via tipc_link_reset() on the establishing branch.

The following trace was captured on 7.2.0-rc5-00284-gaf39eb111ce6:

  BUG: KASAN: slab-use-after-free in tipc_link_is_establishing (net/tipc/link.c:285)
  Read of size 4 at addr ffff88802e2aa068 by task swapper/2/0
   tipc_link_is_establishing (net/tipc/link.c:285)
   tipc_node_link_down (net/tipc/node.c:1076)
   tipc_node_timeout (net/tipc/node.c:843)
  Allocated by task 9549:
   tipc_link_create (net/tipc/link.c:490)
   tipc_node_check_dest (net/tipc/node.c:1279)
   tipc_disc_rcv (net/tipc/discover.c:252)
   tipc_udp_recv (net/tipc/udp_media.c:389)
  Freed by task 9549:
   tipc_node_link_down (net/tipc/node.c:1084)
   tipc_node_delete_links (net/tipc/node.c:1320)
   bearer_disable (net/tipc/bearer.c:414)
   __tipc_nl_bearer_disable (net/tipc/bearer.c:992)

Move the le->link read inside tipc_node_write_lock(), so it is serialised
against the kfree() in the delete path.  A racing teardown now either has
not run yet, and we see a valid link, or has already run, and we see NULL.

The Linux kernel CVE team has assigned CVE-2026-74609 to this issue.


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

	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 5.10.265 with commit 2be741ad565c610871a6a95062c12c39da7168fd
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 5.15.216 with commit 69d209461c110388710e483a130caf051e4fd09a
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 6.1.183 with commit de017c22135f545ca4e65d1eada22887b64958eb
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 6.6.152 with commit 47ba70891b10b2feb52462086b7fcd2ad75d3ce3
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 6.12.104 with commit a714d62513befef37f71f4ae89bb1fe173b65f2e
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 6.18.45 with commit c3f2347a47754eac690967cfd82cb6d559817b07
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 7.1.9 with commit 5558a8312452ddb21eff22b1cbd84302ad951944
	Issue introduced in 4.4 with commit 73f646cec35477b5099d7e952297cb9e1855be45 and fixed in 7.2 with commit cba9ccb47e9fa4cc77692fb896cc5ab57a667882

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-2026-74609
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/tipc/node.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/2be741ad565c610871a6a95062c12c39da7168fd
	https://git.kernel.org/stable/c/69d209461c110388710e483a130caf051e4fd09a
	https://git.kernel.org/stable/c/de017c22135f545ca4e65d1eada22887b64958eb
	https://git.kernel.org/stable/c/47ba70891b10b2feb52462086b7fcd2ad75d3ce3
	https://git.kernel.org/stable/c/a714d62513befef37f71f4ae89bb1fe173b65f2e
	https://git.kernel.org/stable/c/c3f2347a47754eac690967cfd82cb6d559817b07
	https://git.kernel.org/stable/c/5558a8312452ddb21eff22b1cbd84302ad951944
	https://git.kernel.org/stable/c/cba9ccb47e9fa4cc77692fb896cc5ab57a667882