CVE-2026-74493: net/smc: fix socket use-after-free during link group termination

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

net/smc: fix socket use-after-free during link group termination

smclgrterminate() drops connslock after finding a connection in lgr->connsall, but before taking a reference on its socket. The connection is embedded in the socket, and its registration reference protects it only while the connection remains in the tree.

A concurrent close can unregister the connection and drop that reference, freeing the socket before the termination worker reaches sockhold().

The race is reachable when close overlaps link group termination. Local stress testing reproduced the use-after-free and KASAN reported:

BUG: KASAN: slab-use-after-free in smclgrterminate.part.0 [smc] Write of size 4 by task kworker/3:3 Workqueue: events smclgrterminatework [smc] smclgrterminate.part.0 [smc]

The socket was allocated by smccreate(), freed through slabfreeafterrcudebug(), and was followed by:

refcountt: addition on 0; use-after-free. smclgrterminate.part.0 [smc]

Take the socket reference while connslock still protects the tree entry. The unregister path then cannot drop the last reference until termination has finished using the socket.