CVE-2024-26923

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

af_unix: Fix garbage collector racing against connect()

Garbage collector does not take into account the risk of embryo getting
enqueued during the garbage collection. If such embryo has a peer that
carries SCM_RIGHTS, two consecutive passes of scan_children() may see a
different set of children. Leading to an incorrectly elevated inflight
count, and then a dangling pointer within the gc_inflight_list.

sockets are AF_UNIX/SOCK_STREAM
S is an unconnected socket
L is a listening in-flight socket bound to addr, not in fdtable
V's fd will be passed via sendmsg(), gets inflight count bumped

connect(S, addr) sendmsg(S, [V]); close(V) __unix_gc()

NS = unix_create1()
skb1 = sock_wmalloc(NS)
L = unix_find_other(addr)
unix_state_lock(L)
unix_peer(S) = NS
// V count=1 inflight=0

        NS = unix_peer(S)
        skb2 = sock_alloc()
        skb_queue_tail(NS, skb2[V])

        // V became in-flight
        // V count=2 inflight=1

        close(V)

        // V count=1 inflight=1
        // GC candidate condition met

                    for u in gc_inflight_list:
                      if (total_refs == inflight_refs)
                        add u to gc_candidates

                    // gc_candidates={L, V}

                    for u in gc_candidates:
                      scan_children(u, dec_inflight)

                    // embryo (skb1) was not
                    // reachable from L yet, so V's
                    // inflight remains unchanged

__skb_queue_tail(L, skb1)
unix_state_unlock(L)
for u in gc_candidates:
if (u.inflight)
scan_children(u, inc_inflight_move_tail)

                    // V count=1 inflight=2 (!)

If there is a GC-candidate listening socket, lock/unlock its state. This
makes GC wait until the end of any ongoing connect() to that socket. After
flipping the lock, a possibly SCM-laden embryo is already enqueued. And if
there is another embryo coming, it can not possibly carry SCM_RIGHTS. At
this point, unix_inflight() can not happen because unix_gc_lock is already
taken. Inflight graph remains unaffected.