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.

Notes