CVE-2024-26923 af_unix: Fix garbage collector racing against connect()

What is CVE-2024-26923?

CVE-2024-26923 is a high-severity vulnerability in Debian Debian Linux, published on 24 Apr 2024. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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.

Weakness: CWE-362

Which versions are affected by CVE-2024-26923?

It affects 2 products.

Products include Debian Debian Linux, Linux kernel.

How do I fix CVE-2024-26923?

Apply the vendor's patch.

Is CVE-2024-26923 being exploited?

There is no known exploitation of CVE-2024-26923 in the wild and no public exploit code.

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Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as A-336268889, ASB-A-336268889, BELL-CVE-2024-26923, DEBIAN-CVE-2024-26923, GHSA-8f6m-26fj-7fm5, SUSE-SU-2024:2411-1 and 2 more.