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 Vs 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 Vs
					// 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

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/2e2a03787f4f0abc0072350654ab0ef3324d9db3
• https://git.kernel.org/stable/c/343c5372d5e17b306db5f8f3c895539b06e3177f
• https://git.kernel.org/stable/c/47d8ac011fe1c9251070e1bd64cb10b48193ec51
• https://git.kernel.org/stable/c/507cc232ffe53a352847893f8177d276c3b532a9
• https://git.kernel.org/stable/c/a36ae0ec2353015f0f6762e59f4c2dbc0c906423
• https://git.kernel.org/stable/c/b75722be422c276b699200de90527d01c602ea7c
• https://git.kernel.org/stable/c/dbdf7bec5c920200077d693193f989cb1513f009
• https://git.kernel.org/stable/c/e76c2678228f6aec74b305ae30c9374cc2f28a51
• https://git.kernel.org/stable/c/2e2a03787f4f0abc0072350654ab0ef3324d9db3
• https://git.kernel.org/stable/c/343c5372d5e17b306db5f8f3c895539b06e3177f
• https://git.kernel.org/stable/c/47d8ac011fe1c9251070e1bd64cb10b48193ec51
• https://git.kernel.org/stable/c/507cc232ffe53a352847893f8177d276c3b532a9
• https://git.kernel.org/stable/c/a36ae0ec2353015f0f6762e59f4c2dbc0c906423
• https://git.kernel.org/stable/c/b75722be422c276b699200de90527d01c602ea7c
• https://git.kernel.org/stable/c/dbdf7bec5c920200077d693193f989cb1513f009
• https://git.kernel.org/stable/c/e76c2678228f6aec74b305ae30c9374cc2f28a51
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html
• https://lists.debian.org/debian-lts-announce/2024/06/msg00020.html