In the Linux kernel, the following vulnerability has been resolved:
af_unix: Initialise scc_index in unix_add_edge().
Quang Le reported that the AF_UNIX GC could garbage-collect a
receive queue of an alive in-flight socket, with a nice repro.
The repro consists of three stages.
1)
1-a. Create a single cyclic reference with many sockets
1-b. close() all sockets
1-c. Trigger GC
2)
2-a. Pass sk-A to an embryo sk-B
2-b. Pass sk-X to sk-X
2-c. Trigger GC
3)
3-a. accept() the embryo sk-B
3-b. Pass sk-B to sk-C
3-c. close() the in-flight sk-A
3-d. Trigger GC
As of 2-c, sk-A and sk-X are linked to unix_unvisited_vertices,
and unix_walk_scc() groups them into two different SCCs:
unix_sk(sk-A)->vertex->scc_index = 2 (UNIX_VERTEX_INDEX_START)
unix_sk(sk-X)->vertex->scc_index = 3
Once GC completes, unix_graph_grouped is set to true.
Also, unix_graph_maybe_cyclic is set to true due to sk-X's
cyclic self-reference, which makes close() trigger GC.
At 3-b, unix_add_edge() allocates unix_sk(sk-B)->vertex and
links it to unix_unvisited_vertices.
unix_update_graph() is called at 3-a. and 3-b., but neither
unix_graph_grouped nor unix_graph_maybe_cyclic is changed
because both sk-B's listener and sk-C are not in-flight.
3-c decrements sk-A's file refcnt to 1.
Since unix_graph_grouped is true at 3-d, unix_walk_scc_fast()
is finally called and iterates 3 sockets sk-A, sk-B, and sk-X:
sk-A -> sk-B (-> sk-C)
sk-X -> sk-X
This is totally fine. All of them are not yet close()d and
should be grouped into different SCCs.
However, unix_vertex_dead() misjudges that sk-A and sk-B are
in the same SCC and sk-A is dead.
unix_sk(sk-A)->scc_index == unix_sk(sk-B)->scc_index <-- Wrong!
&&
sk-A's file refcnt == unix_sk(sk-A)->vertex->out_degree
^-- 1 in-flight count for sk-B
-> sk-A is dead !?
The problem is that unix_add_edge() does not initialise scc_index.
Stage 1) is used for heap spraying, making a newly allocated
vertex have vertex->scc_index == 2 (UNIX_VERTEX_INDEX_START)
set by unix_walk_scc() at 1-c.
Let's track the max SCC index from the previous unix_walk_scc()
call and assign the max + 1 to a new vertex's scc_index.
This way, we can continue to avoid Tarjan's algorithm while
preventing misjudgments.
What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)
The EG score (7.8): severity, escalated by confirmed exploitation. This CVE's 7.8 is its impact severity (EG Severity), not escalated.
- EG Severity
- 7.8 HIGHCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
- EG Likelihood
- 0.2% within 30 daysSource: FIRST EPSS. FIRST's exploitation probability, not an EchelonGraph model.
- Confirmed exploitation
- None on record (CISA KEV, EG-KEV Tier 1)Reported apart from the likelihood: an observed fact, not a probability.
- EG Priority
- SSVC Track at medium mission impactTrack at low or medium mission impact; Track* at high (mission-essential systems).CISA BOD 26-04: 14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.Each input and its source, and whom BOD 26-04 binds: the SSVC and BOD 26-04 blocks below.
SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Track* at high (mission-essential systems).
A fix is available. Apply it within your standard update timelines.
Exploitation none (no CISA value, KEV listing or exploit code on record) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). EchelonGraph holds no SSVC assessment of this CVE from CISA: none of these inputs is from Vulnrichment. EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table
BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.
The score's inputs: In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). Where an input reads “CISA's BOD 26-04 default”, no one has published it for this CVE, and CISA's implementation guidance says: “If “Automatable” and “Technical Impact” information items about a CVE ID are not available and the CVE ID is not in the KEV Catalog, CISA will treat its values as “no” and “total,” respectively, until CISA or a third party provides metadata about the CVE ID. CISA will always provide the Vulnrichment data for a CVE ID listed in the KEV Catalog.” Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it