In the Linux kernel, the following vulnerability has been resolved:
net/sched: sch_api: fix xa_insert() error path in tcf_block_get_ext()
This command:
$ tc qdisc replace dev eth0 ingress_block 1 egress_block 1 clsact
Error: block dev insert failed: -EBUSY.
fails because user space requests the same block index to be set for
both ingress and egress.
[ side note, I don't think it even failed prior to commit 913b47d3424e
("net/sched: Introduce tc block netdev tracking infra"), because this
is a command from an old set of notes of mine which used to work, but
alas, I did not scientifically bisect this ]
The problem is not that it fails, but rather, that the second time
around, it fails differently (and irrecoverably):
$ tc qdisc replace dev eth0 ingress_block 1 egress_block 1 clsact
Error: dsa_core: Flow block cb is busy.
[ another note: the extack is added by me for illustration purposes.
the context of the problem is that clsact_init() obtains the same
&q->ingress_block pointer as &q->egress_block, and since we call
tcf_block_get_ext() on both of them, "dev" will be added to the
block->ports xarray twice, thus failing the operation: once through
the ingress block pointer, and once again through the egress block
pointer. the problem itself is that when xa_insert() fails, we have
emitted a FLOW_BLOCK_BIND command through ndo_setup_tc(), but the
offload never sees a corresponding FLOW_BLOCK_UNBIND. ]
Even correcting the bad user input, we still cannot recover:
$ tc qdisc replace dev swp3 ingress_block 1 egress_block 2 clsact
Error: dsa_core: Flow block cb is busy.
Basically the only way to recover is to reboot the system, or unbind and
rebind the net device driver.
To fix the bug, we need to fill the correct error teardown path which
was missed during code movement, and call tcf_block_offload_unbind()
when xa_insert() fails.
[ last note, fundamentally I blame the label naming convention in
tcf_block_get_ext() for the bug. The labels should be named after what
they do, not after the error path that jumps to them. This way, it is
obviously wrong that two labels pointing to the same code mean
something is wrong, and checking the code correctness at the goto site
is also easier ]
What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)
The EG score (5.5): severity, escalated by confirmed exploitation. This CVE's 5.5 is its impact severity (EG Severity), not escalated.
- EG Severity
- 5.5 MEDIUMCVSS-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 (model v2026.06.15, score date 2026-10-25). 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 every mission impact level.CISA BOD 26-04: 60 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 every mission impact level.
A fix is available. Apply it within your standard update timelines.
Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (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)60 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 Vulnrichment) · Technical impact partial (CISA Vulnrichment). 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