In the Linux kernel, the following vulnerability has been resolved:
bpf: check_cond_jmp_op(): properly infer if register is null
Nicholas Carlini reported a bug when verifier can incorrectly infer
that a pointer is non-null. The bug occurs when two pointers are
compared and one of them has a type w/o PTR_MAYBE_NULL flag,
but which allows a value to be NULL at runtime.
Here is an example:
// a is PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED
// a is 0 at runtime.
// b is PTR_TO_MAP_VALUE | PTR_MAYBE_NULL
void *a = bpf_rdonly_cast(0, 0);
int *b = bpf_map_lookup_elem(...);
if (a == b)
*b = 42; // verifier does not catch null pointer dereference
This happens because of a special case in check_cond_jmp_op(),
which attempts to strip PTR_MAYBE_NULL flags from pointer types,
when processing comparisons like rA == rB, if either rA or rB can't
be null.
The non-null property is derived based on the absence of
PTR_MAYBE_NULL flag on rA's or rB's type. But that is not sufficient
for types like PTR_TO_MEM, as in the example.
This patch replaces type_may_be_null() call with reg_not_null(),
which contains an allowlist of types for which absence of
PTR_MAYBE_NULL actually means that the value can't be NULL at runtime.
At the moment, the list in the reg_not_null() omits two types for
which PTR_MAYBE_NULL is applicable: PTR_TO_XDP_SOCK and PTR_TO_BUF.
In order to remain backward compatible, and assuming that only
comparison between pointers of the same type makes sense,
this commit extends reg_not_null(). W/o such an extension e.g.
verifier_jeq_infer_not_null/null_ptr_to_map_value fails.
reg_not_null() can be extended further, but I deem that out of scope
for the fix at hand. Explicit base_type(...) != PTR_TO_BTF_ID
checks in the check_cond_jmp_op() can be removed with migration to
reg_not_null(), but that is a behavioural change, as the special case
would start matching for PTR_TO_BTF_ID that is also is_trusted_reg().
I omit the behavioural change from this commit.
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. 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