CVE-2026-98019 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: mark a NULL call argument precise
check_func_arg() allows bpf_register_is_null() for nullable arguments w/o marking the underlying scalar register precise. Hence a checkpoint created on such a path would prune against arbitrary scalar value.
check_helper_call() enforces second parameter of the bpf_get_local_storage() to be zero, w/o marking the underlying scalar register precise. Hence a checkpoint created on such a path would prune against arbitrary scalar value.
Grouping these two into one patch, as they share the same fixes tag.
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Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel's Berkeley Packet Filter subsystem contains a critical logic flaw within its verifier engine that compromises the integrity of state pruning during program analysis. This vulnerability stems from an incorrect handling of nullable arguments in specific helper function calls, specifically affecting how scalar register values are marked as precise or imprecise. The BPF verifier is responsible for ensuring that eBPF programs executed in kernel space are safe and do not lead to memory corruption or privilege escalation. A core mechanism of this safety guarantee involves tracking the precision of registers; when a value is deemed precise, it means its exact bit pattern matters for subsequent operations, whereas imprecise values indicate that only certain properties, such as sign or range, need to be tracked. The flaw arises because the check_func_arg function permits bpf_register_is_null checks on nullable arguments without subsequently marking the underlying scalar register as precise. This oversight creates a discrepancy between the logical requirement of null-checking and the verifier's internal state tracking.
This technical deficiency has direct consequences for checkpoint creation during program verification. When the verifier encounters a conditional branch, it may create checkpoints to save the current state of registers so that paths can be analyzed independently and later merged. If a register involved in such a path is not marked precise after being checked against null, the resulting checkpoint will incorrectly prune or discard valid execution states based on arbitrary scalar values rather than strict precision requirements. This leads to an inaccurate representation of program behavior, potentially allowing maliciously crafted eBPF programs that rely on specific register behaviors to bypass security checks. The issue affects two distinct helper functions: bpf_register_is_null and the second parameter validation for bpf_get_local_storage. In both cases, the verifier enforces constraints but fails to update the precision metadata of the associated scalar registers accordingly.
The operational impact of this vulnerability is significant because it undermines the deterministic nature of BPF program verification. Attackers who can influence or predict register states might exploit these imprecise checkpoints to craft programs that appear valid during static analysis but exhibit undefined behavior at runtime. This could potentially lead to kernel crashes, information leaks, or further exploitation vectors depending on how the eBPF program interacts with other kernel subsystems. The fact that both affected functions share the same fixes tag indicates they were identified as part of a broader pattern in verifier logic where null checks and specific parameter validations are not properly synchronized with precision tracking mechanisms. This synchronization is essential for maintaining the soundness of the verification process, ensuring that all paths through an eBPF program adhere to strict safety constraints before execution is permitted.
Mitigation strategies primarily involve applying the upstream kernel patch that corrects this logic error by explicitly marking scalar registers as precise after null checks and specific parameter validations in check_func_arg and check_helper_call functions. System administrators should ensure their Linux kernels are updated to versions where these verifier corrections have been integrated, particularly those targeting stable releases affected by this issue. From a defensive perspective, organizations deploying eBPF-based security tools or network monitoring solutions must verify that the underlying kernel version includes these fixes to prevent potential bypasses of verification policies. Additionally, developers writing custom BPF programs should be aware that relying on implicit precision assumptions in older kernels may lead to unpredictable behavior and should adhere strictly to verifier guidelines regarding register usage and null checks.
This vulnerability aligns with CWE-20 Improper Input Validation as the kernel fails to properly validate the state of inputs before making critical decisions about program execution paths. It also relates to CWE-697 Incorrect Comparison, where the comparison logic does not correctly update internal state variables reflecting data precision. In terms of MITRE ATT&CK, this flaw could facilitate T1059 Command and Scripting Interpreter abuse if an attacker crafts a malicious BPF program that exploits these verification gaps to execute unauthorized operations within kernel space. The lack of proper precision tracking effectively weakens the sandbox boundaries imposed by eBPF security models, making it crucial for vendors to prioritize patches addressing verifier logic errors in high-assurance environments where container isolation and network policy enforcement rely heavily on correct BPF execution guarantees.