CVE-2026-98041 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Don't predict JMP32 pointer vs zero comparisons
Consider the following program:
r1 = map_value; /* low 32 bits are zero at runtime */ r6 = 0xdead000000000000; if w1 != 0 goto l1; l0: r1 += r6; r2 = *(u64 *)(r1 + 0); exit; l1: r6 = 0; goto l0;
At the moment is_branch_taken() reports the jump as always taken, because it does not distinguish between BPF_JMP and BPF_JMP32 comparisons when processing 'if w1 != 0 ...'.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel's Berkeley Packet Filter (BPF) subsystem includes a verifier that analyzes eBPF programs for safety before they are loaded into the kernel. A critical flaw existed in the branch prediction logic, specifically within the is_branch_taken function, which failed to correctly distinguish between standard 64-bit jump instructions and their 32-bit counterparts when comparing registers against zero. This oversight led to incorrect assumptions about program flow during static analysis, potentially allowing malicious or malformed eBPF programs to bypass security checks that rely on accurate control flow prediction. The vulnerability stems from the verifier treating BPF_JMP and BPF_JMP32 instructions identically in certain contexts, despite their fundamental difference in how they handle register widths and value truncation.
In a typical scenario involving this flaw, an eBPF program might utilize a 64-bit register where only the lower 32 bits are initialized or relevant at runtime. For instance, if a pointer obtained from map_value is stored in a general-purpose register, its upper 32 bits may contain arbitrary data while the lower 32 bits hold valid memory addresses. When such a program executes a conditional jump instruction like BPF_JMP32 with an immediate value of zero to check for null pointers or specific conditions, the verifier incorrectly predicts that the branch is always taken because it does not account for the fact that only the lower 32 bits are being compared against zero in this context. This misprediction causes the verifier to assume certain code paths are unreachable when they might actually be executable under specific runtime conditions, thereby weakening the security guarantees provided by the BPF sandbox.
The operational impact of this vulnerability is significant because it undermines the integrity of the eBPF execution environment. Attackers could potentially craft programs that exploit these incorrect branch predictions to access memory regions or execute code paths that should have been deemed unsafe and rejected by the verifier. This could lead to information disclosure, privilege escalation if the BPF program runs with elevated privileges, or denial of service through kernel crashes caused by invalid memory accesses resulting from flawed control flow analysis. The issue highlights a critical gap in how the kernel handles mixed-width operations within its static analysis engine, which is essential for maintaining system stability and security against untrusted user-space programs that load eBPF code.
To mitigate this vulnerability, it is imperative to apply the latest Linux kernel updates that include patches correcting the branch prediction logic in the BPF verifier. These fixes ensure that BPF_JMP32 instructions are processed with appropriate awareness of their 32-bit scope, preventing false positives in reachability analysis. System administrators should monitor for upstream kernel releases addressing this specific issue and verify that patch levels are current across all systems running eBPF workloads. Additionally, developers writing eBPF programs should adhere to strict coding standards that minimize reliance on implicit type conversions or assumptions about register states, ensuring that comparisons explicitly match the intended width of data being evaluated.
This vulnerability is categorized under CWE-20 as Improper Input Validation because the verifier failed to properly validate the constraints associated with different instruction types during static analysis. It also relates to CWE-841 regarding Improper Enforcement of Behavioral Constraints, as the enforcement mechanism for safe eBPF execution was bypassed due to flawed logic in branch prediction. From a threat intelligence perspective, this aligns with MITRE ATT&CK techniques involving exploitation of software vulnerabilities for initial access or privilege escalation, particularly within environments heavily reliant on containerization and network monitoring tools that utilize eBPF features such as XDP or tc filters. Ensuring robust verification logic is essential to maintaining the trust boundary between user-space applications and kernel space resources.