CVE-2026-98631 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
xsk: Use a 32-bit compare in xsk_map_gen_lookup
xsk_map_gen_lookup() loads a u32 key and compares it with max_entries using BPF_JMP_IMM. BPF immediates are sign-extended to 64 bits, so a max_entries value of 0x80000000 or higher becomes a threshold larger than every zero-extended 32-bit key. An out-of-range index then skips the bounds check and the generated lookup reads past xsk_map[].
Compare with BPF_JMP32_IMM so the check stays in 32-bit unsigned range.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/18/2026
The vulnerability identified within the Linux kernel's XDP socket implementation stems from a subtle type mismatch during Just-In-Time compilation of Berkeley Packet Filter instructions, specifically within the xsk_map_gen_lookup function. This routine is responsible for generating eBPF code that performs bounds checking on array lookups to ensure memory safety. The core technical flaw arises because the function loads a 32-bit unsigned integer key and compares it against max_entries using the BPF_JMP_IMM instruction set, which operates with immediate values sign-extended to sixty-four bits. When the maximum number of entries in the map exceeds or equals two to the power of thirty-one, specifically at hexadecimal value eight zero zero zero zero zero zero zero, the signed interpretation causes this threshold to be treated as a negative number when viewed through certain logical comparisons or simply results in an excessively large positive range if not handled correctly by the verifier logic. Consequently, any index that is zero-extended from its original thirty-two-bit unsigned representation will always appear less than this inflated sixty-four-bit threshold, effectively bypassing the intended bounds check mechanism.
This architectural oversight allows for a critical out-of-bounds read condition where an attacker or malicious eBPF program can access memory locations beyond the allocated boundaries of the xsk_map array structure. By exploiting this logic error in the generated code, it is possible to leak kernel memory contents that were not intended to be accessible through standard socket operations. The operational impact includes potential information disclosure vulnerabilities and could potentially serve as a stepping stone for more severe exploitation scenarios depending on what data resides immediately following the map allocation in kernel space. Such out-of-bounds reads violate fundamental security principles regarding memory isolation between user-space applications and privileged kernel structures, undermining the integrity of the system's execution environment.
From an industry standards perspective, this flaw aligns with CWE-125 Out-of-bounds Read, as it involves accessing memory beyond allocated boundaries due to improper validation of input indices. Furthermore, in the context of attack techniques, this vulnerability facilitates unauthorized data access which can be categorized under ATT&CK technique T1083 File and Directory Discovery if used for reconnaissance or potentially part of broader exploitation chains involving privilege escalation via kernel information leaks. The root cause is fundamentally a type confusion issue where thirty-two-bit unsigned values are incorrectly compared against sixty-four-bit signed immediates, leading to incorrect control flow decisions within the eBPF verifier's generated code paths.
To mitigate this vulnerability and prevent similar issues in future developments, it is imperative that developers utilize BPF_JMP32_IMM instructions for comparisons involving thirty-two-bit keys rather than relying on standard sixty-four-bit immediate operations. This ensures that all arithmetic and logical evaluations remain strictly within the unsigned thirty-two-bit range, preserving the integrity of boundary checks against max_entries values up to four billion two hundred ninety-five million nine hundred sixty-seven thousand three hundred fifty-one. Additionally, rigorous static analysis tools should be employed during code review processes to detect mismatches between variable types and instruction operands in eBPF program generation logic. Updating affected kernel versions with patches that enforce thirty-bit or full thirty-two-bit unsigned comparison semantics will resolve the immediate risk while reinforcing best practices for safe memory access patterns within high-performance networking subsystems like XDP sockets.