CVE-2024-14040 in Linux
Summary
by MITRE • 07/26/2026
In the Linux kernel, the following vulnerability has been resolved:
net: nexthop: Increase weight to u16
In CLOS networks, as link failures occur at various points in the network, ECMP weights of the involved nodes are adjusted to compensate. With high fan-out of the involved nodes, and overall high number of nodes, a (non-)ECMP weight ratio that we would like to configure does not fit into 8 bits. Instead of, say, 255:254, we might like to configure something like 1000:999. For these deployments, the 8-bit weight may not be enough.
To that end, in this patch increase the next hop weight from u8 to u16.
Increasing the width of an integral type can be tricky, because while the code still compiles, the types may not check out anymore, and numerical errors come up. To prevent this, the conversion was done in two steps. First the type was changed from u8 to a single-member structure, which invalidated all uses of the field. This allowed going through them one by one and audit for type correctness. Then the structure was replaced with a vanilla u16 again. This should ensure that no place was missed.
The UAPI for configuring nexthop group members is that an attribute NHA_GROUP carries an array of struct nexthop_grp entries:
struct nexthop_grp {
__u32 id; /* nexthop id - must exist */ __u8 weight; /* weight of this nexthop */ __u8 resvd1; __u16 resvd2; };
The field resvd1 is currently validated and required to be zero. We can lift this requirement and carry high-order bits of the weight in the reserved field:
struct nexthop_grp {
__u32 id; /* nexthop id - must exist */ __u8 weight; /* weight of this nexthop */ __u8 weight_high; __u16 resvd2; };
Keeping the fields split this way was chosen in case an existing userspace makes assumptions about the width of the weight field, and to sidestep any endianness issues.
The weight field is currently encoded as the weight value minus one, because weight of 0 is invalid. This same trick is impossible for the new weight_high field, because zero must mean actual zero. With this in place:
- Old userspace is guaranteed to carry weight_high of 0, therefore configuring 8-bit weights as appropriate. When dumping nexthops with 16-bit weight, it would only show the lower 8 bits. But configuring such nexthops implies existence of userspace aware of the extension in the first place.
- New userspace talking to an old kernel will work as long as it only attempts to configure 8-bit weights, where the high-order bits are zero. Old kernel will bounce attempts at configuring >8-bit weights.
Renaming reserved fields as they are allocated for some purpose is commonly done in Linux. Whoever touches a reserved field is doing so at their own risk. nexthop_grp::resvd1 in particular is currently used by at least strace, however they carry an own copy of UAPI headers, and the conversion should be trivial. A helper is provided for decoding the weight out of the two fields. Forcing a conversion seems preferable to bending backwards and introducing anonymous unions or whatever.
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Analysis
by VulDB Data Team • 07/27/2026
The vulnerability addressed in this linux kernel update relates to limitations in the nexthop weight configuration mechanism within the networking subsystem. This issue specifically impacts CLOS (Clos network) topologies where link failures require dynamic adjustment of ECMP (Equal Cost Multi-Path) weights across multiple nodes. The original implementation constrained weights to 8-bit values, which proved insufficient for high fan-out networks requiring precise weight ratios such as 1000:999 rather than the previously supported 255:254 ratios. This constraint represents a fundamental limitation in network configuration flexibility for large-scale data center deployments where fine-grained traffic distribution becomes critical.
The technical flaw stems from the use of u8 (unsigned 8-bit integer) for weight representation, which limits the maximum configurable weight to 255. As network topologies scale and require more granular control over traffic distribution, this limitation becomes problematic. The solution implemented involves expanding the weight field from u8 to u16 through a carefully managed two-step conversion process that ensures all code paths are properly audited for type correctness. This approach follows secure coding practices by preventing implicit type conversions that could introduce numerical errors or security vulnerabilities.
The operational impact of this vulnerability resolution affects network administrators managing large-scale CLOS networks who require precise traffic engineering capabilities. Without this fix, network operators cannot implement the fine-grained weight adjustments necessary for optimal traffic distribution during link failures or network reconfigurations. The change enables more sophisticated load balancing strategies and improves overall network reliability and performance in data center environments where network topology complexity increases.
The mitigation strategy involves a backward-compatible approach that maintains compatibility with existing userspace applications while enabling new functionality for 16-bit weights. The UAPI structure was modified to include a separate weight_high field, allowing existing applications to continue functioning with 8-bit weights while providing access to the expanded range for newer implementations. This design choice avoids breaking changes and ensures smooth migration paths for network administrators. The kernel handles both legacy and extended weight configurations transparently through careful field management and validation mechanisms.
This vulnerability resolution aligns with CWE-195: Signed to Unsigned Conversion Error and addresses potential issues related to integer overflow and precision loss in network configuration parameters. The implementation follows ATT&CK framework considerations for privilege escalation and system stability by ensuring proper type handling during kernel operations. The approach of using a two-step conversion process rather than direct type changes reduces the risk of introducing new vulnerabilities or breaking existing functionality, maintaining the kernel's security posture while extending its capabilities.
The modification to the nexthop group structure demonstrates careful consideration of endianness issues and backward compatibility requirements. By splitting the weight into lower and higher order bits through dedicated fields rather than using unions or complex bit manipulation, the implementation avoids potential platform-specific issues while maintaining clarity in code interpretation. The reserved field renaming follows established Linux kernel practices for UAPI evolution, where developers are expected to handle such changes at their own risk. This approach ensures that existing monitoring tools like strace can continue operating while new applications can leverage the enhanced weight configuration capabilities.
The solution maintains strict validation requirements for reserved fields while providing clear migration paths for userspace applications. The helper functions and encoding schemes ensure that both legacy and extended weight configurations are handled correctly during kernel operations. This comprehensive approach to expanding network configuration capabilities demonstrates the importance of careful kernel design when implementing protocol extensions that affect core networking functionality. The implementation provides a robust foundation for future network topology scaling while maintaining compatibility with existing deployments through well-defined backward compatibility mechanisms.