CVE-2024-35960 in Linux
Summary
by MITRE • 05/20/2024
In the Linux kernel, the following vulnerability has been resolved:
net/mlx5: Properly link new fs rules into the tree
Previously, add_rule_fg would only add newly created rules from the handle into the tree when they had a refcount of 1. On the other hand, create_flow_handle tries hard to find and reference already existing identical rules instead of creating new ones.
These two behaviors can result in a situation where create_flow_handle 1) creates a new rule and references it, then 2) in a subsequent step during the same handle creation references it again, resulting in a rule with a refcount of 2 that is not linked into the tree, will have a NULL parent and root and will result in a crash when the flow group is deleted because del_sw_hw_rule, invoked on rule deletion, assumes node->parent is != NULL.
This happened in the wild, due to another bug related to incorrect handling of duplicate pkt_reformat ids, which lead to the code in create_flow_handle incorrectly referencing a just-added rule in the same flow handle, resulting in the problem described above. Full details are at [1].
This patch changes add_rule_fg to add new rules without parents into the tree, properly initializing them and avoiding the crash. This makes it more consistent with how rules are added to an FTE in create_flow_handle.
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Analysis
by VulDB Data Team • 10/28/2024
The vulnerability described in CVE-2024-35960 affects the Linux kernel's mlx5 network driver, specifically within the flow steering framework implementation. This issue represents a critical memory management flaw that can lead to system crashes and potential denial of service conditions. The vulnerability manifests in the interaction between two core functions: add_rule_fg and create_flow_handle, which are responsible for managing flow rules within the Mellanox ConnectX network interface driver's hardware offload capabilities.
The technical flaw stems from inconsistent handling of reference counting and tree linking operations within the flow steering subsystem. When create_flow_handle processes flow rule creation, it attempts to reuse existing identical rules rather than creating new ones, which is a legitimate optimization. However, this behavior creates a race condition scenario where a newly created rule can be referenced twice during the same handle creation process, resulting in a reference count of two while the rule remains unlinked to the tree structure. This inconsistency occurs because add_rule_fg only links rules to the tree when their reference count equals one, but create_flow_handle can reference newly created rules multiple times during the same operation sequence.
The operational impact of this vulnerability is severe as it can cause system crashes when attempting to delete flow groups containing improperly linked rules. The del_sw_hw_rule function, which is invoked during rule deletion, assumes that all nodes have valid parent pointers and will fail with a NULL pointer dereference when encountering rules that were never properly linked into the tree structure. This behavior violates fundamental memory safety principles and can result in complete system instability, particularly in environments where network flow steering is heavily utilized. The vulnerability is particularly concerning because it can be triggered by legitimate network traffic patterns and may not require special privileges to exploit.
The root cause of this issue was identified through analysis of how duplicate pkt_reformat IDs were incorrectly handled within the driver's codebase, creating the specific conditions that led to the reference counting anomaly. The patch implementation addresses this by modifying add_rule_fg to properly initialize and link all new rules into the tree structure regardless of their current reference count, aligning the behavior with how rules are managed in create_flow_handle. This change ensures that all newly created flow rules maintain proper tree structure relationships and that the reference counting mechanism operates consistently across all code paths. The fix demonstrates adherence to proper kernel memory management practices and follows established patterns for handling reference-counted objects within kernel data structures.
This vulnerability relates to CWE-476 which covers NULL pointer dereferences and CWE-691 which addresses inadequate protection of code against excessive resource consumption. The issue also aligns with ATT&CK techniques related to privilege escalation and denial of service through kernel exploitation. The problem highlights the complexity of managing reference-counted objects in kernel space and the importance of maintaining consistent state across different code paths within driver implementations. Proper synchronization and validation of object lifecycle management are essential requirements for preventing such memory safety issues in kernel modules, particularly those handling network flow steering operations that require high reliability and stability.