CVE-2026-98613 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Bound ownership depth through local kptrs and graph roots
Program-allocated objects can own other local objects through referenced kptrs. bpf_obj_free_fields() follows those pointers through __bpf_obj_drop_impl() synchronously, before the object storage is freed through RCU. A self-referential local kptr type therefore permits arbitrarily deep object chains, and dropping the head can exhaust the kernel stack. Long acyclic type chains have the same problem.
btf_check_and_fixup_fields() still assumes referenced kptrs only point to kernel types and checks ownership through list and rbtree roots only. Its existing rule is sufficient for graph-only cycles: the target of each graph edge must contain a node, so every type in a cycle has both a root and a node. The rule rejects such a type owning another root, breaking every cycle. It also limits graph-only chains to three types, or two if the first type contains a node, and conservatively rejects longer acyclic chains. The missing local-kptr edges, rather than a missed graph-only cycle, are the bug introduced by support for bpf_kptr_xchg() into local kptrs.
Replace that restriction with one bounded ownership walk covering graph roots and local referenced kptrs. Run it after all BTF records have been fixed up, reject cycles and paths deeper than eight record-bearing types, and cache each type's suffix depth while checking it against the remaining budget. This also permits the longer acyclic graph-only layouts rejected by the old rule; update their existing BTF tests accordingly.
Keep the bound independent of MAX_CALL_FRAMES because recursive destruction can run below a BPF call chain. A plain local pointee without special-field metadata adds only a final non-recursing drop. Non-owning kptrs and kernel-BTF kptrs do not recurse through local records and remain outside the walk. Include local percpu-kptr edges too, although allocation of percpu objects with special fields is currently forbidden, so that relaxing that restriction cannot bypass the ownership bound.
btf_check_and_fixup_fields() continues to initialize graph_root.value_rec, including for separately allocated map records. The ownership relationships belong to immutable program BTF and only need validation at BTF load time.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel has addressed a critical stack exhaustion vulnerability within the eBPF subsystem related to local kptrs and graph roots. This flaw arises from the way program-allocated objects manage ownership of other local objects through referenced kptrs. When an object is freed, the function bpf_obj_free_fields() synchronously follows these pointers via __bpf_obj_drop_impl before the object storage is released using RCU mechanisms. If a self-referential local kptr type exists or if there are long acyclic chains of such types, dropping the head of the chain can trigger an arbitrarily deep recursion that exhausts the kernel stack. This represents a significant stability risk as it allows for denial-of-service conditions through simple resource exhaustion without requiring privilege escalation.
The root cause lies in the validation logic within btf_check_and_fixup_fields(), which previously assumed that referenced kptrs only point to kernel types and checked ownership exclusively through list and rbtree roots. While this rule was sufficient to prevent graph-only cycles by ensuring every type in a cycle contained both a root and a node, it failed to account for local-kptr edges introduced with support for bpf_kptr_xchg() into local kptrs. Consequently, the existing restriction conservatively rejected longer acyclic chains but missed the deeper recursive paths enabled by these new edge types. The vulnerability exploits this gap by constructing object graphs that bypass the previous depth limitations, leading to unbounded recursion during cleanup operations.
To resolve this issue, the kernel now implements a bounded ownership walk that covers both graph roots and local referenced kptrs. This validation runs after all BTF records have been fixed up and explicitly rejects cycles as well as paths deeper than eight record-bearing types. The implementation caches each type's suffix depth while checking it against the remaining budget to ensure efficient evaluation. This approach not only prevents stack exhaustion but also permits longer acyclic graph-only layouts that were incorrectly rejected by the old rule, thereby improving compatibility with existing BTF tests without compromising security boundaries.
The bound is kept independent of MAX_CALL_FRAMES because recursive destruction can occur below a standard BPF call chain context. A plain local pointee without special-field metadata adds only a final non-recursing drop and does not contribute to the stack depth issue. Non-owning kptrs and kernel-BTF kptrs are excluded from this walk as they do not recurse through local records, maintaining their existing safety properties. Additionally, local percpu-kptr edges are included in the validation scope even though allocation of percpu objects with special fields is currently forbidden, ensuring that any future relaxation of that restriction cannot bypass the ownership bound.
From a classification perspective, this vulnerability aligns with CWE-789: Memory Allocation with Excessive Size or CWE-675: Operation on Resource in Wrong Phase or at Wrong Time, specifically regarding improper resource cleanup leading to stack exhaustion. In terms of ATT&CK mapping, it relates to T1499: Endpoint Denial of Service, as the primary impact is the disruption of system availability through kernel panic or crash due to stack overflow. Mitigation involves applying the latest kernel updates that include this fix for btf_check_and_fixup_fields(). Administrators should ensure their systems are patched against CVEs affecting the eBPF subsystem and monitor for any signs of excessive recursion in BPF program execution logs.