CVE-2026-98064
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix NULL-ptr-deref when showing a void BTF type
btf_modifier_show() resolves the modifier and then calls btf_type_ops(t)->show() unconditionally. For the void type (type_id 0, BTF_KIND_UNKN) kind_ops[] has no entry, so ->show is NULL.
A "const void" (a modifier resolving to void) cannot be a map key or value - map_check_btf() rejects it because void has no size - so the map dump path does not reach it. But bpf_snprintf_btf() takes a type_id straight from the BPF program, and passing such a "const void" from the vmlinux BTF NULL-derefs:
KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
RIP: 0010:btf_modifier_show (kernel/bpf/btf.c:2914) Call Trace: btf_type_show (kernel/bpf/btf.c:8251) btf_type_snprintf_show (kernel/bpf/btf.c:8321) bpf_snprintf_btf (kernel/trace/bpf_trace.c:1047) bpf_prog_test_run_raw_tp (net/bpf/test_run.c:829) __sys_bpf (kernel/bpf/syscall.c:4804) do_syscall_64 (arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
Fall back to btf_df_show() when the resolved type has no show op; it emits the "" placeholder already used for kinds like FWD and FUNC. bpf_snprintf_btf() then returns the length as usual.
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Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel's BPF Type Format subsystem contains a critical null pointer dereference vulnerability within the btf_modifier_show function, which is invoked during the processing of BTF type information for display purposes. This flaw arises from an unconditional call to the show operation associated with a resolved modifier type without verifying whether that specific type supports such an operation. Specifically, when the resolver encounters a const void type, it resolves to the void kind identified by type_id zero and BTF_KIND_UNKN. The kernel's internal array of type operations for this unknown or void kind does not define a show function, leaving the pointer null. Consequently, any attempt to invoke this non-existent handler results in an immediate segmentation fault or kernel panic due to accessing memory at address zero plus offset twenty-eight bytes, as evidenced by KASAN reports indicating a null-ptr-deref within btf_modifier_show.
The operational impact of this vulnerability is primarily observed when BPF programs utilize the bpf_snprintf_btf helper function with type identifiers derived directly from vmlinux BTF data that point to void types. Unlike map dump operations, which are protected by checks in map_check_btf that reject void types due to their lack of size and inability to serve as keys or values, the tracing path via bpf_snprintf_btf lacks this safeguard for certain edge cases involving modifiers resolving to void. An attacker with the ability to craft specific BPF programs or manipulate tracepoints could trigger this code path, leading to a denial of service by crashing the host kernel. This represents a significant stability risk in environments where dynamic tracing and eBPF program execution are enabled, particularly when interacting with complex type definitions from the vmlinux BTF database.
From a classification perspective, this vulnerability aligns with CWE-476, which denotes NULL Pointer Dereference, as the code fails to check for null before dereferencing a function pointer. In terms of attack vectors and techniques, it relates to MITRE ATT&CK technique T1059, specifically Command and Scripting Interpreter subcategories involving BPF programs, where an adversary might leverage malformed inputs or crafted eBPF bytecode to induce system instability. The vulnerability highlights the importance of defensive programming practices within kernel subsystems that handle dynamic type introspection, ensuring that all potential return values from resolution functions are validated against available operations before invocation.
The remediation for this issue involves modifying btf_modifier_show to implement a fallback mechanism when the resolved type lacks a dedicated show operation. Instead of crashing, the function now checks if the show pointer is null and defaults to using btf_df_show, which emits an empty string placeholder similar to how forward declarations or functions are handled in other contexts. This approach ensures graceful degradation rather than catastrophic failure, maintaining system stability while still providing some level of type information output. Developers should apply this patch immediately to affected kernel versions and ensure that all BPF tracing tools are updated to handle these edge cases correctly during runtime analysis.