CVE-2026-93135 in Linux
Summary
by MITRE • 09/18/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Reject programs with inlined helpers if JIT is not available
When an architecture (such as LoongArch, ARM64, and RISC-V) implements bpf_jit_inlines_helper_call(), the verifier skips rewriting the helper call offset (insn->imm) in bpf_do_misc_fixups(). This is because the helper is expected to be inlined by the JIT compiler later. Therefore, insn->imm remains as the raw helper enum ID.
However, if JIT is disabled at runtime (net.core.bpf_jit_enable=0) or if JIT compilation fails dynamically (e.g., due to OOM), the program falls back to the BPF interpreter.
When the interpreter executes (__bpf_call_base + insn->imm) with the unpatched raw ID, it jumps into an invalid address space, triggering an instruction alignment fault or a kernel panic.
Although these helpers have valid C implementations in the kernel, the omission of offset rewriting makes runtime interpreter fallback fatal.
Fix this by setting 'prog->jit_required = 1' when helper call rewriting is skipped for JIT inlining. This ensures that such programs are safely rejected if JIT is not available, preventing the runtime kernel panic.
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Analysis
by VulDB Data Team • 09/18/2026
The Linux kernel's Berkeley Packet Filter (BPF) subsystem provides a powerful mechanism for running sandboxed programs within the kernel space, primarily used for networking and observability tasks. A critical flaw exists in how the BPF verifier handles helper function calls when Just-In-Time (JIT) compilation is involved but subsequently unavailable at runtime. Specifically, on architectures such as LoongArch, ARM64, and RISC-V that implement bpf_jit_inlines_helper_call(), the verifier intentionally skips rewriting the immediate field of instruction operands during the fixup phase. This optimization assumes that the JIT compiler will later replace these helper call instructions with inline code or direct jumps to optimized implementations. Consequently, the operand retains its raw helper enumeration ID rather than a valid memory offset or pointer suitable for execution by an interpreter.
The vulnerability manifests when the BPF program is loaded and executed under conditions where JIT compilation cannot be utilized. This occurs either if the user explicitly disables JIT via the net.core.bpf_jit_enable sysctl parameter set to zero, or if dynamic factors such as out-of-memory errors prevent successful JIT code generation. In these scenarios, the kernel falls back to interpreting the BPF bytecode using the standard interpreter rather than executing compiled machine code. The interpreter attempts to execute a call instruction by computing the target address based on the base of the bpf_call function plus the immediate value stored in the instruction operand. Because this value is still the raw helper ID and not a properly calculated offset or pointer, it results in an invalid memory access.
This invalid memory access triggers severe operational impacts, typically resulting in an instruction alignment fault or a complete kernel panic. The system becomes unstable or unresponsive, effectively causing a denial of service for any process relying on that specific BPF program or potentially affecting the entire host if the panic is not contained. Although valid C implementations exist for these helper functions within the kernel source code, the lack of proper offset rewriting means the interpreter cannot locate them safely. The core issue lies in the mismatch between the verifier's assumption of JIT availability and the runtime reality where fallback mechanisms are required but unsupported for this specific class of instructions.
The resolution involves modifying the BPF program structure to enforce strict requirements when helper call rewriting is skipped due to expected JIT inlining. By setting the jit_required flag to one on the program object, the kernel ensures that any attempt to load or run such a program without an available and functional JIT compiler will be rejected at load time rather than allowed through with fatal consequences at runtime. This preventive measure aligns with security best practices by failing securely when preconditions for safe execution are not met. From a vulnerability classification perspective, this issue relates to CWE-823, which covers the use of pointers or references that have been improperly modified, leading to out-of-bounds access or invalid memory operations. It also intersects with ATT&CK technique T1059, Command and Scripting Interpreter, as it involves the execution environment for scripting-like kernel code being compromised due to improper input validation regarding execution context capabilities.
To mitigate this vulnerability in environments where patching is not immediately possible, administrators should ensure that JIT compilation remains enabled if BPF programs utilizing these specific helper calls are required. Disabling JIT while relying on such programs will inevitably lead to system instability. Furthermore, monitoring for OOM events during program loading can help identify instances where dynamic fallback might occur unexpectedly. Long-term mitigation relies entirely on applying the kernel update that enforces the jit_required flag check, thereby ensuring that incompatible BPF programs are rejected before they can cause harm. This approach maintains system integrity by preventing the execution of code paths that lack necessary runtime support structures.