CVE-2026-89906 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
LoongArch: BPF: Refactor jump offset calculation in tail call
The old macro-based jmp_offset calculation derives the jump distance from a stale prior-pass code stride, which can lead to wrong branch offsets and soft lockups under extra JIT passes.
Fix this by calculating the offset directly on the absolute target: "ctx->offset[insn + 1] - ctx->idx".
To avoid a false 16-bit range check abort during size estimation, add a "ctx->image == NULL" guard to inject a safe dummy offset.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel's eBPF Just-In-Time (JIT) compiler for the LoongArch architecture contained a critical logic error in how it calculated jump offsets for tail calls. This vulnerability stems from an outdated macro-based approach that derives the branch distance based on stale code stride information accumulated during previous compilation passes. In complex scenarios involving multiple JIT optimization or translation passes, this reliance on historical state rather than current absolute positions results in incorrect branch target calculations. When the kernel attempts to execute these miscalculated jumps, it can lead to control flow errors within the eBPF program execution environment.
The operational impact of this flaw is severe, primarily manifesting as soft lockups or system hangs. Because the jump offset points to an invalid memory location or a non-existent instruction sequence, the CPU may enter an infinite loop or trigger a fault that halts the scheduler's ability to preempt tasks effectively. This denial-of-service condition affects not only the specific eBPF program but can potentially destabilize the host kernel if the error propagates through critical subsystems relying on BPF for networking, tracing, or security enforcement. The issue is particularly insidious because it may not trigger immediately during initial compilation but rather emerges under conditions that require additional JIT passes to resolve complex instruction sequences.
To remediate this vulnerability, the fix involves refactoring the jump offset calculation logic to determine distances directly from absolute target addresses rather than relative strides. Specifically, the implementation now computes the offset using the formula ctx->offset[insn + 1] minus ctx->idx, which ensures that each branch is calculated based on its actual position in the generated machine code image. This approach eliminates dependency on potentially stale intermediate states and guarantees accurate control flow transfer during tail calls. Additionally, a guard condition checking if ctx->image equals NULL was introduced to handle edge cases during size estimation phases where no valid memory buffer exists yet.
This correction prevents false positives from 16-bit range check aborts that could occur when the system attempts to validate offset sizes before the image buffer is fully allocated or initialized. By injecting a safe dummy offset in these specific scenarios, the kernel avoids premature termination of legitimate compilation processes while maintaining strict bounds checking integrity once actual code generation begins. This dual-layer fix addresses both the logical error in branch calculation and the structural vulnerability related to uninitialized memory access during estimation phases.
From a security taxonomy perspective, this flaw aligns with CWE-682 Incorrect Calculation, as the core issue is a miscalculation of memory offsets leading to erroneous control flow. It also relates to CWE-401 Missing Release of Memory after Successful Allocation if the stale state leads to resource leaks in complex JIT scenarios, though the primary impact is availability rather than confidentiality or integrity. In terms of adversarial tactics, this vulnerability could be leveraged within ATT&CK technique T1203 Exploitation for Defense Evasion, where an attacker might craft malicious eBPF programs to trigger kernel panics and disrupt security monitoring tools that rely on BPF-based tracing.
Mitigation strategies should prioritize applying the upstream Linux kernel patch immediately to affected LoongArch systems. Administrators running custom or out-of-tree kernels must ensure their JIT compiler implementations are updated to reflect this absolute offset calculation method. For environments where immediate patching is not feasible, restricting unprivileged users from loading arbitrary eBPF programs via BPF syscall restrictions can reduce the attack surface. Continuous monitoring for kernel soft lockups associated with network packet processing or tracing activities may help detect exploitation attempts of similar JIT-related flaws in other architectures.