CVE-2026-98151

Medium · 5.5

CVSS

5.5

EPSS

0.1%

Published

Sep 25, 2026

CISA due date

—

Cloudanix analysis

Next patch cycle SSVC: Track

A bug in the Linux kernel's eBPF safety checker could let certain programs cause internal errors when loaded by regular users.

Exploitation: Local Privilege
Attacker needs: Low Privilege
Complexity: Low
Impact: Denial Of Service
Component: OS kernel
Class: Improper Input Validation
Remediation: Upgrade
Effort: Medium
Patch risk: Low

What it actually does

Inconsistent register state (var_off vs 32-bit subregister bounds) in the BPF verifier during sanitize_ptr_alu on speculative pointer arithmetic paths, causing reg_bounds_sanity_check to fail on unprivileged programs.

Why it matters to the business

Local users able to load unprivileged BPF programs can trigger verifier warnings or crashes, leading to availability impact on systems relying on eBPF for observability, networking, or security tooling; widespread in Linux distributions with recent kernels.

How an attack unfolds

An attacker compiles and loads an unprivileged BPF program that performs bounded scalar addition to a map value pointer, triggering the verifier path through adjust_ptr_min_max_vals, __mark_reg32_unbounded before the speculative snapshot, and reg_bounds_sanity_check on the subsequent instruction.

How to fix it

Update the Linux kernel to a version containing the fix that moves __mark_reg32_unbounded after sanitize_ptr_alu; rebuild and deploy any custom kernel packages or wait for distribution updates.

How to confirm the fix worked

Confirm the kernel source contains the moved __mark_reg32_unbounded call after sanitize_ptr_alu; attempt to load the previously triggering BPF program and verify absence of the REG INVARIANTS VIOLATION warning.

Typical attack path

Obtain unprivileged BPF loading capability on the target system
Craft a BPF program containing pointer arithmetic with a bounded scalar followed by a register copy
Load the program via bpf() syscall to reach adjust_ptr_min_max_vals and sanitize_ptr_alu
Trigger the inconsistent snapshot state that reaches reg_bounds_sanity_check

How to detect it

Review kernel logs for 'verifier bug: REG INVARIANTS VIOLATION' messages around bpf program loads; inspect audit logs for bpf syscall activity from non-root users.

What your security team should do

Prioritize kernel updates on systems exposing unprivileged BPF to untrusted users
Restrict unprivileged BPF loading via sysctl kernel.unprivileged_bpf_disabled=1 where feasible
Monitor for distribution kernel advisories and schedule patching within standard change windows

The record

In the Linux kernel, the following vulnerability has been resolved: bpf: Fix REG INVARIANTS VIOLATION on speculative pointer arithmetic Take the following unprivileged program as an example: r0 = bpf_map_lookup_elem(...) /* PTR_TO_MAP_VALUE, offset 0 */ ... 14: r0 += r1 /* r1 is a bounded scalar */ 15: r9 = r0 Loading it triggers a verifier warning from reg_bounds_sanity_check(): verifier bug: REG INVARIANTS VIOLATION (alu): const subreg tnum out of sync with range bounds r64={.base=0x0, .size=0x0} r32={.base=0x0, .size=0xffffffff} var_off=(0x0, 0x0) What happens: 1. Processing insn 14 (r0 += r1) in adjust_ptr_min_max_vals(), the new offset is computed into dst_reg's var_off and 32/64-bit ranges. 2. Because pointer registers do not track 32-bit subregister bounds, __mark_reg32_unbounded() first sets r32 to the full range; r32 is re-derived from the offset at the end of the function by reg_bounds_sync(). 3. On the unprivileged path, sanitize_ptr_alu() is called and, via sanitize_speculative_path() -> push_stack(), snapshots the current register state and schedules the next instruction (insn 15) to be verified directly as a speculative path. 4. That snapshot is taken between step 2 and the final reg_bounds_sync(): at this point dst_reg's var_off still holds the (const) original offset while r32 has just been blanked to the full range, i.e. the two are out of sync. When the speculative path later verifies insn 15 (r9 = r0), the inconsistent state reaches reg_bounds_sanity_check() and trips the warning. var_off and the 32-bit range must always be consistent. There are two ways to keep the snapshot consistent: 1. sync var_off and r32 before the snapshot so they match, or 2. leave r32 at its original (already consistent) value and blank it only after the snapshot. The whole point of sanitize_ptr_alu() is to insert a harmless masking sequence that keeps the access in bounds under speculation, so the state it snapshots should faithfully represent that. Take approach 2: move __mark_reg32_unbounded() to after sanitize_ptr_alu(), so the speculative snapshot keeps the pointer's original, consistent r32. The non-speculative path is unchanged: r32 is still blanked before the offset is applied and re-derived by reg_bounds_sync().

Affected packages

Ecosystem	Package	Introduced	Fixed