CVE-2024-26884: Patch for 32-bit Linux BPF hashtab overflow

A small, surgical change to the Linux kernel’s BPF hashtab code fixed a subtle integer‑overflow check that could be triggered on 32‑bit systems and lead to kernel instability or denial‑of‑service; the defect is tracked as CVE‑2024‑26884 and was introduced by a misplaced overflow test that ran after a dangerous rounding operation, allowing undefined behavior on 32‑bit arches.

Background​

The Linux kernel’s extended Berkeley Packet Filter (eBPF/BPF) subsystem exposes map types and in‑kernel data structures used by tracing, networking and security tooling. One of the internal helpers used to build BPF hash maps is a hashtab allocator that computes the number of hash buckets as the next power of two of the requested maximum entries. On many platforms this is safe, but on 32‑bit architectures the roundup_pow_of_two() operation can itself overflow when shifting a 32‑bit unsigned long left past its width, producing undefined behavior. That undefined behavior can defeat later sanity checks and let a crafted request cause an invalid allocation pattern or kernel crash.
This class of problem — integer overflow leading to out‑of‑bounds allocations or truncated sizes — is well understood in systems programming, but it remains a recurring risk at low levels of the kernel where code commonly manipulates raw sizes and bit shifts for performance reasons. The issue was discovered in practice by automated kernel fuzzing (syzbot) which found the condition while exercising DEVMAP_HASH‑style maps that reuse the same logic.

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What exactly went wrong (technical root cause)​

The vulnerable pattern​

The vulnerable code path computed the bucket count using:

• read max_entries from user attr
• call roundup_pow_of_two(max_entries) to compute n_buckets
• then check if (n_buckets == 0) or if (n_buckets > U32_MAX / sizeof(bucket)) to detect overflow / oversized allocations

On 32‑bit builds, roundup_pow_of_two() implementation performs left‑shifts on an unsigned long. If max_entries is large enough, the internal left shift can shift into or past the high bit of the 32‑bit unsigned long. In C, left‑shifting into or past the width of the integer is undefined behavior, so the result is not guaranteed to be the truncated value the later check expects. The overflow can therefore occur inside roundup_pow_of_two() before the code ever evaluates n_buckets == 0, which leaves the check blind to the overflow.

Why the timing of the check matters​

The vulnerable check was placed after the rounding call. The correct defensive approach is to validate inputs before performing operations that can invoke undefined behavior — in this case, limit the max_entries value to a threshold under which roundup_pow_of_two() cannot overflow on the target type. The upstream fix moves or adds the overflow test so it runs prior to the rounding operation, avoiding undefined left shifts entirely. The change is small but critical: moving a single check before a shift removes the undefined behavior and restores a sound allocation-size computation.

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The upstream patch (what maintainers changed)​

The fix, committed upstream and propagated into stable trees, applies the following logic:

• Add a pre‑roundup guard that rejects max_entries values larger than a safe threshold on 32‑bit unsigned long (for example, if (max_entries > 1UL << 31) return error).
• Then call roundup_pow_of_two(max_entries) only after that guard.
• Retain or adjust later u32‑overflow checks against U32_MAX / sizeof(bucket) to ensure that the total memory request cannot exceed map indexing limits.

The kernel commit message and the stable‑tree note spell this out and include the small diff that performs the pre‑check and removes the earlier post‑roundup zero check. The patch has been cherry‑picked into the stable kernel trees to ensure distributions and OEMs can ship the fix quickly.

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Impact and severity — what the CVE means in practice​

• The National Vulnerability Database (NVD) and several vendor trackers assign high/important impact to CVE‑2024‑26884; the commonly cited CVSS v3.1 base score is 7.8 (High) with an attack vector and privileges consistent with a local attacker who can create BPF maps.
• The practical impact is primarily availability (denial‑of‑service / kernel crash) and integrity (memory corruption leading to more severe failures). Several vendor writeups also flag theoretical privilege escalation as a potential outcome in some exploitation chains, though that requires additional conditions and is not established as widely‑reliable remote code execution. Treat escalation claims cautiously: they are plausible outcomes of memory‑corruption bugs but are not demonstrated universally for this specific defect.
• The defect is architectural: it only triggers on 32‑bit kernels because the undefined left shift arises from using unsigned long sized to 32 bits. On 64‑bit systems the same shift does not overflow in the same range, so the bug is not present there. That said, embedded devices, routers, and specialized appliances still running 32‑bit kernels remain relevant targets.

Because the attack surface requires local ability to issue BPF map creation calls (or a process capable of making bpf() syscalls), the vector is not fully remote in the classic sense, but unprivileged access to BPF has been a real-world concern. Many modern distros and kernel configs already harden or restrict unprivileged BPF usage; those controls materially reduce the exploitation surface for CVE‑2024‑26884.

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Who and what is affected​

Kernel code and versions​

• Multiple vulnerability databases and vendor advisories map the issue to the upstream Linux kernel patch that landed in mainline and was backported into the stable 6.x trees. Distribution patches followed quickly; affected kernels typically include versions before the fix applied (early 6.x kernels and older stable branches). Some tracking services list a wide affected range (kernels compiled from roughly 3.19 up to but not including the patched 6.8.2). Exact affected versions depend on whether a distribution carried the vulnerable code path and whether they backported the upstream fix.

Architectures and products​

• This is a 32‑bit architecture specific problem — ARMv7, older x86/i386 builds, MIPS 32‑bit, and embedded SOCs running 32‑bit kernels are the most relevant targets. Many mainstream desktop and server installations today run 64‑bit kernels, but embedded Linux devices and older appliances still in production can be exposed.

Distributions and vendor advisories​

• Major distribution security trackers and vendors (Ubuntu, SUSE, Amazon Linux / ALAS, Red Hat advisories and others) published remediations and kernel updates after the upstream patch; administrators should consult their distro's security feed for the exact package versions that contain the fix. Several vendor trackers list this CVE and recommend kernel updates as the primary remediation.

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Exploitation scenarios — what an attacker can realistically do​

Below are plausible exploitation models, ordered from most likely to least:

• Denial‑of‑Service (DoS) — the most straightforward impact: a user or unprivileged process that can request a crafted BPF map triggers the overflow path, causing a kernel panic, oops, or memory corruption that takes the host or service offline. This is the most immediate operational risk and the one observed by syzbot’s fuzzing.
• Local privilege escalation (LPE) — using the memory corruption to corrupt BPF internal data structures in a way that permits escalation to higher privileges or code execution. This requires precise control over allocation and layout and successful steering of corrupted pointers; while possible in principle for this class of bug, it is not a guaranteed result in every kernel build or configuration. Several vulnerability summaries list LPE as a possible impact vector but do so with caveats about exploitability.
• Pivoting or chaining — in a multi‑tenant context where an attacker already controls a container or unprivileged process, exploiting the bug to crash host kernel threads or destabilize isolation can be used as a stepping stone in broader attacks. This is more complex but worth defending against in cloud and edge environments.

Note: there’s no authoritative public proof‑of‑concept widely circulated at time of writing that reliably turns CVE‑2024‑26884 into a remote, unauthenticated root exploit against modern 64‑bit hosts. The immediate operational risk remains availability and targeted local attacks on 32‑bit systems.

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Detection, monitoring and incident response guidance​

• Watch for kernel logs and OOPS: investigate repeated "kernel oops", stack traces mentioning bpf, hashtab, htab_map_alloc, or unexplained kernel panics on 32‑bit hosts. The upstream commit names functions and file kernel/bpf/hashtab.c that can appear in traces. Correlate such crashes with recent BPF activity or userspace processes creating BPF maps.
• Audit BPF mmap/map creation activity: log or monitor bpf() syscall invocations where possible. Container runtimes, monitoring agents, or seccomp policies can be instrumented to detect unusual map creation patterns. Look for processes issuing map create operations with anomalously large max_entries values.
• Use host integrity checks: in environments that run 32‑bit kernels you should raise the priority of kernel package updates and consider short maintenance windows to roll out fixes to avoid persistent availability problems. Vendor advisories are the authoritative source for the exact package names and versions.

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Immediate mitigations and hardening steps​

If you cannot immediately apply a vendor kernel update, apply defensive measures that reduce the attack surface and limit the ability of unprivileged users to exercise BPF map creation:

• Patch the kernel: the single best mitigation is to install the vendor kernel update or upstream stable kernel containing the bpf: Fix hashtab overflow check on 32‑bit arches commit. Distributions already published packages once the fix reached stable trees. Prioritize 32‑bit hosts and embedded devices.
• Disable unprivileged BPF syscall access: use the kernel.unprivileged_bpf_disabled sysctl to block unprivileged (non‑CAP_SYS_ADMIN / non‑CAP_BPF) callers from invoking bpf() where your kernel supports it. Setting kernel.unprivileged_bpf_disabled=1 (or 2 in some configurations) will prevent untrusted local users from creating maps and loading programs, significantly lowering the risk from local mischief. This knob is documented and used by security guidance and STIGs; it is a practical, immediate hardening lever. Note: once set to 1 it may require a reboot to reverse on some kernels.
• Tighten capabilities and seccomp: ensure only trusted services/containers have CAP_SYS_ADMIN or CAP_BPF, and apply seccomp filters in containers to forbid the bpf() syscall where it is unnecessary. Reduce the set of processes that can create BPF maps and load programs.
• Constrain user namespaces / container privileges: container orchestration platforms should disallow privileged containers or limit the ability for containers to request capabilities that enable bpf() usage. Where possible, run network and tracing agents in a privileged host context with controlled interfaces rather than granting containers broad kernel operation capability.
• Audit, limit, or patch third‑party tooling: some userland frameworks or tooling that create BPF maps may include parameters that allow large max_entries. Review such tooling (e.g., in-house tracing agents, observability frameworks) to ensure they validate input sizes and do not allow malformed values passed to kernel map creation APIs.

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Recommended remediation playbook (step‑by‑step)​

• Identify 32‑bit hosts and appliances: inventory all systems and firmware that run 32‑bit kernels. These are the primary population at risk.
• Prioritize kernel updates: apply vendor patches for kernels that include the upstream commit that fixes the hashtab overflow check. Use vendor advisories to map package names and fixed versions.
• If immediate patching is impossible, set kernel.unprivileged_bpf_disabled=1 (or 2 where appropriate) on affected systems and document the change. This reduces the attack surface for local unprivileged processes. Remember that on some kernels 1 is irreversible without a reboot.
• Harden container and service configurations: remove unnecessary capabilities, enforce seccomp policies that deny bpf(), and limit CAP_BPF/CAP_SYS_ADMIN usage.
• Monitor logs and alerts: set alerts for kernel oopses and repeated BPF failures as an operational indicator of attempted exploitation. Correlate with process activity and recent software changes.

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Risk analysis: strengths of the fix, remaining concerns​

Strengths​

• The upstream fix is conceptually and practically simple: validating the input before performing the risky rounding operation removes the undefined behavior at its source. Simple fixes are easier to audit and less likely to introduce regressions.
• The patch landed in the stable trees quickly and was distributed via vendor advisories, which reduces the window of exposure for updated systems.
• Existing kernel hardening knobs (like unprivileged_bpf_disabled) and distribution policies to default to stricter BPF settings further reduce exploitation likelihood on many modern systems.

Remaining concerns and caveats​

• The issue only affects 32‑bit builds: many organizations assume their environment is unaffected because most modern servers are 64‑bit. That assumption is risky where legacy or embedded devices exist; inventory gaps will leave those hosts exposed.
• Memory corruption bugs in the kernel are context‑sensitive: whether a given memory corruption leads to a recoverable crash, persistent denial‑of‑service, or a full privilege escalation depends on allocator behavior, kernel config, compiler optimizations, and other subtle factors. Some of those factors vary across distributions and device firmware, making definitive exploitation claims hard to generalize. Flag such escalations as possible but not universal without proof‑of‑concept validation.
• Operational constraints may delay patching on embedded and fielded appliances (air‑gapped devices, vendor‑locked firmware, or devices where kernel upgrades are nontrivial). Those systems require compensating controls (restrict local user access, firewall management ports, physical security) until updates can be applied.

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Long‑term lessons for administrators and developers​

• Validate before you transform: the canonical lesson here is to validate inputs at the earliest safe point — never trust that a later check will catch undefined behavior produced by a previous transformation. This is especially true for bit‑width dependent operations.
• Prefer explicit integer‑width types for arithmetic where overflow matters: using u32, u64, or explicit bounds checks helps make intent clear and reduces reliance on unsigned long width assumptions when code is intended to run on multiple architectures.
• Embrace defense‑in‑depth for subsystems that expose kernel programming surfaces (BPF, io_uring, etc.): combine principle‑of‑least‑privilege (capability restrictions), sysctl knobs to control unprivileged access, and observability for syscall usage to create layered defenses.

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Conclusion​

CVE‑2024‑26884 is a textbook example of how a placement of a single overflow check — before versus after an operation — changes the security posture of kernel code. The bug affected the BPF hashtab code on 32‑bit architectures by letting roundup_pow_of_two() perform an undefined left shift before the code validated the preconditions. That undefined behavior was flagged by automated fuzzing and fixed upstream with a small but important change that moves the guard into the safe position.
For defenders, the prescription is straightforward: patch 32‑bit kernels now, mitigate with kernel.unprivileged_bpf_disabled and capability/ seccomp restrictions where immediate patching is not possible, and audit legacy devices that may still use 32‑bit kernels. For engineers, the bug is a reminder to prefer early validation over late checks and to treat integer‑width dependent operations with particular care.
CVE‑2024‑26884 has a high impact on affected 32‑bit hosts, but the combination of a minimal, correct upstream fix and the availability of distribution patches means risk can be eliminated quickly with standard patch management — provided teams identify and prioritize the vulnerable systems.

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Source: MSRC Security Update Guide - Microsoft Security Response Center