CVE-2024-42075: Linux Kernel BPF Arena Memory Safety Patch

The Linux kernel received a targeted, upstream fix in July 2024 for a memory-safety bug in the BPF arena subsystem — tracked as CVE-2024-42075 — that could produce a use-after-free when memory regions backed by the BPF arena are remapped. The patch adds a reference counter to account for multiple mmap/mremap events and prevents an unsafe free inside arena_vm_close; operators should treat this as an availability-first kernel bug with a real, but limited, exploitation surface and update affected kernels promptly.

Background / Overview​

The Extended Berkeley Packet Filter (eBPF) subsystem has become a core extension mechanism in modern Linux kernels: it lets users provide small programs and data structures that run inside the kernel for packet processing, tracing, security, and more. To allocate and manage BPF program and map storage efficiently, the kernel uses an internal allocator often called an arena — a compact pool of memory managed for frequent short-lived allocations. A subtle interaction between the arena bookkeeping and the kernel’s virtual memory remap (mremap) paths introduced a correctness gap: when a memory region backed by a BPF arena is remapped, the arena code did not increment a per-vma reference count that reflects multiple mmap-like events. Under some sequences this could allow arena_vm_close to free the arena while another vm_area_struct still references it, producing a use-after-free.
At a high level, CVE-2024-42075 is an availability-centered kernel defect: a local actor who can load or influence BPF programs or maps — particularly in environments that allow unprivileged BPF operations or that run multi-tenant workloads — can trigger a kernel crash or other stability failures. The Linux kernel CVE team and downstream sources assigned the issue CVE-2024-42075, described the fix, and point to the upstream patches that correct the logic by adding a refcount for multiple mmap events.

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What exactly went wrong (technical breakdown)​

The arena abstraction and mmap/mremap interactions​

• The BPF arena is an in-kernel allocation region used for compact, cache-friendly storage of BPF program and map data.
• Typical arena usage involves creating an arena object, mapping it into user or kernel address spaces via mmap, and later tearing it down when the owning entity is destroyed.
• The Linux virtual memory subsystem supports remapping (mremap), which can create a new virtual-area mapping that refers to the same underlying memory object without going through the original mmap path.

When the arena code assumed that each mapping corresponds to a single lifecycle and did not increment a counter for additional remap-created mappings, a race or sequencing issue could leave the arena freed while other vm_area_structs still pointed at it. The fix was to make the arena track multiple mmap-like events and only free when the last mapping is closed. This is implemented by adding a refcount (and corresponding increment on open/remap and decrement on close) so arena_vm_close cannot prematurely free a still-referenced arena.

Concrete symptoms and failure mode​

• The practical failure is a use-after-free inside kernel context, which typically manifests as kernel oops, panic, or other instability.
• The defect impacts availability — a local attacker can repeatedly trigger it to crash/deny-service a host.
• There is no public evidence that confidentiality or integrity guarantees are directly affected (the CVSS assessment and public records show Confidentiality: None, Integrity: None, Availability: High for the impact components).

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Affected kernels, introduction point, and upstream fixes​

• The issue was introduced in the 6.9 kernel series (commit identified by the kernel CVE notice) and was fixed in later stable kernels: the official announcement notes a fix in 6.9.8 and in 6.10 via the referenced commits. The kernel CVE announcement documents the affected file (kernel/bpf/arena.c) and the corrective commits.
• Public vulnerability catalogs (NVD, OSV, and multiple distro security trackers) mark the CVE as applying to kernels up to, but not including, 6.9 and to the 6.9.1–6.9.7 range (i.e., 6.9 introduced the regression and 6.9.8 contains the fix). Operators running mainline or distribution kernels that cherry-picked 6.9-era code paths should confirm whether the fix has been backported into their kernel build.
• Upstream patches referenced in public advisories include explicit commit hashes and small modifications to arena bookkeeping; distro vendors have the option of delivering those commits in vendor-specific kernel updates or shipping updated kernel packages.

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Severity and exploitability: what the numbers mean​

Multiple sources converge on a common assessment:

• CVSS v3.1 base score commonly reported: 5.5 (Medium) with vector roughly AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H — local attack vector, low complexity and low privileges required, no confidentiality or integrity impact, but high availability impact.
• The vulnerability is not labeled remotely exploitable; an attacker needs local code-execution capability or the ability to load BPF artifacts (or otherwise exercise the BPF mmap/mremap lifecycle) on the target host. This restricts the threat model primarily to local users, containers, or orchestrated multi-tenant environments where untrusted code can run BPF or cause mmap/mremap on BPF arenas.
• There were no widely published proofs-of-concept or reports of exploitation in the wild at the time the CVE and upstream fixes were published; the primary operational risk is recurrent denial-of-service (kernel crash) rather than stealthy privilege escalation or data theft. Monitoring sources that track exploitation attempts showed low EPSS/PoC activity for this specific CVE. Still, a well-resourced attacker on a multi-tenant system with local code execution could weaponize the failure to repeatedly impact availability.

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Who should be worried (threat model and real-world exposure)​

Not all Linux systems are equally exposed. Consider where the necessary conditions align:

• Hosts that permit unprivileged BPF loading or that run container runtimes with relaxed BPF restrictions are most at risk. Modern container platforms or CNI plugins that rely on BPF for networking (XDP, tc, Cilium, etc.) are places to pay attention.
• Multi-tenant virtualization hosts or cloud images that allow tenant workloads to exercise BPF or cause remap behavior are also higher risk — for example, poorly configured container hosts where untrusted users can load maps or cause remaps.
• Single-user desktop or locked-down servers where only trusted administrators can load BPF prove lower risk, but kernel failures on production servers are always disruptive and warrant patching.
• Cloud vendors and Linux distributions may have issued advisories or backports; operators must verify whether their specific images or vendor kernels include the fix even when the canonical kernel version number might not change. Historically, vendor attestations (for example, Microsoft’s Azure Linux mapping) are product-scoped inventory statements — useful, but not exhaustive — so customers should confirm per-artifact coverage in their environment.

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Practical detection: how to tell if you were hit​

There is no single forensic signature that guarantees exploitation, but defenders should look for these indicators:

• Repeated kernel oops/panic logs that mention BPF paths, arena_vm_close, or memory corruption in BPF-related code. Check dmesg, journalctl -k, and system crash dumps for late-stage references to kernel/bpf/arena.c or similar function names.
• Unplanned kernel restarts or live migration failures correlated with workloads that use BPF (packet filters, XDP, eBPF-based telemetry).
• In containerized environments, look for correlations between container lifecycle events and host kernel oopses — repeated crashes triggered by containers that exercise BPF map creation or use shared map memory may be suspicious.
• Absence of a public PoC reduces the probability of stealthy exploitation, but log-based detection is still valuable for identifying attempted or accidental triggers. If you see repeated, reproducible kernel crashes from local test cases, apply vendor patches immediately.

Caveat: Kernel crash logs may be noisy and do not always include human-friendly function names when compiled with optimizations or when symbolization is incomplete. When in doubt, collect vmcore and the exact kernel build string and consult your vendor’s kernel-team or distribution security advisory for guidance.

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Mitigation and remediation checklist (recommended actions)​

• Inventory: Determine the kernel builds in use across your estate. Gather kernel version strings (uname -a), distribution patches, and vendor kernel package versions.
• Confirm exposure: Check whether your hosts permit unprivileged BPF load or run workloads that use BPF maps and XDP. Container runtimes, CNIs, and observability agents are common BPF users.
• Patch: Update to a kernel that includes the upstream fix — mainstream guidance points to stable kernels that include the fix (e.g., 6.9.8 and kernels in 6.10 with the referenced commits). Where vendors have backported the patch, apply vendor-supplied kernel updates. If a rolling upgrade to the latest stable kernel is feasible, prefer that route.
• Workarounds (temporary): If immediate patching is impossible:
• Restrict who can load BPF programs or create maps. Tighten kernel.feature or capability controls (e.g., restrict CAP_BPF and CAP_PERFMON to trusted users).
• Harden container runtimes: disallow unprivileged BPF features in container policy, or run problematic workloads on dedicated nodes that are isolated from sensitive multi-tenant services.
• Implement resource/time limits for processes that could repeatedly exercise mmap/mremap paths to make denial-of-service harder to achieve.
• Monitor: Add dmesg and journal alerts for bpf/arena-related oops messages and watch for repeat kernel crashes correlated with BPF activity.
• Validate vendor attestations: If you rely on cloud images (for example, Azure Linux), check vendor security advisories and per-image attestations for the presence of the fix. Vendor product-level attestations are helpful but not a replacement for verifying the kernel build running in your environment.

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Vendor and distribution response — what operators should ask​

• Has my distribution backported the patch into its kernel package? Vendors sometimes backport fixes without changing the upstream version string; rely on vendor security advisories rather than kernel version numbers alone.
• For cloud instances and images, confirm whether the image kernel includes the commit or the backport. Product attestations (e.g., “this image is potentially affected”) are useful starting points, but must be validated against the actual kernel in use. Microsoft’s approach to attesting which Microsoft artifacts include an upstream library or kernel component has been product-scoped in prior advisories; treat these attestations as inventory signals and verify per-image kernel content when possible.
• If you maintain custom or vendor-packaged kernels (WSL, Azure image kernels, OEM kernels), request concrete VEX/CSAF or SBOM-style attestations that confirm whether the specific kernel build contains the fix or the backport.

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Attack scenarios and risk assessment (practical lens)​

• Low-skill DoS: A local user or unprivileged container that can load or coerce BPF artifacts into creating particular mmap/mremap sequences could produce repeated kernel oopses and effectively deny service. This requires local access or a workload that already has BPF-loading privileges.
• Multi-tenant risk: On a shared node, attackers who can run containers with BPF capabilities or who can cause file-backed mappings to be remapped may force the host kernel into instability, potentially affecting all tenants on the node.
• Privilege escalation: Public advisories and CVE analyses did not indicate a direct privilege escalation or remote-code execution path from this flaw alone. The primary operational impact is availability; however, any kernel memory corruption theoretically raises the risk surface in complex exploit chains, so defenders should not dismiss the potential for chaining with other bugs.

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Why this matters: the operational cost of kernel availability failures​

Kernel instability has outsized operational cost compared with user-space failures:

• Crashes and oopses can bring down multiple services, cause state loss, or require disruptive host restarts that trigger failover or migration events.
• In tightly coupled systems — distributed storage, networking nodes, hypervisor hosts — a single host kernel crash can ripple through the cluster.
• Even when the vulnerability is local by design, modern infrastructure often runs large fleets where local privilege may be easier to obtain (misconfigured containers, overly-permissive capabilities, or third-party software). That makes availability bugs like CVE-2024-42075 operationally important even when the CVSS base score is “medium.”

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Developer and maintainer notes (for kernel teams and packagers)​

• The upstream fix is minimal and focuses on correct reference counting for multiple mmap-like events; maintainers should prefer applying the upstream commits to preserve semantics rather than attempting ad-hoc rewrites. The kernel CVE team specifically recommends updating to the latest stable kernel if feasible.
• When backporting, ensure that the surrounding arena lifecycle code in kernel/bpf/arena.c is consistent with the rest of the BPF subsystem in the target branch; missing supportive changes can introduce regressions.
• QA expectation: incorporate stress tests that exercise BPF map lifecycle, mmap, and mremap sequences under concurrency to prevent similar regressions from slipping in unnoticed.

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Cross-references and independent confirmation​

Multiple independent tracking and advisory sources describe this defect consistently:

• The Linux kernel CVE announcement (linux-cve-announce) and upstream commit notes document the introduction point, the affected file (kernel/bpf/arena.c), and the fix.
• Public vulnerability databases, including NVD and OSV, list CVE-2024-42075 with CWE-416 (Use-After-Free) and describe the fix and affected version ranges.
• Security vendors and vulnerability trackers (Wiz, cvefeed, and others) provide corroborating CVSS and exploitability assessments and link to the upstream patches; many note the availability impact and recommend patching.
• For operational context on vendor attestations and product-scope statements (how vendors like Microsoft describe Linux component inclusion in Azure images), see discussion and past examples where vendor advisories identify Azure Linux as a carrier of upstream components — such statements are useful but should be validated per-artifact.

If you need proof that the fix exists in upstream history, the kernel CVE announcement and public git commit references supply the authoritative commits to review or cherry-pick for backporting.

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Recommended timeline and priorities​

• Immediate (within 24–72 hours): Inventory kernels and confirm whether hosts are running affected versions. If you operate multi-tenant nodes or permit unprivileged BPF, prioritize those systems.
• Short-term (1–2 weeks): Apply vendor-supplied kernel updates that include the patch. If vendor updates are not available, consider targeted mitigations (restrict BPF capabilities or isolate workloads).
• Medium-term (1–3 months): Integrate BPF-related lifecycle tests into regression suites, and verify that backported patches are present in vendor kernels through SBOM/VEX artifacts where available.
• Long-term: Adopt a policy to track upstream fixes for kernel subsystems used in critical infrastructure (BPF, networking, storage) and ensure quick backport/validation workflows for security fixes.

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Final analysis: strengths of the fix and residual risks​

The upstream remedy for CVE-2024-42075 is surgically focused and corrects the fundamental bookkeeping mistake: adding a refcount for multiple mmap events is the canonical approach to preventing premature free during vm_area lifecycle transitions. That minimal, well-scoped change reduces the risk of regression and keeps the kernel semantics straightforward.
Strengths:

• The fix addresses the root cause — missing accounting for mremap — rather than patching symptoms.
• Upstream release (6.9.8 and 6.10) and vendor advisories provide clear upgrade or backport pathways for operators.

Residual risks and cautions:

• The exploitability requires local access, but that can be obtainable in cloud or container environments; multi-tenant hosts should be considered higher risk.
• Backports into vendor kernels must be verified. Distribution-specific builds or OEM kernels may lag; do not rely on version numbers alone. Validate the precise patch presence in the kernel build.
• While this CVE’s primary impact is availability, any kernel memory-corruption bug increases the risk surface for chained exploits; defenders should treat kernel memory bugs seriously even when initial impact appears limited to stability.

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CVE-2024-42075 is a good example of how small lifecycle bookkeeping errors in high-frequency kernel subsystems (like BPF arenas) can translate into real operational risk. The remedy is clear, the upstream community identified and fixed the regression quickly, and the operational path forward is straightforward: confirm whether your kernels are affected, apply vendor or upstream fixes, or implement temporary hardening to limit untrusted BPF or mapping operations. Doing so will remove the direct availability risk and reduce the chance that this class of memory-safety issue can be weaponized in your environment.

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Source: MSRC Security Update Guide - Microsoft Security Response Center