The Linux kernel received a low‑to‑medium severity vulnerability report identified as
CVE-2024-0639, a subtle locking bug in the
SCTP subsystem that can trigger a kernel deadlock on the per‑net workqueue lock
net->sctp.addr_wq_lock, allowing a local attacker to cause a denial‑of‑service (DoS) by making the system unresponsive or crash. Patches landed in the upstream kernel to change the locking context in the SCTP codepath — a surgical change from
spin_lock()/
spin_unlock() to
spin_lock_bh()/
spin_unlock_bh() — but the incident underscores how a single mismatched lock variant between process and bottom‑half (timer/softirq) contexts can turn a benign initialization path into a system‑wide availability failure.
Background
SCTP (Stream Control Transmission Protocol) is a transport‑layer protocol supported by the Linux kernel for specialized networking scenarios (multi‑homing, telephony signaling, etc.). Its kernel implementation uses per‑network (per‑net) data structures and timers to maintain address lists and perform periodic housekeeping. One such structure is the address workqueue with an associated lock named
addr_wq_lock; by design, that lock must be taken consistently across all codepaths that access the workqueue — whether from process context, softirq/timer callbacks, or other bottom‑half contexts. When those contexts disagree about lock semantics (for example, process code uses a plain
spin_lock() while a timer callback expects bottom‑half handling to be disabled), a deadlock or soft‑lockup becomes possible.
The vulnerability was publicly cataloged as CVE‑2024‑0639 and given a medium severity by several vulnerability databases, reflecting the fact that exploitation requires local access and that the impact is
availability (DoS) rather than confidentiality or integrity compromise. Upstream kernel maintainers accepted a narrowly scoped fix that prevents the deadlock by ensuring the lock disables bottom halves while held from the process path.
What happened: the technical root cause
Where the deadlock occurs
At the heart of CVE‑2024‑0639 is the function
sctp_auto_asconf_init() in
net/sctp/socket.c. This function conditionally inserts sockets into a global per‑net list (
auto_asconf_splist) while acquiring
net->sctp.addr_wq_lock using the plain
spin_lock()/
spin_unlock() pair. Elsewhere in the SCTP subsystem, timer handlers and timeout code run in a bottom‑half/softirq context and may also take
addr_wq_lock. If a timer callback runs while the process code is holding the lock — or vice versa — the mismatched expectations about disabling bottom halves can create a circular wait condition and a hard deadlock on the CPU. The mismatch is precisely what upstream maintainers corrected.
Why spin_lock_bh() fixes it
spin_lock_bh() is the kernel’s spinlock variant that disables bottom halves (softirq/softirqs) on the current CPU before taking the lock, and
spin_unlock_bh() re‑enables bottom halves after releasing the lock. Using the
_bh variants ensures that timer callbacks and other bottom‑half handlers cannot interrupt and try to reacquire the same lock while it is held in process context. The upstream patch replaces the plain
spin_lock()/
spin_unlock() pair with
spin_lock_bh()/
spin_unlock_bh() in exactly the small code fragment that was observed to create the deadlock window. The change is intentionally minimal to avoid broader behavior changes while eliminating the race.
History and context: why SCTP has complex locking
SCTP’s kernel code has a long history of subtle concurrency issues because the protocol performs asynchronous operations (timers, heartbeats, retransmits, address management) while also exposing sockets to blocking process contexts (accept(), bind(), etc.). Previous patches in the SCTP tree have adjusted lock usage and orderings to avoid races between accept/listen and timeout handlers, demonstrating that these interactions are brittle and easily mis‑synchronized. The CVE fix is therefore consistent with prior work: protecting a shared data structure consistently across process and bottom‑half contexts.
Affected systems and scope
- The underlying bug is in the Linux kernel upstream source inside
net/sctp/ (the SCTP subsystem). Upstream commits reference kernel versions up to the affected stable branch; vendors mapped and shipped fixes into their distribution kernels.
- Several major distributions and vendor advisory pages list the issue and their remediation status; Amazon Linux, SUSE, and vendor feeds reported the flaw and provided packaged kernel updates or backports where appropriate. The CVSS assessment varies among sources but consistently identifies the impact as availability (DoS) and the attack vector as local.
- Exploitation requires local access (unprivileged or low‑privileged users that can open/bind SCTP sockets or otherwise exercise the affected code paths). That means multi‑tenant hosts, containers, shared CI runners or virtual machines where untrusted users get shell access are higher‑risk scenarios. Network‑only remote attackers cannot trigger the condition unless local code execution is available.
Risk is concentrated on servers or appliances where multiple users share kernel resources. Cloud instances or hosted VMs that permit user logins — or systems that expose guest environments — should prioritize mitigation.
Impact and exploitability analysis
Impact: total loss of availability
The vulnerability results in a
deadlock, which means CPU(s) can be hung waiting on a lock that will never be released, leading to a system hang, kernel soft‑lockup, or crash depending on workload and timing. That is a classic denial‑of‑service impact: critical processes and networking may stop responding, requiring a reboot. Datacenter and cloud services that rely on high availability can be negatively affected. Multiple advisories classify the impact as high for availability while noting no confidentiality or integrity effects.
Exploitability: local, timing dependent, practical but not trivial
- Local access required. The attacker needs the ability to exercise SCTP codepaths (socket operations) or manipulate conditions that let the code hit
sctp_auto_asconf_init() concurrently with timeout handlers. This reduces the remote attack surface.
- Timing/race required. The issue is a concurrency race between process and timer contexts. Reliable exploitation typically requires precise timing or a fast, repeated trigger loop (syzkaller/fuzzer or crafted local workload), making widespread exploitation less trivial than a simple crash‑on‑connect bug. That said, automated local exploit scripts are feasible in controlled environments and syzkaller reports and mailing list threads indicate fuzzers can reproduce these classes of races.
- Net effect in practice. On multi‑tenant or development machines, untrusted users or containerized processes can trigger a DoS with modest effort; on single‑user desktops the practical exploitation opportunity is limited. Cloud providers and ISPs should regard the vulnerability as meaningful for tenant isolation guarantees.
Because the vulnerability only affects availability, exploitability does not immediately allow privilege escalation or code execution. However, availability failures in production systems lead to operational and business risk, so the practical severity is frequently escalated by operations teams responsible for uptime.
The fix: what changed in the kernel
Upstream maintainers applied a targeted patch that switches the lock pair around the
auto_asconf_list modification in
sctp_auto_asconf_init() from plain spinlock calls to the bottom‑half disabling variants. The diff is tiny — two lines changed — but it addresses the crux of the deadlock by preventing softirq/timeout handlers from interrupting the protected critical section. Upstream commit metadata and changelogs note the patch as "sctp: fix potential deadlock on &net->sctp.addr_wq_lock", and the change was backported into stable trees and vendor kernels.
There were follow‑up tests and discussion on kernel mailing lists and automated fuzzers (syzkaller) about reproductions and whether additional related codepaths required fixes or testing iterations. That is normal for concurrency fixes: testers try to stress the change and confirm the race window is eliminated across architectures and kernel configurations. Some reports show syzkaller‑style runs continued to find adjacent issues until a second patch or follow‑up was landed. Administrators should therefore prioritize vendor kernel updates that include the full set of related commits and QA backports, not just a single cherry‑picked patch.
Vendor responses and patch availability
Multiple vendors and distribution security teams published advisories or incorporated the upstream fixes into their kernel packages:
- Upstream commit and stable tree entries were recorded and tagged in the kernel Git history; the commit replaces
spin_lock()/spin_unlock() with spin_lock_bh()/spin_unlock_bh() at the critical site.
- Distribution advisories (Amazon Linux, SUSE, Red Hat forks) listed the issue and provided CVE entries, often with variant CVSS scoring reflecting each vendor’s view of attack complexity and exposure. Some vendors backported fixes to LTS kernels; others noted the issue was fixed in newer kernel series. Administrators must consult their distribution's advisory and install the supplied kernel update rather than attempting arbitrary cherry‑picks.
- The NVD and CVE aggregation pages list the vulnerability with the canonical description and timeline. Those records help security teams track systems by CVE ID in asset management tools.
Because kernel upgrades require reboots, patching cadence should be planned to minimize downtime — but given the DoS risk and the local exploit vector applicable to multi‑tenant systems, scheduling an early maintenance window is advised.
Mitigations and recommended actions for administrators
Apply these steps in the order that fits your environment. Where possible, coordinate with provider or vendor guidance.
- Identify affected systems.
- Search inventory for kernels built from upstream trees or vendor kernels prior to the patch date. Use your asset management tools to detect kernels containing the vulnerable
net/sctp tree revision or the CVE tag.
- Patch promptly.
- Install vendor‑provided kernel updates that include the SCTP fix and reboot the hosts. Prefer vendor packages and kernels tested for your distribution rather than upstream tarball builds unless your team can fully QA a custom kernel.
- If immediate patching is impossible, apply temporary mitigations.
- Disable the SCTP kernel module or remove SCTP support if your workloads do not use it: unload
sctp module or disable CONFIG_SCTP in custom kernels. That removes the attack surface but impacts any legitimate SCTP traffic. Use modprobe -r sctp (with caution) or blacklist the module until a patch is applied.
- Harden tenant isolation — reduce the number of untrusted users allowed to create sockets or bind network devices; restrict container capabilities so containers cannot exercise network stack operations that reach the vulnerable paths.
- Monitor for exploit attempts.
- Watch for kernel OOPS/soft‑lockup messages referencing
sctp_addr_wq_timeout_handler, sctp_auto_asconf_init, or “deadlock”/“soft lockup” patterns in kernel logs. Fuzzer reports suggested these are reproducible with stress harnesses; real‑world attempts will mirror that behavior.
- Validate fixes after upgrade.
- After patching and rebooting, run workload validation and, for high‑risk systems, run stress tests or the vendor‑recommended test harness to confirm the system is stable under load and timers. Concurrency fixes sometimes expose other timing windows, so confirmation is necessary.
Practical guidance for cloud providers and multi‑tenant hosts
Cloud operators and hosting providers should treat this CVE as an availability and multi‑tenant isolation issue:
- Prioritize hypervisors, VM hosts and container host kernels for patching, even if guest images are not immediately affected, because local guest code may be able to trigger the bug in the host kernel via shared kernel interfaces.
- If patching hosts requires host reboots, consider live migration to move tenants off hosts during maintenance windows, and communicate clearly with tenants about the DoS risk and maintenance plan.
- Ensure that tenant‑facing images or templates do not enable unnecessary kernel modules (like SCTP) if not required by tenants, to reduce the exposed attack surface.
Why this kind of bug still matters
Concurrency and lock‑context mismatches are a perennial source of kernel reliability issues. Modern kernels are highly concurrent and rely on precise discipline around locking semantics:
- A one‑line mismatch (using
spin_lock() where spin_lock_bh() is required) can escalate from benign inconsistency to a full system hang under the right timing.
- Patches that change bottom‑half/softirq behavior are delicate; they must be narrowly targeted and validated on architectures and configurations used in production.
- The presence of active fuzzers (syzkaller) and continuous integration testbeds increases the chance such bugs are found before attackers weaponize them — but discovery does not guarantee immediate vendor patch availability for every distribution or kernel branch.
For security teams, the lesson remains: even medium CVSS vulnerabilities that only affect availability deserve urgent attention when they can be triggered by untrusted local users in multi‑tenant contexts.
Caveats, unknowns, and verification notes
- Public exploit code for CVE‑2024‑0639 was not in wide circulation at the time of the vendor advisories; the vulnerability is timing/race dependent and generally reproduced by fuzzers and stress tools rather than by a single trivial command. However, the absence of public PoC code today does not guarantee adversaries cannot craft reliable triggers in the future. Treat the vulnerability as a live operational risk until all affected hosts are patched.
- The patch is small and low risk in scope, but follow‑on testing revealed adjacent timing windows in some testbeds. That means administrators should prefer vendor QAed packages. If you must backport a minimal patch yourself, test under stress on non‑production hardware first.
- The mapping of affected versions differs slightly between vendor advisories. Some vendors score the bug with a higher attack complexity or different CVSS vector strings; consult the specific vendor advisory for how your distribution’s kernels were scored and which package version contains the remediation.
Final recommendations and a short checklist
- For production servers, especially multi‑tenant or shared systems: schedule an immediate kernel update that includes the SCTP
addr_wq_lock fix and reboot.
- For development or single‑user machines: patch when convenient but prioritize systems with untrusted users or CI runners.
- If you cannot immediately patch: consider unloading or blacklisting the
sctp module and strengthening local user restrictions.
- After patching: run kernel sanity checks and stress tests, and monitor kernel logs for signs of lingering or related race conditions.
- Keep an eye on vendor advisories and stable tree notes: concurrency fixes may have follow‑ups and distributors will bundle the correct backports for your kernel series.
CVE‑2024‑0639 is a textbook reminder that kernel concurrency is unforgiving: the fix is small but the consequences of delay can be severe for availability. Systems that host untrusted workloads should treat this as a priority patch, and operations teams should incorporate this case into their playbooks for managing timing‑dependent kernel bugs — from fast detection to careful patch validation and rollout.
Source: MSRC
Security Update Guide - Microsoft Security Response Center