Understanding CVE-2024-23849 Linux RDS kernel off-by-one DoS

The Linux kernel flaw tracked as CVE-2024-23849 is a classic off-by-one bounds-check error in the RDS receive path that can produce an out‑of‑bounds memory access and a denial‑of‑service (system crash) on affected kernels up to and including 6.7.1.

Background / Overview​

Reliable Datagram Sockets (RDS) is a niche, high‑performance transport used primarily in clustered and RDMA‑backed environments. The vulnerability resides in the kernel function rds_recv_track_latency in net/rds/af_rds.c and stems from an incorrect comparison against the RDS trace-array limit (an RDS_MSG_RX_DGRAM_TRACE_MAX check). When the bounds test fails to exclude the highest valid index, a code path reads past the end of an array, producing an out‑of‑bounds access that can trigger a kernel panic. Multiple vulnerability databases and distribution advisories recorded the defect and its impact.
Kernel maintainers incorporated the corrective patch into the 6.7.y stable series during the 6.7.3 update process, and downstream distributions backported fixes into their kernel packages (Debian, Ubuntu, SUSE, Amazon Linux and others). Distribution advisories and trackers list the CVE and corresponding security updates.

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What the bug is — technical summary​

• The vulnerable code path is rds_recv_track_latency() in net/rds/af_rds.c. The function maintains a small per‑socket trace array (receive‑latency traces).
• The code used a comparison that failed to correctly limit an index prior to reading inc->i_rx_lat_trace[j + 1]. Because the check allowed the maximum index value through, the subsequent read could step one slot beyond the array. This is an off‑by‑one bounds failure (CWE‑193).
• The out‑of‑bounds access is a read; it does not, under the published descriptions, directly disclose confidentiality or enable integrity violations. Its primary impact is availability — a read past the end of a kernel array can cause unpredictable behavior including kernel oopses and panics, which yield denial of service.

Why off‑by‑ones matter in kernel code​

The kernel runs with full system privileges and is memory‑unsafe by design (C). Any out‑of‑bounds read or write in kernel space risks crashing the host or corrupting kernel state. Even a single unchecked index can cascade into a system‑wide outage on a production server. Because this flaw is an out‑of‑bounds read that can trigger an oops, the immediate threat is denial of service rather than silent data theft — but availability impacts in multi‑tenant or clustered environments can be severe.

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Exploitability and real‑world risk​

• Attack vector: Local (AV:L). The attacker needs the ability to invoke the vulnerable RDS receive path on the target kernel. Published analyses rate the complexity as low and privileges required as low in the CVSS vector used by many trackers.
• Remote exploitation: not directly remote; the vulnerability is reachable via code paths that process RDS messages. In practice, effective exploitation requires either that the RDS kernel modules are loaded or that the kernel image was built with RDS compiled in. Many distributions compile RDS as a module that is only loaded when RDS sockets are used; that fact reduces the practical attack surface for hosts that never run cluster/RDMA workloads.
• Privileges and triggers: a local, low‑privileged user or a local process that can trigger RDS receive handling is sufficient to reach the bug in many configurations. Where the RDS module is not present or is blacklisted, the vulnerability cannot be triggered. Historically, the recommended short‑term mitigation for kernel RDS issues has been to prevent the RDS modules from loading until a kernel update is applied.

Risk assessment for different deployment profiles:

• Standard desktop or cloud instances that do not run RDMA/clustering software: low risk, because RDS modules are typically unused.
• HPC, database clusters, Oracle Exadata, and environments that explicitly use RDS or InfiniBand/RDMA: higher risk — these systems commonly load or compile RDS support and therefore are directly exposed until patched.

EPSS and exploit evidence

• Public vulnerability intelligence shows no reliable proof of weaponized exploit kits for this CVE at the time of disclosure. EPSS scores reported by enrichment services were low, consistent with a local‑vector kernel bug that is hard to chain remotely. Nevertheless, local exploitation is realistic and repeated triggering can cause sustained availability loss.

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Vendor and distribution responses — who patched and how​

Multiple distributors tracked and patched the issue; the following is a summary of notable responses:

• Upstream Linux kernel: the stable 6.7.y series received the corresponding correction in the 6.7.3 stable release cycle. The kernel stable announcement and patch set include a large series of fixes, one of which resolves this RDS bounds check. Administrators using upstream kernels should upgrade to a fixed stable release.
• SUSE: SUSE issued security updates that reference an array‑index‑out‑of‑bounds fix for RDS and linked it to their bug tracker (bsc#1219127). SUSE classified the update as important and shipped fixes in their kernel updates.
• Debian / Ubuntu: Debian and Ubuntu published advisories and security updates (Debian DLA entries and Ubuntu USN notices); Ubuntu’s security page documents the issue and assigns a medium CVSS (5.5) while describing the practical risk and its low priority for non‑RDS users. Administrators should consult distro‑specific advisories and install the vendor kernel updates.
• Amazon Linux (ALAS): Amazon’s ALAS advisories list the issue in their kernel security announcements and backported patches for Amazon Linux 2 and related kernel variants. Administrators using Amazon Linux AMIs should install the listed ALAS kernel updates (ALAS-2024-2475 and related advisories).
• Other trackers and scanners (OpenCVE, Rapid7, Snyk, Tenable) all recorded the CVE and pointed to distribution fixes and the upstream patch reference; Snyk and other vulnerability aggregators also store the kernel commit identifier used in the upstream correction. Where stable upstream access to the kernel.org commit page is hampered for some users, multiple distribution advisories and independent trackers corroborate the patch details.

Important verification note: the kernel commit referenced by many advisories (commit id cited in aggregation sites) is repeatedly listed in trackers but direct browsing of some kernel.org commit pages may be blocked or rate‑limited from automated scraping; administrators should rely on vendor advisories or directly fetch the stable branch from official kernel mirrors for the authoritative patch text.

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Practical mitigation and mitigation checklist​

If you are responsible for Linux servers, especially in clustered, RDMA, or HPC environments, follow the prioritized steps below. The numbered steps are sequenced for pragmatic operational deployment.

• Inventory and identify affected hosts
• Check kernel version: uname -r. Any kernel version <= 6.7.1 is in the upstream affected range unless your distribution has already applied an out‑of‑tree patch.
• Check if RDS modules are loaded: lsmod | grep rds. Also check /proc/modules and dmesg for rds/rds_tcp/rds_rdma entries.
• Search running processes and service stacks for RDMA/InfiniBand/cluster workloads (Oracle Exadata, HPC daemons, NFS over RDMA, MPI jobs).
• Apply vendor kernel updates and reboot
• Primary remediation is to install the security update your distribution provides and then reboot to load the patched kernel. Distributors that backported fixes include Debian (DLA), Ubuntu (USN/DLA mappings), SUSE, Amazon Linux (ALAS), and others — follow your vendor’s kernel update path.
• Short‑term workaround — prevent module autoloading
• If you cannot immediately patch, you can prevent the vulnerable code from being loaded by blacklisting the RDS modules. Use a file under /etc/modprobe.d/ (example):
  

  # Prevent automatic loading of RDS modules until kernel update is applied
  echo "blacklist rds" > /etc/modprobe.d/blacklist-rds.conf
  echo "blacklist rds_tcp" >> /etc/modprobe.d/blacklist-rds.conf
  echo "blacklist rds_rdma" >> /etc/modprobe.d/blacklist-rds.conf

• Beware: blacklisting RDS will break any workloads that legitimately require RDS (cluster communication, RDMA stacks). Validate operational impact before applying widely.
• Consider kernel config / boot‑time options for immutable builds
• In environments where you build kernels yourself, remove or disable CONFIG_RDS and related options, or ensure the build includes the upstream fix. Rebuild, test, and deploy as you would for any kernel change.
• Monitor and test
• After patching, watch your host logs for kernel oopses and unexpected reboots. Validate that cluster services behave normally and that no new regressions were introduced by the updated kernel.
• If you blacklisted RDS as a workaround, plan an accelerated patch window so the blacklist can be safely removed after kernel upgrades are installed.

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Operational impact and prioritization guidance​

• For the majority of servers that do not run RDS or RDMA software, the operational priority is moderate: the theoretical exposure exists only if RDS is loaded; Ubuntu’s own team rated priority low for default desktop/cloud hosts, but still advises administrators to patch per normal security practice.
• For hosts in HPC, database clusters, InfiniBand or Oracle environments, treat this as high‑priority for patching. Those systems are more likely to have RDS modules present or to rely on kernel builds that include RDS by default. The practical consequence of a kernel panic on such systems ranges from immediate application downtime to cascading cluster failover events.
• If you run multi‑tenant services, cloud hypervisors, or containers on top of a vulnerable host: even though the vulnerability requires local trigger, the impact of a host kernel panic can extend across tenants and services — so remediation must be scheduled as a cross‑service operational priority.

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For security teams: detection and telemetry​

• Log sources: kernel oops traces in dmesg, journalctl -k, or system crash collectors are the primary detection signals for attempted or successful triggering of this defect.
• SIEM/EDR alerts: configure rules to surface repeated kernel OOPS messages originating from networking/RDS subsystems and to mark unexpected rds module loads on hosts that should not use it.
• Vulnerability scanning: ensure your asset inventory and vulnerability scanners catch kernel package versions and distribution advisory IDs (Debian DLA references, Ubuntu USNs, ALAS IDs). Many scanners and patch‑management tools already identify CVE‑2024‑23849; coordinate scans with your distro patching pipeline.

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Why this CVE matters to WindowsForum readers and enterprise operators​

Although this vulnerability sits in the Linux kernel rather than in Windows, many Windows shops run Linux workloads in mixed environments: virtualization hosts, cloud instances, backups, database appliances, and specialized appliances (including cloud RDS instances). The operational lesson is familiar: even small, bounded correctness bugs in kernel networking code can produce high‑impact availability failures if the module is present or if the workload exercises that code path.
Microsoft’s product attestations for Linux components (for example, Azure Linux) have been a recurring theme in recent disclosure cycles. When vendors declare that a particular Microsoft distribution includes an affected upstream component, that is an authoritative product‑scope statement; however, it does not automatically identify every other product or image that may ship the same vulnerable code. Administrators should therefore verify the kernel builds and configurations used across their images rather than assuming a single attestation describes all of their infrastructure. The forum’s internal discussion on vendor attestation best practices highlights this nuance and why operators must confirm patching across product variants rather than relying solely on a single vendor notice.

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Strengths and limitations of the upstream response​

Strengths

• The upstream kernel stable process incorporated the fix into the 6.7.3 stable release quickly during the normal stable‑review cycle; this ensured a canonical patch for downstream distributions to backport.
• Major distributions produced security advisories and backports, providing a clear remediation path for administrators (ALAS, SUSE, Debian, Ubuntu).

Limitations / Risks

• The immediate attack surface is dependent on whether RDS is loaded. Many default systems are not directly exposed, which can lull operators into de‑prioritizing the update; this is risky for environments that do use RDS or where modules may be autoloaded by other components.
• Some public aggregators reference the kernel commit id for the patch, but direct browsing of kernel.org or mailing‑list archives may be blocked or inconsistent for some users; administrators should rely on vendor advisories if upstream browsing fails. Where direct patch text is needed for auditing, fetch the stable tree from an authoritative git mirror and confirm the change.

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Recommendations — executive summary for operations and security teams​

• Immediate: identify all hosts with kernel versions ≤ 6.7.1 and discover whether RDS modules are present or RDS‑dependent services exist.
• Short window (24–72 hours): apply vendor kernel security updates; schedule reboots as required by your maintenance windows. Use rolling updates and node‑by‑node patching for clustered services to avoid mass downtime.
• If you cannot patch immediately: temporarily blacklist rds, rds_tcp, and rds_rdma to prevent module loading — but only after validating that your production workloads do not rely on RDS. This is a defensive stopgap, not a substitute for a patch and reboot.
• Post‑patch: monitor kernel logs for oops, ensure crash collectors/telemetry capture any residual instability, and remove blacklists only after patched kernels are in place and tested.

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Conclusion​

CVE‑2024‑23849 is a textbook kernel off‑by‑one that produces an out‑of‑bounds read in the RDS receive path; its primary danger is availability loss through kernel oopses and panics. The exposure is concentrated in systems that load or compile RDS support — HPC, InfiniBand, and cluster appliances — but any environment that autoloads the module is at risk until patched. Upstream and downstream vendors produced patches and backports; the operational imperative is straightforward: inventory, patch, and reboot. Where immediate patching is impossible, preventing the RDS modules from loading is an effective temporary mitigation, but it must be applied with careful operational validation because it will disable legitimate RDS workloads. In short: treat the issue seriously in clustered/RDMA environments, patch promptly everywhere, and use module blacklisting only as a controlled, temporary stopgap.

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Source: MSRC Security Update Guide - Microsoft Security Response Center