Linux Kernel rxrpc CVE-2023-53218 Fix: Abortable Sendmsg for Availability

A narrowly targeted but important Linux kernel fix addressing CVE-2023-53218 changes how the rxrpc subsystem handles a queued call so a waiting process can be aborted instead of leaving subsequent calls blocked with EBUSY — a behavior that could cause sustained denial-of-service for local users or guests until the connection assignment completes. The patch restores safer cancellation semantics around sendmsg’s wait-for-connection logic and has been picked up by multiple upstream and distribution trackers; administrators should treat this as a patch-and-verify priority for systems that run the rxrpc stack or expose kernel sockets to untrusted/guest code.

Background / Overview​

rxrpc is a kernel transport used for RPC-style communications in certain kernel subsystems and by specific userspace stacks. The vulnerability described as CVE-2023-53218 arises in the code path exercised when a userland call invokes sendmsg to create an rxrpc call: the call is queued and waits for a connection and channel to be assigned before it can begin moving encrypted DATA packets that embed connection summary information. Under certain timing conditions a waiting sendmsg can be interrupted before a connection is assigned; after that interruption further sendmsg attempts for the same logical call can fail with EBUSY until assignment completes. That behavior effectively lets an attacker or misbehaving local/guest process produce a sustained availability impact against the affected endpoint. The issue and the rationale for change are summarized by mainstream vulnerability trackers and distribution advisories. Why this matters: a blocked sendmsg that cannot be cleanly aborted is a reliability and availability problem. In multi-tenant hosts, containers, or virtual machines where untrusted code may exercise socket or RPC operations, an attacker with local or guest privilege could repeatedly trigger the condition to prevent legitimate callers from establishing or using the transport. Multiple distributors (Ubuntu, Debian, SUSE, Amazon Linux advisories) have ingested the CVE entry and flagged it for remediation.

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Technical anatomy: what the bug actually is​

The vulnerable sequence (plain language)​

• A userspace sendmsg creates an rxrpc call structure and queues it while waiting for a connection and channel to be allocated.
• The call cannot start sending its encrypted DATA packets until a connection is assigned (those packets embed connection parameters).
• Under a narrow timing window, the waiting sendmsg can be interrupted before the assignment occurs.
• After that interruption, subsequent sendmsg calls can fail with EBUSY until the connection assignment completes — effectively blocking progress for the caller and producing application-level hangs or service degradation.

Why this is dangerous (threat model)​

The primary impact is availability: attackers who can run local code (or untrusted guests on a host that exposes kernel rxrpc paths) can cause repeated EBUSY returns and force callers to stall or fail. Because this is not a straightforward remote network daemon vulnerability, the attack vector is assessed as local/guest rather than unauthenticated remote. However, in hosted environments where guests are intentionally isolated but allowed to interact with kernel transports or device emulation, the practical exposure can be non-trivial. Multiple vendor trackers emphasize the local/guest vector and rate the issue as moderate-to-important depending on distribution and scoring assumptions.

The upstream fix: make the waiting call abortable​

Upstream kernel maintainers changed the rxrpc send/queue logic so that a waiting call can be aborted cleanly if it is interrupted before a connection is assigned, and they were careful not to start the call timer before an assignment exists (starting the timer too early could lead to timer-based oopses). The fix is intentionally localized: it adjusts the call-queue/wakeup/cancellation semantics so the call does not leave the transport in a state where future calls are rejected with EBUSY indefinitely. The kernel stable commit metadata and tracking entries list the short patch and show it applied to the relevant stable trees.

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Verified facts and cross-checks​

• The vulnerability description is present in the NVD entry for CVE-2023-53218 and matches the upstream commit message and distribution advisories.
• Ubuntu and Debian security trackers published advisories describing the same behavior and listing fixed or affected package mappings; Debian’s tracker includes package mapping and fixed-version guidance for their releases.
• SUSE and Amazon Linux advisory portals also list the issue and classify it as resolved or flagged in distribution trackers; Amazon’s ALAS page shows a CVSS v3 base score presentation and indicates vendor-specific status for various Amazon Linux channels. Use their platform advisory entries to determine whether your Amazon (or RHEL-derived) kernel receives a backport.

Cross-check rationale: the description and fix details appear consistently across NVD, major Linux distribution trackers, and CVE aggregation sites. That multi-source confirmation reduces the chance that this is a mischaracterized or duplicate entry and confirms this is an upstream kernel change (small, surgical, and focused on abort semantics).
Caveat: some metadata (e.g., authoritative CVSS vector string in NVD) may lag; distributions often attach their own severity ratings and backport decisions. Administrators should use distribution advisories (Ubuntu/Debian/SUSE/Red Hat) to map upstream commit IDs to shipped kernel packages in their environment.

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Practical impact assessment​

Likely real-world effects​

• Most likely outcome on unpatched systems: application-level failures or hangs where callers repeatedly observe EBUSY or blocked sendmsg calls. In many cases this will present as service unresponsiveness rather than a kernel panic or exploit chain.
• Less likely but plausible: complex exploitation where a local attacker combines this race with other bugs to escalate impact or cause memory corruption. Public trackers note that while race conditions can be amplified by skilled exploit authors, no public proof-of-concept exploitation was available at disclosure. Treat the absence of a PoC as limited comfort, not immunity.
• Operational severity is proportionate to exposure: hosts that run user workloads which interact with rxrpc (or guests/containers that are permitted to exercise the relevant socket paths) should prioritize patching. For ordinary desktops and servers that do not load rxrpc, exposure will be low to none.

Scoring and distribution perspectives​

Different trackers classify severity differently because scoring depends on assumptions (local vs. adjacent vector, privilege requirements, and whether the affected host is network-exposed). Amazon’s ALAS listing shows a CVSS-like presentation for Amazon-specific packages and marks the issue as Important in their schema, while SUSE lists the issue as moderate overall. Organizations should reconcile vendor ratings with their own risk posture.

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Detection and hunting guidance​

Although this is primarily a correctness/availability fix rather than a high-confidence remote exploitation vector, the following detection steps will help you find affected hosts or hit patterns:

• Search kernel logs (dmesg and journalctl -k) for error traces involving rxrpc, sendmsg, and repeated EBUSY returns tied to RPC socket activity. Log entries that correlate blocked sendmsg calls or stack traces involving rxrpc functions are high-signal indicators.
• For hosts running kernel debugging tools, inspect oops or KASAN outputs for calls originating in rxrpc send/queue logic. KASAN/UBSAN traces can show up if other timing faults are present; while CVE-2023-53218 is not primarily a memory-corruption issue, look for correlated traces.
• Enumerate systems for kernel modules or subsystems that suggest rxrpc is in use. On Linux: check for relevant modules, systemd socket activation units, or applications known to use rxrpc-style transports. If rxrpc usage is absent, the practical exposure is zero.

Short hunting commands (examples to adapt to your environment):

• journalctl -k | grep -i rxrpc
• dmesg | egrep -i 'rxrpc|sendmsg|EBUSY'
• lsmod | grep rxrpc

Add contextual fields (hostnames, container/VM ID) to ensure you can link suspicious traces to the right tenant or workload.

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Remediation: what administrators should do now​

The fix is upstream in the kernel stable trees and distribution vendors are mapping those commits into their stable releases. Practical remediation steps:

• Identify scope
• Inventory hosts that might be running kernels with rxrpc enabled or that run software known to use the rxrpc transport. Use configuration management and package inventories to locate relevant kernels.
• Apply vendor patches
• Install updated kernel packages from your distribution vendor that include the upstream fix. Reboot as required to load the new kernel. Distribution trackers (Debian/Ubuntu/SUSE/Red Hat/Amazon) list fixed package versions or per-release status; consult those vendor advisories to map upstream commit IDs to your package versions.
• If you maintain custom kernels
• Merge the upstream commit(s) referenced by the CVE into your kernel tree, build, test on representative hardware, and schedule rollout. Keep the change minimal and validate that timers and abort semantics are correct under your workloads.
• Short-term mitigations if patching cannot be immediate
• If rxrpc is not needed, consider unloading the rxrpc kernel module or disabling features that expose the transport to unprivileged or guest code. Reduce host exposure by restricting which processes or guests may open RPC channels to kernel sockets. Use host-level policies to prevent untrusted code from triggering teardown/assignment races.
• Validate and monitor
• After patching, reproduce previously observed blocked-call scenarios in a controlled test to confirm that sendmsg can be aborted and that EBUSY no longer persists. Monitor kernel logs for a window (7–14 days recommended) to detect any remaining oopses or regressions. Best-practice checklists for kernel patch rollouts — testing on representative hardware and staged deployments — apply here.

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Strengths of the upstream response​

• The upstream change is surgical and narrowly scoped: maintainers adjusted the abort/cancellation semantics and timer-start logic rather than performing a risky rewrite — that reduces regression risk while addressing the correctness issue. Multiple stable-kernel commits reference the intent and the minimal nature of the patch.
• Distribution trackers quickly ingested the CVE metadata and began mapping the upstream commits to package updates; this means downstream patches are likely to appear as typical kernel stable updates rather than emergency, large-scale rewrites.

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Potential risks, limitations, and blind spots​

• Vendor lag and backporting: embedded devices, vendor appliances, and custom kernels sometimes lag upstream. If rxrpc is used in vendor-supplied images, those appliances may remain vulnerable until vendors release patched images. This is a common operational blind spot for kernel fixes.
• Local attack vector can be non-trivial in multi-tenant or cloud hosts: while the vulnerability requires local or guest ability to trigger the condition, cloud and virtualization deployments frequently provide such primitives to tenants (VM syscalls, emulated devices, or socket interactions), so risk is higher in shared environments.
• Absence of PoC ≠ safe: there is no widely published PoC as of disclosure, but race conditions are frequently weaponized once details are public; maintainers’ intentional minimalism in the patch reduces regression risk, but administrators should still patch promptly in exposed environments.
• Monitoring may be noisy: hunting for EBUSY in production logs could generate false positives if other legitimate subsystems return transient EBUSY; contextual correlation (which PID, which socket path, which workload) is essential.

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Recommended step-by-step action plan (prioritized)​

• High-priority: inventory
• Identify hosts that might have rxrpc in use (kernel config, modules, application-provided RPC transports). Use automated inventory tools where possible.
• High-priority: patch pilot
• Deploy vendor-supplied kernel updates in a test/pilot cohort and validate that the sendmsg/abort behavior no longer triggers persistent EBUSY. Reboot schedule as required.
• Medium-priority: broader rollout
• Stage updates across environments (test → pilot → production) with rollback plans and logs-inplace for post-deploy monitoring. Monitor dmesg/journalctl for rxrpc/eBUSY traces.
• Compensating controls
• If immediate patching is impossible: unload rxrpc where not needed, restrict module load/unload rights, and quarantine/rate-limit untrusted tenants or guests. Document these mitigations as temporary and track them until patching is complete.
• Post-remediation verification
• After deployment, run the previously failing or synthetic tests to ensure calls are abortable, and continue to monitor for a period (suggest 7–14 days) for unexpected regressions.

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Final analysis and verdict​

CVE-2023-53218 is a correctness/concurrency vulnerability in the Linux kernel rxrpc path that primarily affects availability: waiting sendmsg calls could be interrupted and subsequent calls might return EBUSY until connection assignment completes, producing sustained or repeated denial-of-service-like behavior in affected environments. The upstream patch is intentionally minimal, focused on abortability and not starting call timers before assignment; multiple independent trackers (NVD, Debian, Ubuntu, SUSE, Amazon ALAS) corroborate the issue and the remediation path. Administrators should treat this as a real but targeted risk: prioritize patching for systems that actually expose or use the rxrpc subsystem, and apply standard kernel rollout discipline (test, staged deploy, monitor). Key takeaways:

• This is an availability-first bug — patch promptly in exposed environments.
• Upstream fixes are small and backportable, but vendor lag is the main operational risk.
• Use inventory and targeted detection to avoid broad, unnecessary kernel churn where rxrpc is not used.

Administrators and packagers should map upstream commit IDs to their distribution kernels, apply vendor patches, validate in a test ring, and monitor kernel logs afterward; if vendor updates are not yet available for vendor-supplied kernels, apply compensating controls (module unloading, tenant isolation) until an official backport is installed.
Conclusion: CVE-2023-53218 is not a headline-grabbing remote RCE — it’s a subtle but meaningful kernel correctness fix that prevents a reliable denial-of-service condition in environments where rxrpc is exercised. The remedy is an upstream-supplied, intentionally small patch; successful defense relies on timely distribution backports, sensible inventory, and prudent rollout discipline.

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Source: MSRC Security Update Guide - Microsoft Security Response Center