Linux Kernel n_gsm MSC Deadlock Fix CVE-2025-40071

A narrowly scoped but practically important fix landed in the Linux kernel this October to prevent the n_gsm tty driver from hanging the input processing path while it waits for a Modem Status Command (MSC) response — tracked as CVE-2025-40071 — and it exemplifies how small timing and protocol-handling errors in kernel drivers translate directly into system availability risks.

Background​

The affected code lives in the n_gsm driver under drivers/tty, the kernel-side implementation of GSM multiplexing used to expose GSM serial/telephony channels via DLC (Data Link Connection Identifier) channels. The driver parses incoming frames in a main queue routine and performs control operations when DLC channels open. In basic mode encoding, the driver previously attempted to synchronously update modem status by waiting for an MSC response on the same input queue it was processing. That wait could deadlock input processing because no other queue reader could consume the response while the caller itself was blocked. The upstream patch changes that behavior by sending an initial MSC without waiting for the remote to reply, implemented as a new helper function named gsm_modem_send_initial_msc().
This change is defensive and pragmatic: MSC (Modem Status Command) is only meaningful when the link is using basic encoding, and waiting for a response on the same queue created an implicit dependency that violated the driver's input-processing contract. The new approach simply sends the MSC proactively and continues processing, avoiding the risk of deadlock.

---

Why this matters: availability over confidentiality​

At first glance this is not a glamorous bug — it does not allow arbitrary code execution nor theft of secrets — but in kernel-space software availability issues are frequently the most operationally painful. A blocked input queue inside a tty driver can manifest as:

• Hung processes that rely on that tty device.
• Device subsystems that stop responding to new frames or control events.
• Higher-level services dependent on the modem channel suffering stalls or cascading failures.
• In virtualized or embedded contexts, operational appliances requiring manual reboots.

Because the kernel operates with global privileges, a local logic bug that produces a deadlock or indefinite wait in a core input path is, in practice, a denial-of-service (DoS) vector against the host’s availability. Several public trackers classify the issue as a medium-severity availability bug in the kernel.

Technical anatomy — what the patch changes​

The vulnerable sequence​

• The driver’s gsm_queue() routine processes frames from the incoming buffer.
• On DLC channel open, gsm_dlci_open() triggers a modem-status check via gsm_modem_update().
• In basic mode, gsm_modem_update() called gsm_modem_upd_via_msc(), which attempted to wait for an MSC reply on the same input queue that gsm_queue() is currently iterating.
• Because the queue-processing function blocked waiting for the response to arrive in that same queue, nothing could consume the incoming reply; the processing stalled.

This is a classical self-inflicted blocking problem: a consumer tries to wait for an item in the queue while the caller holds the execution context responsible for consuming or delivering it, producing a deadlock or indefinite wait. The upstream commit removes the wait and replaces the synchronous update path with a fire-and-forget initial MSC send.

The fix in code terms​

• A new helper, gsm_modem_send_initial_msc(), was added in drivers/tty/n_gsm.c; it assembles the MSC payload and calls gsm_control_command() to send it.
• The helper is a no-op unless the DLCI is in basic adaptation mode and the GSM stack is configured for GSM_BASIC_OPT encoding; it deliberately avoids action for advanced encodings and for convergence layer type 2.
• The patch is small (around two dozen insertions in the single file) and intentionally minimal: change behavior only where required, preserve existing contracts elsewhere.

This surgical approach follows typical kernel practice: for protocol/driver-level timing errors, the low-risk remedy is to stop synchronous waits in contexts that cannot safely block, and convert those waits to asynchronous or non-blocking actions. The upstream commit and its inclusion in stable patch branches show that maintainers view this as a correctness/robustness fix rather than a new feature.

---

Scope and affected systems​

• Component: Linux kernel — tty: n_gsm driver (drivers/tty/n_gsm.c).
• Affected builds: kernels that include the upstream commit before the stable backport was applied are vulnerable; distributions and appliances that shipped earlier kernel revisions may still be affected.
• Encoding/usage specifics: the behavior only matters when the modem channel or remote end is configured for basic mode encoding (MSC is only relevant there). Systems using advanced encoding or convergence layer type 2 are not functionally impacted by this MSC change.

Distribution trackers and vulnerability databases have indexed the CVE and begun mapping affected package versions (for example, Debian’s security tracker lists kernel package versions compiled from the affected upstream trees). Operators should assume any kernel release prior to the stable backport is potentially affected until they confirm the package contains the fixed commit.

---

Risk assessment and exploitability​

• Attack vector: local. The logic bug is triggered by normal modem/DLCI control flows and is not a remote unauthenticated exploit in the classic sense.
• Privileges required: low to medium depending on system configuration — a local process that can manipulate or drive the relevant tty/GSM device or that can cause DLC channel open events could trigger the condition.
• Primary impact: availability (Denial of Service). Public vulnerability summaries and scoring tools place CVSS v3 in the medium range when modeled as a local availability bug. Some lifecycle trackers record different scores, but the consensus is that this is operationally relevant, not a catastrophic confidentiality or integrity compromise.

At present there are no credible public reports of weaponized exploits or in-the-wild campaigns abusing this specific bug. That does not change the operational urgency: DoS conditions that manifest as hangs or blocked device paths can have significant real-world impact in embedded, telecom, or industrial deployments where devices are designed to run unattended and reboots are costly. The remediation required — a kernel update — is straightforward; the hard part often lies in update distribution to embedded appliances or vendor-forked kernels.

---

Validation: verifying the claim and the patch​

Two independent verification points corroborate the technical story:

• The upstream stable-review emails and patch notes that include the patch summary and the diff excerpt showing insertion of gsm_modem_send_initial_msc() and the accompanying comment about not blocking the input queue. These messages carry the commit identifiers used by stable branches.
• Public vulnerability aggregators and OSV-style trackers that have ingested the CVE and map it to kernel commits and distribution package ranges — these record the same description and link to the stable commit references. Debian/OSV entries list the CVE and map affected package versions compiled from vulnerable upstream code.

Where available, operators should confirm the presence of the fix by matching kernel package changelogs against the upstream commit IDs included in distribution advisories or by inspecting the tree for the new helper function in drivers/tty/n_gsm.c. If package changelogs are opaque (for vendor forks), contact your vendor or inspect the kernel source tree provided with the shipped image.
Caveat: not every public tracker offers a CVSS vector or EPSS rating for this CVE at time of writing; that means scoring and exploit-prediction numbers may vary or be absent until databases synchronize. Treat reported scores as guidance and prioritize by operational context.

---

Practical remediation and operational guidance​

• Inventory: identify hosts that use the n_gsm driver or expose GSM tty devices. Use tools like lsmod, uname -r, and package-manager queries to find candidate systems.
• Map package versions: check your distribution’s security tracker or kernel change log to confirm whether the shipped kernel includes the upstream commit that adds gsm_modem_send_initial_msc().
• Patch: install the kernel package that contains the stable-tree backport or the upstream merge containing the commit; schedule reboots as required. For distributions that provide signed stable updates, use their normal patch channels.
• For embedded appliances and vendor-forked kernels: open a vendor support ticket requesting a backport of the upstream commit, or plan device replacement if vendor updates are not available.
• Compensations if patching is delayed:
• Restrict who can access the device nodes for GSM tty devices.
• Isolate affected hosts from high-value networks and services where a DoS might escalate operational consequences.
• Monitor kernel logs for hangs or repeated blocked reads on tty devices and add SIEM/alerting for tty driver anomalies.
• Post-patch validation: confirm that DLC open sequences no longer block the input queue; if possible, reproduce the original scenario in a test system to ensure the driver continues to behave correctly with both basic and advanced encodings.

---

Why the upstream approach is right (and what to watch for)​

The upstream fix is intentionally minimal and conservative: it avoids heavy refactors and limits behavior change to precisely the unsafe synchronous wait, converting it to an asynchronous send in the narrow case where MSC makes sense. This reduces regression risk while restoring robust input-processing semantics. Kernel maintainers typically favor such narrowly scoped patches for device-protocol correctness; they are low risk and backportable.
Potential risks and operational caveats:

• Embedded vendors with long-lived, forked kernel trees may not push backports promptly; these devices can remain vulnerable until vendor action.
• Although the fix targets the self-blocking scenario, downstream integration or testing might reveal corner cases in specific modems or dial plans; validate with representative hardware.
• Monitoring and detection for this kind of bug is inherently noisy: a blocked input queue may be logged as a generic hang without a clear fingerprint, so proactive inventory and patching are the most reliable mitigation.

---

Short checklist for IT teams (actionable)​

• Inventory all Linux hosts that could load drivers/tty/n_gsm: (1) list running kernels and modules, (2) search fleet for tty device names used by GSM devices.
• Confirm whether your distribution kernel package includes the upstream commit or stable backport; if unclear, check package changelog or kernel source.
• Deploy the fixed kernel package during the next maintenance window and reboot hosts in controlled batches.
• For unreachable or vendor-locked embedded devices, prioritize isolation or device replacement; escalate to vendor support to obtain a backport.
• Add monitoring rules for kernel log patterns indicating blocked tty reads, repeated DLC open attempts, or hung modem control flows.

---

Broader lessons for Linux kernel operators​

• Small timing and protocol handling issues cause outsized operational pain when they live in kernel input paths; they deserve the same triage priority as more headline-grabbing remote code execution bugs in system protection plans.
• Upstream review and stable-tree backports are effective for low-risk correctness fixes; the practical problem is distribution and vendor propagation rather than code complexity.
• Maintain an inventory of kernel driver usage patterns in your estate. Drivers that are rarely used but present can still be triggered by unexpected workloads, fuzzers, or diagnostic tools — and they are often the slowest to be patched in embedded fleets.

---

Conclusion​

CVE-2025-40071 is a textbook example of a kernel robustness fix: the vulnerability was not a memory-corruption exploit but a logic/timing error in tty n_gsm’s handling of Modem Status Commands that could block the driver’s input queue and produce service hangs. The upstream patch is small, low-risk, and already present in stable kernel update streams; the practical work for operators is ensuring affected kernel packages are patched across distributions and, critically, vendor-supplied embedded images. Prioritize updates for hosts that expose or depend on GSM tty channels, validate fixes with representative hardware, and treat kernel input-path correctness as a running operational priority rather than a theoretical edge case.

---

Source: MSRC Security Update Guide - Microsoft Security Response Center