A race-condition bug in the Linux kernel’s sc16is7xx serial driver can corrupt transmit (TX) FIFO data so that packets received on one channel are erroneously transmitted on another — a serious availability and integrity regression tracked as CVE‑2024‑44951 that was introduced by an earlier locking change and later fixed in the stable kernel trees.
Background
The sc16is7xx family is a kernel driver for a common multi‑channel UART-like serial chip used on embedded boards and SoCs. The device presents multiple logical channels sharing on‑chip FIFO buffers; driver code must carefully synchronize register access and FIFO reads/writes across channels and interrupt handlers.
In September 2024 the Linux kernel CVE team assigned CVE‑2024‑44951 to a regression in that driver: under a precise timing window, a packet arriving on channel A could be observed on channel B’s transmit line because the TX buffer had been corrupted with Rx data from the other channel. The condition is not a subtle loss of a byte or a statistical glitch — it represents a cross‑channel data corruption that directly undermines both
integrity and
availability of serial communications. I reviewed the user-supplied thread collection you provided while preparing this analysis to ensure nothing in your uploads contradicted or materially changed these findings. The provided thread metadata highlights how vendors map upstream fixes into distribution kernels and how cloud images (for example, Azure Linux) are tracked by vendors.
What happened: the technical root cause, in plain terms
The regression: a lock was made too fine-grained
A change to the driver moved an EFR (Extended Function Register) mutex from being
chip‑wide to
per‑channel locking. That change appeared intended to reduce unnecessary contention when accessing EFR registers for a single channel, but it had an unexpected, undocumented side effect: the EFR lock had been implicitly relied upon to also serialize access to a shared data buffer used by transmit and receive paths and across channels. Once the lock was split by channel, that implicit protection disappeared and a rare interleaving between a receive path on channel A and a transmit path on channel B could leave the shared buffer in an inconsistent state.
The observable failure mode
When the race happens, logic‑analyzer captures show the bits that were intended to be transmitted on channel B are instead the contents of a recently received packet from channel A. In other words, kernel bookkeeping wrote Rx bytes into a buffer that a concurrent Tx path then read and sent — effectively sending
someone else’s received data on a different channel’s transmit line. This is not just a lost packet — it is cross‑channel data leakage and corruption.
Why the fix works
Upstream maintainers fixed the problem in two stages:
- First, the Tx handler was changed to use safe, linear kfifo access (specifically kfifo_out_linear_ptr, removing the requirement that a single shared buffer must be protected by the EFR lock — the Tx path obtains a linear pointer to the data to copy out safely. This eliminates the implicit coupling between EFR locking and FIFO data safety.
- Second, the driver’s Rx buffering was changed from a single chip‑wide buffer to separate per‑channel Rx buffers, removing shared mutable state across channels and therefore closing the race surface. Together these changes remove reliance on an undocumented locking contract and make the synchronization explicit and local.
Both changes are small and surgical — the kind of kernel fixes that replace fragile, implicit protections with explicit data‑structure and API-level guarantees.
Timeline and affected kernel versions
- The risky change that introduced the regression is identified by commit 4409df5866b7, which altered the EFR lock semantics to operate per channel instead of chip‑wide.
- The regression was disclosed and assigned CVE‑2024‑44951 on public kernel CVE channels in early September 2024.
- The upstream fix was merged into the stable kernel series and included in stable releases; one stable merge is referenced as commit 09cfe05e9907 and was included in the 6.10.5 stable update (maintainers list the fix being present in kernels after 6.10.5).
Vulnerability trackers and vendor advisories map the fix into multiple kernel series; the databases show affected ranges that include select 6.x stable releases introduced after the initial change and prior to the stable fix. As always with kernel CVEs, exact exposure depends on the specific distribution and whether their kernel package includes the upstream backport.
Severity, exploitability, and attacker model
Severity
Scoring and vendor assessments place the bug in the
High band for combined impact: the NVD/trackers list a CVSSv3 score near 7.8 driven by high impacts to confidentiality, integrity, and availability when the condition is triggered. That reflects the fact that the bug can:
- Corrupt transmitted data (integrity).
- Cause cross‑channel leakage of received data (confidentiality).
- Cause operational failures or service disruption on affected devices (availability).
Some vendor pages and distribution trackers present a lower numeric score (medium) for specific packaging contexts; the final remediation priority should be driven by actual exposure in your environment (whether the affected driver is compiled in and in use, and whether devices rely on multi‑channel FIFO semantics).
Exploitability and attack vector
- Attack vector: Local. The bug is triggered by carefully‑timed concurrent channel activity within the driver. That typically means an adversary needs local access to generate the overlapping Rx/Tx operations (for example, a malicious local process or an untrusted tenant in multi‑tenant environments where serial devices are accessible). Trackers consistently model the vector as Local with Low complexity.
- Privileges required: Low — a local process that can open and operate the serial channels may be sufficient in many configurations. On managed hosts where serial ports are restricted, the barrier is higher; on embedded platforms or appliances that expose serial interfaces it is lower.
- Remote exploitation: Unlikely in standard configurations. The sc16is7xx driver is a kernel device driver interacting with physical hardware lines. There is no broad remote network‑exposed path that directly exercises the device unless a higher‑level service exposes device access remotely (for example, a management interface or a misconfigured container that exposes device nodes). Treat exposure as environment‑dependent.
Practical risk scenarios
- Embedded appliances and IoT devices that use sc16is7xx chips and run unpatched kernels are at highest practical risk because they tend to lag on kernel updates and often expose serial channels directly to application stacks.
- Development boards, factory test rigs, routers, or gateways that present serial devices to unprivileged users or containerized workloads could be exploited by a local process to cause cross‑channel corruption or denial‑of‑service.
- Virtualized hosts and mainstream server deployments are less likely to be affected unless the kernel image includes the vulnerable driver and the host exposes real hardware serial devices to guests or containers.
How to detect whether you are affected
Perform these checks on systems that you manage; prioritize devices where serial ports are in use or where embedded images are in production.
- Confirm the driver exists and is loaded:
- lsmod | grep sc16is7xx
- modinfo sc16is7xx (shows build info)
- Check kernel version and distribution advisories:
- uname -r
- Consult your vendor’s security tracker or package changelog to see if the kernel package includes the stable fix/backport that references CVE‑2024‑44951 or the stable commit IDs. Vendor pages and OSV entries map the affected/stable commit ranges.
- Look for symptomatic logs:
- dmesg or journalctl -k for unexpected serial driver warnings, IRQ handler traces, or unusual FIFO/tx/rx messages during periods of heavy serial activity.
- For embedded devices, consult vendor firmware/changelogs — many OEMs ship custom kernels and must publish their own advisories or updated images.
If you find the driver present and your kernel package predates the upstream stable backport, treat the system as vulnerable until patched.
Mitigations and remediation steps
- Patch: The definitive fix is to install kernels that include the upstream stable commits which implement the kfifo change and per‑channel Rx buffers. Vendors and distributions have incorporated the fix into their kernel updates; install the vendor‑supplied kernel update and reboot. Trackers and advisories list the stable commits and which kernel package ranges were updated.
- If you cannot patch immediately, apply compensating mitigations:
- Restrict access to serial device nodes (/dev/ttyS, /dev/ttyUSB, /dev/serial/by-id/*) to trusted users and processes.
- Remove unnecessary device exposure from containers or sandboxed environments (do not bind /dev/tty* to untrusted containers).
- For embedded fleets, plan accelerated firmware/kernel rollouts; avoid manual module unload/load workarounds that can themselves be hazardous.
- Verify after patching:
- Boot into the patched kernel on a test device and run high‑throughput, concurrent Rx/Tx tests across channels to exercise the code paths and confirm no cross‑channel corruption appears in logic captures or logs.
- Inventory and monitoring:
- Create a simple inventory query for systems that include sc16is7xx in kernel builds (package presence or module list) and prioritize remediation for systems that have the driver and expose serial access to untrusted contexts.
Distribution pages and vendor advisories (Debian, SUSE, Amazon Linux) provide details on which package releases contain the fix; rely on those advisories for exact package names and recommended upgrade procedures.
Operational guidance for Windows-centric environments
Even though CVE‑2024‑44951 is a Linux kernel driver issue, mixed environments with Linux guests, WSL instances, or cloud images matter. Windows administrators should:
- Inventory Linux guests and appliances in their estate to determine whether any run kernels with sc16is7xx and require patching.
- For Azure users: Microsoft’s inventory attestations (Azure Linux images and related advisories) are a pragmatic place to start; cloud images that Microsoft maintains are tracked and updated, but additional managed services or third‑party images may still carry vulnerable kernels. Check image manifests and apply updated images as provided by the cloud vendor.
- For device fleets: coordinate with hardware vendors to push kernel/firmware updates; many embedded vendors must release updated images for field devices.
Developer and maintainer lessons — why this bug mattered
This regression is instructive for kernel developers and maintainers:
- Locks mean more than the obvious: the EFR lock in this driver was not solely protecting EFR register accesses — maintainers had come to rely on it as a synchronization point for shared FIFO data. Changing a lock’s scope without auditing all implicit dependencies can introduce subtle races. Make locking contracts explicit, and document which data is protected by which lock.
- Shared, undocumented buffers are a liability: when transmit and receive handlers and multiple channels share mutable buffers, races and surprising aliasing become likely. Prefer per‑consumer buffers or clear producer/consumer APIs that don't rely on locks that were originally protecting unrelated invariants.
- Use safe kfifo primitives: switching to kfifo_out_linear_ptr is a concrete pattern — if you need to copy out contiguous data from a circular buffer without temporarily sharing state with other handlers, use kernel kfifo helpers that provide safe linear access where possible. The upstream fix demonstrates this approach.
- Backport discipline: vendors backport fixes to stable branches; maintainers should ensure that small, defensive fixes like this one are cherry‑picked into long‑lived kernel branches used by distributions and appliances.
Potential risks and caveats
- No widely documented remote exploit chain exists as of the public trackers' last updates — practical exploitation requires local access in common setups. That said, the impact of uncontrolled kernel memory and data‑path corruption can be severe; in some environments a local DoS translates to service outages with business impact. Treat this as a high‑priority operational patch if you have affected devices accessible to untrusted code.
- Vendor patch timing varies. Enterprise and embedded vendors may take longer to produce an image or backport. If you rely on vendor kernels (OEM images, firmwares), coordinate with the vendor’s security advisories and document timelines for remediation.
- Public PoC claims exist in some aggregator pages but are not authoritative; treat public exploit claims as unverified until they are reported by a trusted maintainer or vendor advisory. Avoid running untrusted PoC code in production — it can cause device crashes or data loss.
Quick checklist for administrators (actionable)
- Identify systems with sc16is7xx:
- lsmod | grep sc16is7xx
- rpm/dpkg/apt/yum queries for kernel packages compiled with sc16is7xx.
- Check vendor advisories for your distribution and cloud images; plan kernel package updates accordingly.
- For embedded fleets, coordinate image updates with manufacturers and schedule an OTA rollout where possible.
- Restrict access to serial device nodes until patches are applied.
- After patching, run multi‑channel Tx/Rx stress tests in a test lab and monitor kernel logs for residual warnings.
Conclusion
CVE‑2024‑44951 is a classic example of how a seemingly safe internal refactor — splitting a lock to reduce contention — can introduce a latent concurrency regression when the lock had been serving undocumented secondary purposes. The impact is real for systems that use the sc16is7xx driver: cross‑channel TX corruption undermines integrity and can cause service outages. The upstream fix is small, defensive, and has been merged into stable kernel branches; the practical remediation for operators is straightforward: inventory devices, apply the vendor/distribution kernel updates that include the backport, and restrict access to serial devices where immediate patching isn’t possible. Operators should treat this as a high‑priority patch for affected devices exposed to untrusted code paths and as a reminder to treat lock‑scope changes with a thorough audit of all implicit data protections.
Source: MSRC
Security Update Guide - Microsoft Security Response Center