The Linux kernel CVE-2025-38483 disclosure fixes a small but meaningful defensive-programming error in the COMEDI das16m1 driver that could lead to an out‑of‑bounds left-shift when a user-supplied IRQ number is used without sanity checks. The upstream patch enforces explicit bounds on the user-provided IRQ selection before performing a bit-shift test and requesting an interrupt line; the change is surgical, low-risk, and has already been merged into multiple stable kernel trees and picked up by distributions.
Background / Overview
COMEDI (Control and Measurement Device Interface) is a Linux kernel subsystem that exposes data-acquisition boards to user space. The das16m1 driver supports a family of older analog/digital I/O boards and includes logic to parse board options supplied from userland. In das16m1_attach, the driver historically tested whether a requested IRQ number was supported using a bitmask-based test that shifted a literal 1 by an
unchecked user-provided value. That test looked like:
- / only irqs 2, 3, 4, 5, 6, 7, 10, 11, 12, 14, and 15 are valid /
- if ((1 << it->options[1]) & 0xdcfc) { ... }
Because it->options[1] is an integer derived from userland, an attacker or malformed input could cause a negative or excessively large shift amount, producing undefined behavior in C, sanitizer traps, or unpredictable runtime results. The upstream remedy is to validate the IRQ value is within the valid range before performing the shift. This class of bug — a shift amount that is not validated before use — is not unusual in low-level kernel code, but it is precisely the kind of corner case that sanitizers (UBSAN) and automated fuzzers (Syzbot) catch and that maintainers prefer to fix with a minimal, conservative change. The fix has been merged and backported into stable kernels; distributions have mapped the CVE to patched kernel package versions.
What changed: the patch in plain terms
The unsafe test
The original test used a literal left shift of 1 by a user-supplied amount and then masked the result against 0xdcfc (the bitmask of
allowed IRQs):
- if ((1 << it->options[1]) & 0xdcfc) { ... }
If it->options[1] was e.g. -1 or 31 or another out-of-range value, the left-shift becomes undefined and could trigger sanitizer traps or architecture-dependent results.
The minimal, correct fix
The upstream commit adds an explicit bounds check ahead of the shift so the shift is only done when the user-supplied value is within the intended IRQ range:
- if (it->options[1] >= 2 && it->options[1] <= 15 &&
- (1 << it->options[1]) & 0xdcfc) {
- ret = request_irq(...;
- ...
- }
This ensures the shift operand is within [2, 15] before evaluating (1 << it->options[1]), eliminating negative or out-of-bounds shifts while preserving original behavior for valid options. The exact commit and stable backports are publicly recorded in the kernel stable trees.
Why the small change is preferred
- It is localized and easy to reason about: adding input validation is less likely to introduce regressions than larger rewrites.
- It preserves intended semantics (the same IRQ acceptance logic) while closing an undefined-behavior hole.
- Small fixes are straightforward to cherry-pick into multiple stable branches for rapid distribution-level rollouts.
Technical analysis: root cause, exploitability, and real-world impact
Root cause: unchecked user input used as a shift amount
C left-shift semantics require that the shift amount be non-negative and less than the width in bits of the promoted left operand. Shifting by a negative or an out-of-range amount is undefined. In kernel drivers where small integer fields are supplied by user space, failing to validate those values before using them in bitwise arithmetic invites undefined behavior and sanitizer traps on instrumented builds. The das16m1 code used a user-provided value directly in a shift expression, producing exactly that risk.
Practical exploitability
- Attack surface: Local. COMEDI device ioctl/initialization paths are required to reach the vulnerable code; a remote attacker with only network access cannot trigger this directly.
- Privileges required: Typically unprivileged userland can open device nodes and pass options depending on device node permissions. On systems where /dev/comedi* is world-accessible or accessible to untrusted containers, the attack vector is broader.
- Impact class: Availability / correctness. The obvious, most-likely outcome of triggering undefined left-shift behavior in the kernel is a crash (oops), sanitizer trap under debug builds, or unpredictable logic that may cause the driver to misconfigure interrupt handling. Public trackers and vendor advisories classify this CVE as a correctness/availability issue rather than an immediate privilege-escalation vector.
As with many kernel DoS primitives, a deterministic crash primitive is valuable in multi-stage escalation chains in certain environments, but on its own this issue is not a reliable remote code-execution exploit. There is no authoritative public evidence of active in‑the‑wild exploitation tied to this specific CVE at disclosure time. That said, local DoS vectors deserve attention — especially on multi‑tenant hosts, CI runners, or shared workstations where an unprivileged user could cause host instability.
Patch timeline and distribution status
- Upstream commit: The kernel commit fixing the issue (commit id ed93c6f6... and related stable-tree entries) was submitted to the kernel mailing lists and merged into upstream and stable branches. The change appears in multiple stable series patches prepared for backport.
- NVD and distro trackers: NVD summarizes the CVE and the fix; distribution trackers (Debian, Ubuntu, SUSE and others) have recorded the CVE and published fixed package versions. Administrators should consult their vendor security advisories for the exact fixed package for the release in use.
- Microsoft / Cloud vendors: Microsoft’s Security Response Center publishes machine-readable attestation-style notices for Linux components used in Azure images; such product attestations indicate Azure Linux images were inventoried and mapped for exposure. For other vendors' kernel builds (especially embedded vendors), timelines vary and may lag upstream.
Because kernel packaging and backport choices differ between distributions, the authoritative source for whether a given host is fixed is the vendor package changelog or security advisory that explicitly lists the upstream commit or references CVE-2025-38483. Debian and Ubuntu trackers already document fixed package versions that map the upstream commit into a distribution package.
How to detect if you are exposed
Inventory checks and quick verification commands:
- Confirm whether the COMEDI driver is present / loaded:
- uname -r
- lsmod | grep -i comedi
- zcat /proc/config.gz | grep -i comedi
- Search for COMEDI device nodes:
- ls -l /dev/comedi* || true
- Inspect running kernel package changelog or vendor advisory to ensure your installed kernel package includes the upstream fix (look for the upstream commit hash, or for references to CVE-2025-38483).
- If you build kernels from source, inspect drivers/comedi/drivers/das16m1.c and confirm the bounds-check patch is present (look for the it->options[1] >= 2 && it->options[1] <= 15 condition).
If you see kernel oops or sanitizer traces referencing das16m1, request_irq, or call stacks that include drivers/comedi/drivers/das16m1.c, treat those systems as high-priority for patching and investigation. Public triage guidance for COMEDI-related CVEs emphasizes this same inventory and log-hunting approach.
Recommended remediation and mitigations
Definitive remediation
- Install a vendor-supplied kernel update that explicitly lists CVE-2025-38483 or includes the upstream stable commit referenced in advisories.
- Reboot into the updated kernel to make the fix active.
Always verify the package changelog or vendor advisory includes the upstream commit hash or a mention of this specific CVE; distribution packages can vary in which stable commits they include. Debian and Ubuntu security trackers list fixed package revisions that administrators can match against their systems.
Short-term compensating controls (when patching is not immediately possible)
- Restrict access to COMEDI device nodes via udev rules or file-permission changes so only trusted users can open /dev/comedi. Example udev snippet to restrict a device to a trusted group:
1. Create a group (e.g., comediadmin) and add only administrators to it.
2. Add a udev rule: ACTION=="add", KERNEL=="comedi", GROUP=="comediadmin", MODE=="0660"
- Unload the COMEDI modules where feasible:
- sudo modprobe -r das16m1 comedi
- Note: only remove modules if you confirm that no critical services depend on COMEDI support.
- For containerized environments, ensure containers are not bind-mounted to host /dev/comedi* and that device pass-through is not allowed for untrusted workloads.
These mitigations reduce the attack surface by limiting the set of actors who can supply malformed options to the driver.
Operational guidance for SOCs and administrators
- Prioritize multi-tenant hosts, CI runners, remote-build hosts, and shared workstations where unprivileged users may have access to device nodes.
- Add detections in SIEM for kernel oops traces mentioning das16m1, comedi, or call stacks that include drivers/comedi; collect and retain dmesg/journal logs for forensic review.
- For embedded appliances and vendor kernels, initiate vendor support requests: many industrial and embedded images lag in upstream backports and may require vendor-supplied updated firmware or kernel images.
- When validating patches in staging, exercise COMEDI device functionality and reproduction test-cases to confirm there are no regressions in board behavior after patching.
Practical triage checklist:
- Inventory hosts with COMEDI support.
- Cross-reference running kernel package with vendor advisories for mapped fixes.
- Apply kernel update; reboot in a controlled maintenance window.
- Validate device functionality and monitor logs post-patch.
Strengths of the upstream response — and remaining risks
Strengths
- The fix is small, reviewing and merging cleanly into multiple kernel stable branches. Small, conservative fixes minimize regression risk and simplify vendor backports.
- The patch addresses the true root cause (unchecked user input used as a shift amount) rather than attempting a surprising or invasive refactor.
- The bug was discovered and reported via fuzzing/automation (Syzbot traces are referenced in upstream comments), demonstrating the effectiveness of continuous testing for kernel robustness.
Remaining risks and caveats
- Long-tail devices: vendor kernels and embedded appliances commonly lag in backporting fixes; these are the highest operational risk and typically require vendor coordination to remediate.
- Inventory complexity: some images ship kernels with COMEDI compiled in, even if most cloud workloads do not use it. Administrators must verify kernel configuration and device node exposure rather than assume absence.
- Local DoS primitives are valuable in targeted environments; remediate hosts in high-value roles (multi-tenant CI, multi-user desktops, virtualization hosts) promptly rather than deferring to low priority.
Where claims about "exploitation in the wild" appear, treat them cautiously: public advisories and tracking pages for this CVE did not document verified active exploitation at disclosure. That absence of public PoCs should not be read as zero risk; local DoS is trivial to weaponize in some scenarios.
Quick reference: checklist for administrators
- Inventory:
- uname -r
- lsmod | grep -i comedi
- ls -l /dev/comedi*
- Patch:
- Install vendor-supplied kernel that references CVE-2025-38483 or the upstream commit ed93c6f6... and reboot.
- Mitigate (if you cannot patch immediately):
- Restrict /dev/comedi* access with udev rules.
- Unload COMEDI modules if safe: sudo modprobe -r das16m1 comedi
- Monitor:
- journalctl -k | grep -i das16m1
- dmesg | grep -i comedi
- Vendor coordination:
- Open support tickets for embedded appliances and vendor-supplied kernels that cannot be patched locally.
Conclusion
CVE-2025-38483 is a textbook example of a small input-validation defect causing undefined behavior: an unchecked userspace-supplied shift amount used to test supported IRQ lines in the das16m1 COMEDI driver. The fix is straightforward and low-risk — validate the IRQ before shifting — and it has been merged upstream and backported into stable trees. Administrators should treat the issue pragmatically: inventory systems for COMEDI support, prioritize patching in hosts that expose device nodes to untrusted users or containers, and apply compensating controls for systems that cannot be patched immediately. The long-tail risk remains in vendor kernels and embedded appliances; those devices require active vendor engagement to ensure backports are applied.
Source: MSRC
Security Update Guide - Microsoft Security Response Center