A small code hygiene fix landed in upstream stable trees this week to close an information‑disclosure hole in the Linux input subsystem: CVE‑2025‑40035 addresses an uninitialized‑stack leak in the uinput compat path by explicitly zeroing an instance of uinput_ff_upload_compat before it is copied to userspace. The change is tiny — a single memset — but it closes a reliable channel for leaking kernel stack contents into userland, and it underlines a perennial lesson for kernel and driver authors: uninitialized padding inside nested compatibility structs is an unexpected source of secrets if left unchecked.
The vulnerability arises from the way the Linux kernel implements 32‑bit compatibility helpers for force‑feedback (FF) upload operations in the uinput driver. The compatibility structure uinput_ff_upload_compat embeds versions of ff_effect_compat twice, and because of alignment requirements a small padding or “hole” appears in memory after struct ff_replay. If the compatibility struct is not cleared before filling its valid fields, leftover stack bytes — potentially containing pointers, sensitive data, or other kernel residues — can be copied to userland by copy_to_user, producing an information leak. The NVD summary captures this succinctly: initialize ff_up_compat to zero before filling valid fields. This is not an exotic exploitation primitive: the bug is a classic case of “uninitialized stack data returned to userspace.” The practical risk depends on two things: whether an attacker can invoke the vulnerable interface locally, and whether the leaked contents are useful (kernel pointers, token fragments, cached keys). Kernel information leaks are valuable reconnaissance primitives because they can defeat KASLR and accelerate privilege‑escalation work. Independent trackers and vendors quickly categorized the flaw and linked it to stable‑tree patches that add a memset to the affected code path.
In developer terms the moral is familiar but worth repeating: when code marshals kernel structures into user buffers, initialize everything you return — including padding. The cost of a memset is negligible; the potential payoff for an attacker harvesting kernel bytes is not. The upstream patch, merged into stable kernels and mirrored by vulnerability aggregators, completes the fix cycle for this particular leak and restores the simple hygiene that prevents these quiet but effective primitives from becoming the first step in a full compromise.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
The vulnerability arises from the way the Linux kernel implements 32‑bit compatibility helpers for force‑feedback (FF) upload operations in the uinput driver. The compatibility structure uinput_ff_upload_compat embeds versions of ff_effect_compat twice, and because of alignment requirements a small padding or “hole” appears in memory after struct ff_replay. If the compatibility struct is not cleared before filling its valid fields, leftover stack bytes — potentially containing pointers, sensitive data, or other kernel residues — can be copied to userland by copy_to_user, producing an information leak. The NVD summary captures this succinctly: initialize ff_up_compat to zero before filling valid fields. This is not an exotic exploitation primitive: the bug is a classic case of “uninitialized stack data returned to userspace.” The practical risk depends on two things: whether an attacker can invoke the vulnerable interface locally, and whether the leaked contents are useful (kernel pointers, token fragments, cached keys). Kernel information leaks are valuable reconnaissance primitives because they can defeat KASLR and accelerate privilege‑escalation work. Independent trackers and vendors quickly categorized the flaw and linked it to stable‑tree patches that add a memset to the affected code path. What changed (technical summary)
The root cause, plain and simple
- The compatibility struct contains embedded FF effect structures and a union that triggers alignment padding.
- The kernel code created a local instance of uinput_ff_upload_compat on the stack.
- Some padding bytes inside that struct were never explicitly initialized.
- When copy_to_user was used to export the filled structure to userland, those uninitialized bytes could leak whatever residual stack data happened to be present.
The patch
- The fix adds one line early in the compat code path:
- memset(&ff_up_compat, 0, sizeof(ff_up_compat);
- That guarantees all padding and otherwise unused bytes are zeroed before the code proceeds to populate the fields the caller expects and before copying to userspace. The change is present in stable updates posted for multiple kernel versions.
Why the vulnerability matters in practice
Local-only, but high leverage
Exploitation requires local code execution — an attacker (or a malicious local process) must be able to call the uinput interface and trigger the compat path. This excludes remote, unauthenticated exploitation by itself; however, the presence of a reliable information leak significantly raises the value of any local foothold.What leaked bytes can enable
- Kernel pointers that defeat Kernel Address Space Layout Randomization (KASLR).
- Small fragments of kernel memory that might include handles, object identifiers, or other recognizable tokens.
- Residual data from other kernel operations that an attacker can correlate and exploit in follow‑on chains.
Who should worry most
- Virtual desktop infrastructure (VDI) hosts, terminal servers, and other multi‑user systems where many distinct user processes share the same kernel address space.
- Build servers, CI runners, and developer workstations that execute untrusted code.
- Systems that expose local devices to untrusted guests (for example, VMs with USB passthrough).
- Any environment where local code execution is a realistic risk (public kiosks, shared lab machines, classroom labs).
Where the fix landed and distribution considerations
The upstream changes were accepted into the kernel stable trees and have been included in stable update releases across multiple kernel branches. The patch series was prepared for inclusion in stable updates (examples show entries against 5.10 and 6.17 stable patch sets) and the fix is a straightforward zero‑initialization in drivers/input/misc/uinput.c. Distributors typically pick up these stable patches and backport or bundle them into distribution kernel packages; timing varies by vendor and branch. Operational guidance:- Map the kernel version(s) in your estate to the stable trees and check your vendor’s advisories for the presence of the patch.
- If you run vendor kernels (Red Hat, Ubuntu, Debian, SUSE, Canonical, etc., wait for and apply the vendor‑supplied kernel update rather than blindly compiling upstream kernels (unless you manage kernel builds).
- If a vendor has not yet published a patch, consider kernel or package‑level backports only in controlled test rings, and validate that the change does not produce regressions for your hardware and input stacks.
Detection and hunting guidance
Because this flaw is an uninitialized memory leak rather than a crash or RCE, detection is behavioral and forensic rather than signature‑based.- Hunt for processes that call the uinput ioctl paths or otherwise interact with /dev/uinput more often than expected.
- Expand EDR rules to catch repeated invocations of the FF upload ioctl from non‑privileged processes — attackers often brute‑force or iterate the vulnerable path to harvest enough bytes for analysis.
- Capture and retain memory images and kernel logs in suspected incidents: leaked pointer values are most useful when correlated with system memory artifacts.
- Instrument test harnesses in staging: run the compat code path before and after patching to confirm that returned buffers no longer contain non‑zero padding bytes.
Practical mitigation and patching checklist
- Identify affected systems
- Inventory hosts that expose input devices, that run custom or vendor kernels, or that offer USB passthrough to VMs.
- Prioritize patch deployment
- Patch systems in the following order: (a) multi‑user / VDI hosts, (b) build/CI hosts, (c) developer machines, (d) general workstations.
- Apply vendor updates
- Use your usual package management (apt, yum/dnf, zypper, pacman) or vendor channel (kernel livepatch, errata) to install kernels that include the stable‑tree fix.
- Test for regressions
- Validate input and FF behavior for devices used in production workloads (game controllers, haptics, MFDs, capture devices) before broad rollout.
- Compensating controls while patching
- Restrict the set of users who can open /dev/uinput (udev rules, file permissions).
- Disable USB passthrough for untrusted VMs.
- Use kernel lockdown mechanisms and least privilege where available.
Developer and maintainer takeaways
- Always zero‑initialize stack structures that may contain padding before copying them to userland. The one‑line memset added in this fix is cheap insurance against subtle alignment‑related leaks.
- Be suspicious of nested/embedded structs and unions in compatibility code paths — padding often appears to satisfy alignment for union members and can be overlooked during manual marshalling.
- Add regression tests that exercise compatibility marshalling and perform deterministic checks that no bytes outside documented fields are returned to userland.
- Use static analysis tools and sanitizers where feasible (for example, kernelkov or other memory analysis tools during development) to catch uses of uninitialized stack memory before commit.
Exploitability: how worried should administrators be?
- Exploitation prerequisites: local code execution or ability to invoke the vulnerable uinput ioctl; this is not a remotely exploitable kernel vulnerability on its own.
- Practical usefulness: medium to high in multi‑tenant contexts. An attacker with a local foothold can use leaked kernel information to defeat KASLR and speed up privilege escalation.
- Public exploit evidence: as of the initial disclosures and stable patches, there were no widely reported proof‑of‑concept exploits for this specific CVE; the patch was a defensive fix to prevent data leakage. That absence reduces immediate mass‑exposure risk but does not remove the value of patching — history shows adversaries weaponize kernel information leaks quickly once they understand what the leak reveals.
- For single‑user desktops with strict local controls: lower immediate risk, but still recommend patching in regular maintenance cycles.
- For shared or multi‑user hosts, CI/build servers, and virtualized hosts with passthrough: treat as high priority and apply patches promptly.
A brief technical walk‑through (what reviewers saw)
The review and patch discussion show maintainers focused on correctness and minimal surface‑area changes. The chosen fix — memset the stack structure — is intentionally conservative and avoids changing visible ABI or marshalling semantics. The patch was submitted to the stable streams and included in the standard stable update series for affected kernel trees. The patch diffs and commit messages explicitly state the cause: an alignment‑introduced hole after struct ff_replay, and copy_to_user could leak stack data if the structure was not zeroed first. This corroborates the straightforward nature of the fix and confirms that the vulnerability is not a complicated memory corruption but a hygiene issue.Longer‑term defensive lessons
- Treat any structure that will be copied to userspace as sensitive: make the initialization of the entire structure deterministic, not just the documented fields.
- Add unit tests and fuzzing harnesses for compat marshalling code: fuzzers that exercise copy_to_user-adjacent code paths are effective at exposing these classes of issues.
- Maintain a strict policy for userland‑facing structs: prefer explicit zero initialization or use of helper macros that guarantee the initial state.
- For distributors and integrators: automate mapping between upstream stable patches and your packaging backports to reduce the window between upstream fix and distribution deployment.
Verification notes and caveats
- The National Vulnerability Database (NVD) entry and multiple independent vulnerability trackers summarize the issue as an uninitialized memory leak and point to upstream stable commits that add the zeroing line. Those public entries are consistent with the upstream patch diffs and stable‑tree messages.
- Some vulnerability feeds report different severity estimates (for example, independent assessments that compute CVSS v3.x scores can vary). If your risk‑threshold or patch priority is driven by CVSS scoring, confirm a final scoring from your organizational risk model rather than relying on one aggregator. One vendor feed provided a CVSS v3 estimate indicating higher confidentiality and availability impacts, which highlights the subjectivity in scoring this class of flaw. Treat CVSS values as one input among many for prioritization.
- Distribution mapping (which exact distro kernel packages include the fix) changes quickly as vendors backport patches into supported branches. Confirm per‑distro advisories and the package lists in your environment before mass rollout. Where vendor advisories are not yet available, use temporary mitigations (restrict access to /dev/uinput) while preparing for a controlled patch. This mapping is environment‑specific and therefore flagged as something administrators must verify against their vendor channels.
Conclusion
CVE‑2025‑40035 is a compact but meaningful fix: a one‑line zeroing of uinput’s compat upload structure removes a simple, reliable avenue for leaking kernel stack contents to userland. The vulnerability is local‑only, not a remote RCE, yet the practical utility of information leaks in multi‑tenant environments makes this a non‑trivial operational risk. System administrators should identify systems that expose the uinput interface or accept untrusted local code, apply vendor kernel updates promptly, and implement short‑term mitigations (udev rules, reduced device access, disabled passthrough) where immediate patching is not feasible.In developer terms the moral is familiar but worth repeating: when code marshals kernel structures into user buffers, initialize everything you return — including padding. The cost of a memset is negligible; the potential payoff for an attacker harvesting kernel bytes is not. The upstream patch, merged into stable kernels and mirrored by vulnerability aggregators, completes the fix cycle for this particular leak and restores the simple hygiene that prevents these quiet but effective primitives from becoming the first step in a full compromise.
Source: MSRC Security Update Guide - Microsoft Security Response Center
