CVE-2025-40035 in Linux
Summary
by MITRE • 10/28/2025
In the Linux kernel, the following vulnerability has been resolved:
Input: uinput - zero-initialize uinput_ff_upload_compat to avoid info leak
Struct ff_effect_compat is embedded twice inside uinput_ff_upload_compat, contains internal padding. In particular, there is a hole after struct ff_replay to satisfy alignment requirements for the following union member. Without clearing the structure, copy_to_user() may leak stack data to userspace.
Initialize ff_up_compat to zero before filling valid fields.
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Analysis
by VulDB Data Team • 05/19/2026
The vulnerability identified as CVE-2025-40035 resides within the Linux kernel's uinput subsystem, specifically addressing a critical information disclosure issue in the force feedback upload compatibility handling mechanism. This flaw manifests when the kernel processes force feedback effect data through the uinput interface, creating a potential pathway for sensitive stack data to be inadvertently exposed to unprivileged userspace applications. The vulnerability stems from improper initialization of the uinput_ff_upload_compat structure, which serves as a compatibility layer for force feedback effect uploads between different kernel versions or application interfaces.
The technical root cause involves the struct ff_effect_compat data structure which is embedded twice within uinput_ff_upload_compat, creating a complex memory layout with intentional padding for alignment purposes. Specifically, there exists a memory hole following the struct ff_replay component to satisfy alignment requirements for subsequent union members within the structure. When the kernel fails to properly zero-initialize the entire uinput_ff_upload_compat structure before populating it with valid data, this padding space retains its original stack contents. During the copy_to_user() operation, which transfers data from kernel space to userspace, this uninitialized padding inadvertently leaks kernel stack information to userspace applications, creating a potential information disclosure vector.
The operational impact of this vulnerability extends beyond simple information leakage, as it represents a serious security concern that could enable attackers to extract sensitive kernel memory contents including cryptographic keys, session tokens, or other confidential data residing in the kernel stack. This type of information leak could potentially be exploited by malicious actors to perform further attacks such as privilege escalation or system compromise. The vulnerability affects systems running Linux kernels that implement the uinput subsystem with force feedback capabilities, particularly those that support compatibility layers for older force feedback effect formats. The issue is particularly concerning in environments where untrusted userspace applications have access to uinput devices, as they could potentially leverage this information leak to gain insights into kernel memory layout and internal state.
Security mitigations for this vulnerability involve implementing proper zero-initialization of the uinput_ff_upload_compat structure before populating valid fields, ensuring that all padding and unused memory regions are cleared before data transfer operations. This approach directly addresses the underlying cause by preventing the accidental exposure of stack data through the copy_to_user() mechanism. System administrators should prioritize applying kernel updates that contain the patched implementation, which typically involves adding a memset or similar initialization operation to clear the entire structure before processing force feedback effect data. The fix aligns with security best practices for memory management and information flow control, preventing unintended data exposure through proper initialization procedures. This vulnerability demonstrates the importance of careful memory management in kernel space operations and highlights the potential consequences of insufficient zero-initialization of complex data structures containing padding and alignment requirements. The issue relates to CWE-248, Uncontrolled Format String, and CWE-125, Out-of-bounds Read, while potentially mapping to ATT&CK techniques involving privilege escalation through information gathering and memory disclosure methods.