CVE-2025-39684 in Linux
Summary
by MITRE • 09/05/2025
In the Linux kernel, the following vulnerability has been resolved:
comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()
syzbot reports a KMSAN kernel-infoleak in `do_insn_ioctl()`. A kernel buffer is allocated to hold `insn->n` samples (each of which is an `unsigned int`). For some instruction types, `insn->n` samples are copied back to user-space, unless an error code is being returned. The problem is that not all the instruction handlers that need to return data to userspace fill in the whole `insn->n` samples, so that there is an information leak. There is a similar syzbot report for `do_insnlist_ioctl()`, although it does not have a reproducer for it at the time of writing.
One culprit is `insn_rw_emulate_bits()` which is used as the handler for `INSN_READ` or `INSN_WRITE` instructions for subdevices that do not have a specific handler for that instruction, but do have an `INSN_BITS` handler. For `INSN_READ` it only fills in at most 1 sample, so if `insn->n` is greater than 1, the remaining `insn->n - 1` samples copied to userspace will be uninitialized kernel data.
Another culprit is `vm80xx_ai_insn_read()` in the "vm80xx" driver. It never returns an error, even if it fails to fill the buffer.
Fix it in `do_insn_ioctl()` and `do_insnlist_ioctl()` by making sure that uninitialized parts of the allocated buffer are zeroed before handling each instruction.
Thanks to Arnaud Lecomte for their fix to `do_insn_ioctl()`. That fix replaced the call to `kmalloc_array()` with `kcalloc()`, but it is not always necessary to clear the whole buffer.
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Analysis
by VulDB Data Team • 02/09/2026
The vulnerability described in CVE-2025-39684 resides within the Linux kernel's Comedi subsystem, specifically affecting the handling of ioctl operations for hardware device communication. This issue manifests as a use of uninitialized memory condition that could potentially lead to information leakage, making it particularly concerning for systems requiring high security standards. The vulnerability impacts the do_insn_ioctl() and do_insnlist_ioctl() functions which are responsible for processing instruction-based ioctl calls to communicate with various hardware devices through the Comedi framework.
The technical flaw stems from improper memory initialization within the kernel's Comedi driver handling code. When processing hardware instructions, the kernel allocates a buffer to store n samples where each sample is an unsigned integer. However, not all instruction handlers properly populate the entire buffer, leaving portions uninitialized. This occurs particularly when instruction types that should return data to user-space only partially fill the allocated buffer. The problem is exacerbated by specific handler functions like insn_rw_emulate_bits() which for INSN_READ operations only fills at most one sample even when the buffer size indicates multiple samples should be populated. Additionally, vm80xx_ai_insn_read() in the vm80xx driver fails to properly handle buffer filling failures, potentially returning uninitialized data to user-space applications.
This vulnerability creates a significant operational impact by enabling potential information leakage through kernel memory that should remain private. The KMSAN (Kernel Memory Sanitizer) detection confirms that uninitialized kernel data is being copied to user-space, which could expose sensitive information including kernel stack contents, memory layout details, or other confidential data. This information leak could be exploited by malicious actors to gain insights into the kernel's internal state, potentially aiding in more sophisticated attacks. The vulnerability affects systems using Comedi drivers for hardware device communication, particularly those involving industrial I/O subsystems where precise control and security are paramount. According to CWE-457, this represents a use of uninitialized variable vulnerability, while the ATT&CK framework would categorize this under privilege escalation techniques that leverage kernel memory corruption to extract sensitive information.
The fix implemented addresses the root cause by ensuring that uninitialized memory portions are properly zeroed before instruction handling completes. The solution involves modifying the do_insn_ioctl() and do_insnlist_ioctl() functions to guarantee that all allocated buffer space is initialized before data is returned to user-space. The approach taken by Arnaud Lecomte specifically replaces kmalloc_array() with kcalloc() in do_insn_ioctl(), which provides the necessary zero-initialization of memory. However, the solution is designed to be selective in its application, avoiding unnecessary full buffer clearing when only partial initialization is required. This approach maintains performance while ensuring security, and follows the principle of least privilege by preventing information leakage without overly impacting system efficiency. The fix applies to both the primary function handling individual instructions and the list handling function, addressing the reported information leak in both code paths.