CVE-2025-39684
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-09-05

Last updated on: 2025-11-03

Assigner: kernel.org

Description
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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Meta Information
Published
2025-09-05
Last Modified
2025-11-03
Generated
2026-05-07
AI Q&A
2025-09-05
EPSS Evaluated
2026-05-05
NVD
CVE-2025-39684
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel 6.1.153
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-UNKNOWN
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AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability is a kernel information leak in the Linux kernel's comedi subsystem. It occurs because certain instruction handlers in the do_insn_ioctl() and do_insnlist_ioctl() functions do not fully initialize the memory buffer before copying data back to user-space. Specifically, when handling instructions that request multiple samples, some handlers only fill part of the buffer, leaving uninitialized kernel memory exposed to user-space. This can leak sensitive kernel data. The issue was fixed by ensuring that uninitialized parts of the buffer are zeroed before use.


How can this vulnerability impact me? :

This vulnerability can lead to an information leak where uninitialized kernel memory is exposed to user-space applications. An attacker or malicious user-space program could potentially read sensitive kernel data, which may include confidential information or internal kernel state, leading to privacy and security risks.


What immediate steps should I take to mitigate this vulnerability?

The vulnerability is fixed by ensuring that uninitialized parts of the allocated kernel buffer are zeroed before handling each instruction in the affected functions. To mitigate this vulnerability, you should update your Linux kernel to a version that includes the fix for CVE-2025-39684, which replaces the use of kmalloc_array() with kcalloc() in do_insn_ioctl() and do_insnlist_ioctl().


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