CVE-2025-39684
comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()
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.
INFO
Published Date :
Sept. 5, 2025, 6:15 p.m.
Last Modified :
Sept. 8, 2025, 4:25 p.m.
Remotely Exploit :
No
Source :
416baaa9-dc9f-4396-8d5f-8c081fb06d67
Solution
- Ensure uninitialized buffer parts are zeroed.
- Update the Linux kernel to the latest version.
- Apply provided security patches for the kernel.
Public PoC/Exploit Available at Github
CVE-2025-39684 has a 1 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
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CVE-2025-39684.
CWE - Common Weakness Enumeration
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associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
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approaches employed by adversaries to exploit the CVE-2025-39684
weaknesses.
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New CVE Received by 416baaa9-dc9f-4396-8d5f-8c081fb06d67
Sep. 05, 2025
Action Type Old Value New Value Added 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. Added Reference https://git.kernel.org/stable/c/3cd212e895ca2d58963fdc6422502b10dd3966bb Added Reference https://git.kernel.org/stable/c/868a1b68dcd9f2805bb86aa64862402f785d8c4a Added Reference https://git.kernel.org/stable/c/aecf0d557ddd95ce68193a5ee1dc4c87415ff08a Added Reference https://git.kernel.org/stable/c/d84f6e77ebe3359394df32ecd97e0d76a25283dc Added Reference https://git.kernel.org/stable/c/f3b0c9ec54736f3b8118f93a473d22e11ee65743 Added Reference https://git.kernel.org/stable/c/ff4a7c18799c7fe999fa56c5cf276e13866b8c1a