linux-cve-announce - CVE-2025-39684: comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist

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Message-ID: <2025090546-CVE-2025-39684-fcce@gregkh>
Date: Fri,  5 Sep 2025 19:20:58 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-39684: comedi: Fix use of uninitialized memory in do_insn_ioctl() and do_insnlist_ioctl()

From: Greg Kroah-Hartman <gregkh@...nel.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.

The Linux kernel CVE team has assigned CVE-2025-39684 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 2.6.29 with commit ed9eccbe8970f6eedc1b978c157caf1251a896d4 and fixed in 5.15.190 with commit 868a1b68dcd9f2805bb86aa64862402f785d8c4a
	Issue introduced in 2.6.29 with commit ed9eccbe8970f6eedc1b978c157caf1251a896d4 and fixed in 6.1.149 with commit ff4a7c18799c7fe999fa56c5cf276e13866b8c1a
	Issue introduced in 2.6.29 with commit ed9eccbe8970f6eedc1b978c157caf1251a896d4 and fixed in 6.6.103 with commit d84f6e77ebe3359394df32ecd97e0d76a25283dc
	Issue introduced in 2.6.29 with commit ed9eccbe8970f6eedc1b978c157caf1251a896d4 and fixed in 6.12.44 with commit f3b0c9ec54736f3b8118f93a473d22e11ee65743
	Issue introduced in 2.6.29 with commit ed9eccbe8970f6eedc1b978c157caf1251a896d4 and fixed in 6.16.4 with commit aecf0d557ddd95ce68193a5ee1dc4c87415ff08a
	Issue introduced in 2.6.29 with commit ed9eccbe8970f6eedc1b978c157caf1251a896d4 and fixed in 6.17-rc3 with commit 3cd212e895ca2d58963fdc6422502b10dd3966bb

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-39684
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	drivers/comedi/comedi_fops.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/868a1b68dcd9f2805bb86aa64862402f785d8c4a
	https://git.kernel.org/stable/c/ff4a7c18799c7fe999fa56c5cf276e13866b8c1a
	https://git.kernel.org/stable/c/d84f6e77ebe3359394df32ecd97e0d76a25283dc
	https://git.kernel.org/stable/c/f3b0c9ec54736f3b8118f93a473d22e11ee65743
	https://git.kernel.org/stable/c/aecf0d557ddd95ce68193a5ee1dc4c87415ff08a
	https://git.kernel.org/stable/c/3cd212e895ca2d58963fdc6422502b10dd3966bb