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=============================================================================
FreeBSD-SA-26:58.sound                                      Security Advisory
                                                          The FreeBSD Project

Topic:          Kernel use-after-free via the SNDCTL_DSP_SYNCSTART ioctl

Category:       core
Module:         sound
Announced:      2026-08-25
Credits:        Hazley Samsudin of GovTech CSG
Affects:        All supported versions of FreeBSD.
Corrected:      2026-08-25 15:59:48 UTC (stable/15, 15.1-STABLE)
                2026-08-25 16:01:23 UTC (releng/15.1, 15.1-RELEASE-p3)
                2026-08-25 16:01:02 UTC (releng/15.0, 15.0-RELEASE-p13)
                2026-08-25 16:00:01 UTC (stable/14, 14.4-STABLE)
                2026-08-25 16:00:25 UTC (releng/14.4, 14.4-RELEASE-p9)
CVE Name:       CVE-2026-58091

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

FreeBSD provides audio support through the sound(4) driver, which
presents each audio device as a set of character device nodes such as
/dev/dsp.  The SNDCTL_DSP_SYNCSTART ioctl(2) allows multiple audio
channels to start playback or capture in unison.

II.  Problem Description

The implementation of this ioctl attempts to acquire locks on all
channels in a sync group.  If locking a channel would block, it
releases the sync group list lock and sleeps.  Upon reawakening,
it is possible that the sync group structure is freed, but the
implementation did not handle this possibility.

III. Impact

On a system with a multiple audio devices, an unprivileged local user
can exploit this use-after-free to escalate privileges.

IV.  Workaround

No workaround is available.  Systems with zero or one sound devices are
unaffected.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date,
and reboot the system.

Perform one of the following:

1) To update your vulnerable system installed from base system packages:

Systems running a 15.0-RELEASE or later version of FreeBSD on the amd64 or
arm64 platforms, which were installed using base system packages, can be
updated via the pkg(8) utility:

# pkg upgrade -r FreeBSD-base
# shutdown -r +10min "Rebooting for a security update"

2) To update your vulnerable system installed from binary distribution sets:

Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms
which were not installed using base system packages can be updated via the
freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for a security update"

3) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-26:58/sound.patch
# fetch https://security.FreeBSD.org/patches/SA-26:58/sound.patch.asc
# gpg --verify sound.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch -E -p0 < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

This issue is corrected as of the corresponding Git commit hash in the
following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/15/                              edff72e5f64c    stable/15-n285144
releng/15.1/                            48c25324ff1c  releng/15.1-n283603
releng/15.0/                            0e8a1c849c3b  releng/15.0-n281106
stable/14/                              e6c1c9215557    stable/14-n275046
releng/14.4/                            8668302cb731  releng/14.4-n273759
- -------------------------------------------------------------------------

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

VII. References

<URL:https://www.cve.org/CVERecord?id=CVE-2026-58091>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:58.sound.asc>
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