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FreeBSD-SA-26:51.ktimer                                     Security Advisory
                                                          The FreeBSD Project

Topic:          Kernel stack disclosure via timer_settime(2)

Category:       core
Module:         ktimer
Announced:      2026-07-29
Credits:        Hazley Samsudin of GovTech CSG
Affects:        FreeBSD 15.1 and 15.0
Corrected:      2026-07-27 19:15:01 UTC (stable/15, 15.1-STABLE)
                2026-07-29 17:50:30 UTC (releng/15.1, 15.1-RELEASE-p2)
                2026-07-29 17:50:05 UTC (releng/15.0, 15.0-RELEASE-p12)
CVE Name:       CVE-2026-58084

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

POSIX interval timers, managed by timer_create(2) and timer_settime(2), allow
a process to schedule periodic or one-shot notifications based on a specified
clock source.  When timer_settime(2) is called with a non-NULL old_value
argument, the kernel returns the timer's previous setting.

II.  Problem Description

To retrieve the previous timer value, the kernel calls realtimer_gettime(),
which obtains the current time for the timer's clock.  For a timer using
CLOCK_TAI this can fail when no TAI offset has been configured, but the error
return was not checked, so the uninitialized output buffer was copied to
userspace.

III. Impact

An unprivileged local user can obtain uninitialized kernel stack memory by
creating a POSIX timer with CLOCK_TAI and calling timer_settime(2),
potentially disclosing sensitive kernel data.

IV.  Workaround

No workaround is available.

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-26:51/ktimer.patch
# fetch https://security.FreeBSD.org/patches/SA-26:51/ktimer.patch.asc
# gpg --verify ktimer.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/                              a4b5ff57ef85    stable/15-n284618
releng/15.1/                            e1c9b0b13a29  releng/15.1-n283589
releng/15.0/                            3254ef000750  releng/15.0-n281092
- -------------------------------------------------------------------------

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-2026-58084>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:51.ktimer.asc>
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