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FreeBSD-EN-26:06.timerfd                                        Errata Notice
                                                          The FreeBSD Project

Topic:          Periodic timerfd(2) timers may produce incorrect results

Category:       core
Module:         timerfd
Announced:      2026-04-21
Affects:        FreeBSD 14.3 and later.
Corrected:      2026-04-03 15:26:14 UTC (stable/15, 15.0-STABLE)
                2026-04-21 15:44:25 UTC (releng/15.0, 15.0-RELEASE-p6)
                2026-04-03 15:27:26 UTC (stable/14, 14.4-STABLE)
                2026-04-21 15:45:30 UTC (releng/14.4, 14.4-RELEASE-p2)
                2026-04-21 15:46:00 UTC (releng/14.3, 14.3-RELEASE-p11)

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

I.   Background

The timerfd(2) family of system calls provides a file descriptor-based
interface for managing timers.

II.  Problem Description

timerfd(2) implements periodic timers.  The implementation had a bug which
caused it to fire too early in some cases.

III. Impact

The bug has been observed to cause excessive CPU usage in some applications,
particularly in some KDE desktop programs.

IV.  Workaround

No workaround is available.

V.   Solution

Upgrade your 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 system installed from base system packages:

Systems running a 15.0-RELEASE 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 now

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

Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms,
or the i386 platform on FreeBSD 13, 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 now

3) To update your 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/EN-26:06/timerfd.patch
# fetch https://security.FreeBSD.org/patches/EN-26:06/timerfd.patch.asc
# gpg --verify timerfd.patch.asc

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

# cd /usr/src
# patch < /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/                              9b785380f307    stable/15-n282826
releng/15.0/                            b0be1af0c48b  releng/15.0-n281020
stable/14/                              3c00f603a280    stable/14-n273878
releng/14.4/                            df8d2f945028  releng/14.4-n273684
releng/14.3/                            f37c6e3a133e  releng/14.3-n271484
- -------------------------------------------------------------------------

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://bugs.freebsd.org/bugzilla/show_bug.cgi?id=293368>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-26:06.timerfd.asc>
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