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FreeBSD-EN-22:16.kqueue                                         Errata Notice
                                                          The FreeBSD Project

Topic:          kevent(2) timers fire too often

Category:       core
Module:         kqueue
Announced:      2022-08-09
Affects:        FreeBSD 13.1
Corrected:      2022-06-08 00:42:21 UTC (stable/13, 13.1-STABLE)
                2022-08-09 20:01:21 UTC (releng/13.1, 13.1-RELEASE-p1)

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

kevent(2) is a system call which provides a generic method of notifying
the caller when a caller-specified event happens or a condition holds.
One use for kevent(2) is to wait for a specified timeout to elapse.
This is implemented by the EVFILT_TIMER filter type.

II.  Problem Description

In FreeBSD 13.1, periodic events of type EVFILT_TIMER will return at
only half of the requested frequency, following the first event.

III. Impact

The bug may cause misbehaviour in software that makes use of periodic
kevent(2)-based timers.

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.

Perform one of the following:

1) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the amd64, i386, or
(on FreeBSD 13 and later) arm64 platforms can be updated via the
freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for an errata update"

2) 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-22:16/kqueue.patch
# fetch https://security.FreeBSD.org/patches/EN-22:16/kqueue.patch.asc
# gpg --verify kqueue.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 by the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/13/                              129112f80d2b    stable/13-n251040
releng/13.1/                            c48048ebdbed  releng/13.1-n250150
- -------------------------------------------------------------------------

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=264131>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-22:16.kqueue.asc>
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