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FreeBSD-EN-24:12.killpg                                         Errata Notice
                                                          The FreeBSD Project

Topic:          Lock order reversal in killpg causing livelock

Category:       core
Module:         kern
Announced:      2024-06-19
Credits:        Peter Holm <pho@FreeBSD.org>
                Konstantin Belousov <kib@FreeBSD.org>
                Michael Gmelin <grembo@FreeBSD.org>
Affects:        FreeBSD 13.3
Corrected:      2024-05-22 23:47:55 UTC (stable/13, 13.3-STABLE)
                2024-06-19 20:37:09 UTC (releng/13.3, 13.3-RELEASE-p3)

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

killpg(2) is a system call that sends signals to process groups.
In order to prevent race conditions with the fork(2) system call,
locking is required.

II.  Problem Description

The code designed to prevent lock order reversals between killpg(2)
and fork(2) did not wait for lock availability before retrying to
acquire a lock, which could result in a livelock causing very high
system load.

III. Impact

Affected machines are practically unusable. They recover if the
processes causing the load are killed.

IV.  Workaround

No workaround is available. Systems that fork a lot are more likely to
to hit this problem.

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 or arm64 platforms,
or the i386 platform on FreeBSD 13, can be updated via the freebsd-update(8)
utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10m "Rebooting for a stability patch"

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-24:12/killpg.patch
# fetch https://security.FreeBSD.org/patches/EN-24:12/killpg.patch.asc
# gpg --verify killpg.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/13/                              cd73b38955f6    stable/13-n257926
releng/13.3/                            eb410545d00d  releng/13.3-n257435
- -------------------------------------------------------------------------

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=279203>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-24:12.killpg.asc>
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