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FreeBSD-SA-26:54.sysvsem                                    Security Advisory
                                                          The FreeBSD Project

Topic:          Heap out-of-bounds access in semctl(2)

Category:       core
Module:         sysvsem
Announced:      2026-07-29
Credits:        Maik Muench of Secfault Security
Affects:        All supported versions of FreeBSD.
Corrected:      2026-07-29 17:48:44 UTC (stable/15, 15.1-STABLE)
                2026-07-29 17:50:36 UTC (releng/15.1, 15.1-RELEASE-p2)
                2026-07-29 17:50:10 UTC (releng/15.0, 15.0-RELEASE-p12)
                2026-07-29 17:49:03 UTC (stable/14, 14.4-STABLE)
                2026-07-29 17:49:37 UTC (releng/14.4, 14.4-RELEASE-p8)
CVE Name:       CVE-2026-58087

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

System V semaphores provide a set-based inter-process communication (IPC)
semaphore facility.

The semctl(2) system call performs control operations on semaphore sets,
including the GETALL and SETALL commands which read or write the values of
every semaphore in a set.

II.  Problem Description

The GETALL and SETALL commands in semctl(2) recorded the number of semaphores
in the target set, dropped the lock protecting the set, allocated a buffer
sized for that count, and reacquired the lock.  A sequence-number check was
used to verify that the set had not been replaced in the interim, but the
sequence number wraps after 0x8000 create/destroy cycles.  By rapidly
destroying and recreating semaphore sets at the same index, another process
can cause the sequence number to wrap, allowing a set with a different number
of semaphores to pass validation.  The subsequent copy then reads or writes
past the end of the allocated buffer.

III. Impact

An unprivileged local user can trigger out-of-bounds reads and writes on
kernel heap memory, potentially leading to privilege escalation.

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-26:54/sysvsem.patch
# fetch https://security.FreeBSD.org/patches/SA-26:54/sysvsem.patch.asc
# gpg --verify sysvsem.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/                              e2585687890e    stable/15-n284647
releng/15.1/                            5eb50510c6b3  releng/15.1-n283594
releng/15.0/                            20c738692c61  releng/15.0-n281097
stable/14/                              8b08ee989506    stable/14-n274645
releng/14.4/                            b5eaa00cdba3  releng/14.4-n273752
- -------------------------------------------------------------------------

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-2026-58087>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:54.sysvsem.asc>
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