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FreeBSD-SA-26:63.posixshm                                   Security Advisory
                                                          The FreeBSD Project

Topic:          TOCTOU race in POSIX shared memory large page configuration

Category:       core
Module:         posixshm
Announced:      2026-08-25
Credits:        tsune of GMO Cybersecurity by Ierae, Inc.
                working with TrendAI Zero Day Initiative
Affects:        All supported versions of FreeBSD.
Corrected:      2026-08-25 15:59:53 UTC (stable/15, 15.1-STABLE)
                2026-08-25 16:01:30 UTC (releng/15.1, 15.1-RELEASE-p3)
                2026-08-25 16:01:10 UTC (releng/15.0, 15.0-RELEASE-p13)
                2026-08-25 16:00:04 UTC (stable/14, 14.4-STABLE)
                2026-08-25 16:00:31 UTC (releng/14.4, 14.4-RELEASE-p9)
CVE Name:       CVE-2026-58094

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

The POSIX shared memory object module supports "largepage" objects,
which are shared memory objects backed by physically contiguous memory.
Such objects can be accessed more efficiently in some cases.  Largepage
objects can be created using shm_create_largepage(3).

II.  Problem Description

The FIOSSHMLPGCNF ioctl(2) operation configures the page size for a
largepage shared memory object.  This is intended to be used immediately
after creating the object, before any memory is allocated for the object.
The handler checked whether a page size had already been configured
without holding the rangelock.  Two concurrent callers could both
observe an unconfigured object and set conflicting page sizes, leaving
the object in an inconsistent state.

III. Impact

An unprivileged local user can exploit this race to escalate
privileges.

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-26:63/posixshm.patch
# fetch https://security.FreeBSD.org/patches/SA-26:63/posixshm.patch.asc
# gpg --verify posixshm.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/                              5990860ca841    stable/15-n285148
releng/15.1/                            a372e73712cf  releng/15.1-n283610
releng/15.0/                            cec488c1669b  releng/15.0-n281113
stable/14/                              6bfd894c8b14    stable/14-n275049
releng/14.4/                            840fd9dc7de2  releng/14.4-n273765
- -------------------------------------------------------------------------

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-2026-58094>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:63.posixshm.asc>
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