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============================================================================= FreeBSD-SA-26:44.posixshm Security Advisory
The FreeBSD Project

Topic: Multiple vulnerabilities in POSIX largepage objects

Category: core
Module: posixshm
Announced: 2026-06-30
Credits: Chris Jarrett-Davies of the OpenAI Codex Security Team
Affects: All supported versions of FreeBSD.
Corrected: 2026-06-30 17:20:14 UTC (stable/15, 15.1-STABLE)
2026-06-30 17:22:03 UTC (releng/15.1, 15.1-RELEASE-p1)
2026-06-30 17:21:31 UTC (releng/15.0, 15.0-RELEASE-p11)
2026-06-30 17:19:55 UTC (stable/14, 14.4-STABLE)
2026-06-30 17:21:04 UTC (releng/14.4, 14.4-RELEASE-p7)
2026-06-30 17:20:37 UTC (releng/14.3, 14.3-RELEASE-p16)
CVE Name: CVE-2026-49427, CVE-2026-49428

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. For most purposes, they behave the same as ordinary POSIX shared memory objects,
but the underlying implementation is quite different, and certain
operations cannot be performed on largepage objects.

Largepage objects can be created using shm_create_largepage(3).

II. Problem Description

Pages belonging to largepage shared memory objects were not explicitly
wired. When sendfile(2) transmitted such an object with the SF_NOCACHE
flag, it freed the underlying pages after transmission even though
existing mappings still referred to them. [CVE-2026-49427]

Separately, certain system calls, such open(2) with the O_TRUNC flag set,
and fspacectl(2), could incorrectly free memory in largepage objects.
These operations are not permitted on largepage objects, but the
implementation did not verify this. [CVE-2026-49428]

III. Impact

An unprivileged local user can abuse the bug to access freed kernel
memory. This can be exploited 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.

[FreeBSD 15.1]
# fetch https://security.FreeBSD.org/patches/SA-26:44/posixshm-15.1.patch
# fetch https://security.FreeBSD.org/patches/SA-26:44/posixshm-15.1.patch.asc
# gpg --verify posixshm-15.1.patch.asc

[FreeBSD 15.0]
# fetch https://security.FreeBSD.org/patches/SA-26:44/posixshm-15.0.patch
# fetch https://security.FreeBSD.org/patches/SA-26:44/posixshm-15.0.patch.asc
# gpg --verify posixshm-15.0.patch.asc

[FreeBSD 14.x]
# fetch https://security.FreeBSD.org/patches/SA-26:44/posixshm-14.patch
# fetch https://security.FreeBSD.org/patches/SA-26:44/posixshm-14.patch.asc
# gpg --verify posixshm-14.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/ 702f4c829c17 stable/15-n284330 releng/15.1/ b15971f462b6 releng/15.1-n283575 releng/15.0/ 8d086f03b9be releng/15.0-n281077 stable/14/ f30052c16dba stable/14-n274455 releng/14.4/ 0848cdea83fd releng/14.4-n273737 releng/14.3/ 5272af920126 releng/14.3-n271537
- -------------------------------------------------------------------------

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-2026-49427> <URL:https://www.cve.org/CVERecord?id=CVE-2026-49428>

The latest revision of this advisory is available at <URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-26:44.posixshm.asc> -----BEGIN PGP SIGNATURE-----

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