FreeBSD Security Advisory FreeBSD-SA-21:10.jail_mount

============================================================================= FreeBSD-SA-21:10.jail_mount Security Advisory The FreeBSD Project
Topic: jail escape possible by mounting over jail root
Category: core Module: jail Announced: 2021-04-06 Credits: Mateusz Guzik Affects: All supported versions of FreeBSD. Corrected: 2021-04-06 18:50:48 UTC (stable/13, 13.0-STABLE) 2021-04-06 19:18:59 UTC (releng/13.0, 13.0-RC5-p1) 2021-04-06 19:20:50 UTC (stable/12, 12.2-STABLE) 2021-04-06 19:21:33 UTC (releng/12.2, 12.2-RELEASE-p6) 2021-04-06 19:22:31 UTC (stable/11, 11.4-STABLE) 2021-04-06 19:22:59 UTC (releng/11.4, 11.4-RELEASE-p9) CVE Name: CVE-2020-25584
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I. Background
The jail(2) system call allows a system administrator to lock a process and all of its descendants inside an environment with a very limited ability to affect the system outside that environment, even for processes with superuser privileges. It is an extension of, but far more powerful than, the traditional UNIX chroot(2) system call.
II. Problem Description
Due to a race condition between lookup of “..” and remounting a filesystem, a process running inside a jail might access filesystem hierarchy outside of jail.
III. Impact
A process with superuser privileges running inside a jail configured with the allow.mount permission (not enabled by default) could change the root directory outside of the jail, and thus gain full read and write access to all files and directories in the system.
IV. Workaround
As a workaround, disable allow.mount permission for all jails with untrusted root users; see jail(1) and jail.conf(5) manual pages for details.
Note that this permission is not enabled by default.
V. Solution
Upgrade your vulnerable 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 vulnerable 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 a security update”
2) 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 13.0] # fetch # fetch # gpg –verify jail_mount.13.patch.asc
[FreeBSD 12.2] # fetch # fetch # gpg –verify jail_mount.12.patch.asc
[FreeBSD 11.4] # fetch # fetch # gpg –verify jail_mount.11.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 and reboot the system.
VI. Correction details
The following list contains the correction revision numbers for each affected branch.
Branch/path Hash Revision – ————————————————————————- stable/13/ 3ae17faa3704 stable/13-n245118 releng/13.0/ 4710439ec594 releng/13.0-n244729 stable/12/ r369552 releng/12.2/ r369557 stable/11/ r369560 releng/11.4/ r369562 – ————————————————————————-
For FreeBSD 13 and later:
Run the following command to see which files were modified by a particular commit:
# git show –stat
Or visit the following URL, replacing HHHHHH with the hash:

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
For FreeBSD 12 and earlier:
Run the following command to see which files were modified by a particular revision, replacing NNNNNN with the revision number:
# svn diff -cNNNNNN –summarize svn://
Or visit the following URL, replacing NNNNNN with the revision number:

VII. References

The latest revision of this advisory is available at —–BEGIN PGP SIGNATURE—–
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