FreeBSD Security Advisory FreeBSD-SA-21:08.vm

============================================================================= FreeBSD-SA-21:08.vm Security Advisory The FreeBSD Project
Topic: Memory disclosure by stale virtual memory mapping
Category: core Module: vm Announced: 2021-04-06 Credits: Ryan Libby, Dell Inc. Affects: All supported versions of FreeBSD. Corrected: 2021-04-06 18:50:46 UTC (stable/13, 13.0-STABLE) 2021-04-06 19:18:49 UTC (releng/13.0, 13.0-RC5-p1) 2021-04-06 19:20:46 UTC (stable/12, 12.2-STABLE) 2021-04-06 19:21:30 UTC (releng/12.2, 12.2-RELEASE-p6) 2021-04-06 19:22:31 UTC (stable/11, 11.4-STABLE) 2021-04-06 19:22:56 UTC (releng/11.4, 11.4-RELEASE-p9) CVE Name: CVE-2021-29626
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I. Background
Memory mappings shared between processes are a feature of the FreeBSD virtual memory system. They may be established by unprivileged processes with the mmap(2), fork(2), and other system calls.
II. Problem Description
A particular case of memory sharing is mishandled in the virtual memory system. It is possible and legal to establish a relationship where multiple descendant processes share a mapping which shadows memory of an ancestor process. In this scenario, when one process modifies memory through such a mapping, the copy-on-write logic fails to invalidate other mappings of the source page. These stale mappings may remain even after the mapped pages have been reused for another purpose.
III. Impact
An unprivileged local user process can maintain a mapping of a page after it is freed, allowing that process to read private data belonging to other processes or the kernel.
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.
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 vm_fault.13.patch.asc
[FreeBSD 12.2] # fetch # fetch # gpg –verify vm_fault.12.patch.asc
[FreeBSD 11.4] # fetch # fetch # gpg –verify vm_fault.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/ 2e08308d62f3 stable/13-n245117 releng/13.0/ 724bc23da1a9 releng/13.0-n244728 stable/12/ r369551 releng/12.2/ r369556 stable/11/ r369559 releng/11.4/ r369561 – ————————————————————————-
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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