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FreeBSD-SA-21:01.fsdisclosure                               Security Advisory
                                                          The FreeBSD Project

Topic:          Uninitialized kernel stack leaks in several file systems

Category:       core
Module:         fs
Announced:      2021-01-29
Credits:        Syed Faraz Abrar
Affects:        All supported versions of FreeBSD.
Corrected:      2021-01-06 14:58:41 UTC (stable/12, 12.2-STABLE)
                2021-01-29 01:20:59 UTC (releng/12.2, 12.2-RELEASE-p3)
                2021-01-29 01:06:09 UTC (releng/12.1, 12.1-RELEASE-p13)
                2021-01-18 19:16:24 UTC (stable/11, 11.4-STABLE)
                2021-01-29 00:20:09 UTC (releng/11.4, 11.4-RELEASE-p7)
CVE Name:       CVE-2020-25578, CVE-2020-25579

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 FreeBSD kernel exports file system directory entries to userspace
using the generic "dirent" structure.  Individual file systems implement
VOP_READDIR to convert from the file system's internal directory entry
layout to the generic form.  dirent structures can be fetched from
userspace using the getdirentries(2) system call.

II.  Problem Description

Several file systems were not properly initializing the d_off field of
the dirent structures returned by VOP_READDIR.  In particular, tmpfs(5),
smbfs(5), autofs(5) and mqueuefs(5) were failing to do so.  As a result,
eight uninitialized kernel stack bytes may be leaked to userspace by
these file systems.  This problem is not present in FreeBSD 11.

Additionally, msdosfs(5) was failing to zero-fill a pair of padding
fields in the dirent structure, resulting in a leak of three
uninitialized bytes.

III. Impact

Kernel stack disclosures may leak sensitive information which could be
used to compromise the security of the system.

IV.  Workaround

Systems that do not have any of the affected file systems mounted are
not affected.  To trigger the leaks, an unprivileged user must have read
access to a directory belonging to one of the mounted file systems.

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 i386 or amd64
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 12.x]
# fetch https://security.FreeBSD.org/patches/SA-21:01/fsdisclosure.12.patch
# fetch https://security.FreeBSD.org/patches/SA-21:01/fsdisclosure.12.patch.asc
# gpg --verify fsdisclosure.12.patch.asc

[FreeBSD 11.x]
# fetch https://security.FreeBSD.org/patches/SA-21:01/fsdisclosure.11.patch
# fetch https://security.FreeBSD.org/patches/SA-21:01/fsdisclosure.11.patch.asc
# gpg --verify fsdisclosure.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
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r368969
releng/12.2/                                                      r369175
releng/12.1/                                                      r369165
stable/11/                                                        r369047
releng/11.4/                                                      r369156
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-25578>

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-25579>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-21:01.fsdisclosure.asc>
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