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FreeBSD-EN-20:01.ssp                                            Errata Notice
                                                          The FreeBSD Project

Topic:          Imprecise ordering of SSP canary initialization

Category:       core
Module:         libc
Announced:      2020-01-28
Credits:        Kyle Evans
Affects:        All supported versions of FreeBSD.
Corrected:      2019-11-25 03:49:38 UTC (stable/12, 12.1-STABLE)
                2020-01-28 18:53:14 UTC (releng/12.1, 12.1-RELEASE-p2)
                2020-01-28 18:53:14 UTC (releng/12.0, 12.0-RELEASE-p13)
                2019-11-25 03:49:38 UTC (stable/11, 11.3-STABLE)
                2020-01-28 18:53:14 UTC (releng/11.3, 11.3-RELEASE-p6)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

The Stack Smashing Protector ("SSP") relies on a stack canary being
initialized early on in application startup.  On FreeBSD, this is
accomplished with a constructor in libc.

II.  Problem Description

When a binary is statically linked, constructor invocation order is based on
priority and sorted arbitrarily within a priority level across all
constructors present in the single statically linked object.  The stack
canary guard constructor had no priority, so statically linked binary could
not predictably order their constructors to avoid bad interactions with
respect to the stack canary constructor leading to false-positive detection
of a stack overflow condition and erroneous process abort in some rare cases.

Dynamically linked binaries are generally not affected, since the stack
canary is initialized in libc and libc is ordered very early in constructor
invocation.

III. Impact

Affected programs will abort and log a "stack overflow detected" message to
syslog(3).

IV.  Workaround

No workaround is available, but dynamically linked binaries are not affected.

V.   Solution

Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date.  Statically linked binaries
should be relinked against the updated base system.

Perform one of the following:

1) To update your 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

2) To update your 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/EN-20:01/ssp.patch
# fetch https://security.FreeBSD.org/patches/EN-20:01/ssp.patch.asc
# gpg --verify ssp.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.

VI.  Correction details

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

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r355080
releng/12.1/                                                      r357215
releng/12.0/                                                      r357215
stable/11/                                                        r355080
releng/11.3/                                                      r357215
- -------------------------------------------------------------------------

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://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241905>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-20:01.ssp.asc>
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