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FreeBSD-SA-20:18.posix_spawnp                               Security Advisory
                                                          The FreeBSD Project

Topic:          posix_spawnp(3) buffer overflow

Category:       core
Module:         libc
Announced:      2020-07-08
Credits:        Andrew Gierth
Affects:        FreeBSD 11.4
Corrected:      2020-06-17 16:22:08 UTC (stable/12, 12.1-STABLE)
                2020-06-17 16:22:08 UTC (stable/11, 11.4-STABLE)
                2020-07-08 20:08:05 UTC (releng/11.4, 11.4-RELEASE-p1)
CVE Name:       CVE-2020-7458

Note: This vulnerability was introduced after the release of FreeBSD 11.3 and
FreeBSD 12.1; FreeBSD 11.4 is the only affected release.

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

posix_spawnp(3) is a lightweight process creation mechanism provided by libc
for general application usage.

II.  Problem Description

posix_spawnp spawns a new thread with a limited stack allocated on the heap
before delegating to execvp for the final execution within that thread.

execvp would previously make unbounded allocations on the stack, directly
proportional to the length of the user-controlled PATH environment variable.

III. Impact

Long values in the user-controlled PATH environment variable cause 
posix_spawnp to write beyond the end of stack that was allocated, ultimately
overflowing the heap-allocated stack with a direct copy of the value stored
in PATH.

IV.  Workaround

No workaround is available.  Few applications in the base system use
posix_spawnp(3) and none of them are particularly viable candidates for an
exploit.  Use by third-party applications has not been investigated.

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 11.4]
# fetch https://security.FreeBSD.org/patches/SA-20:18/posix_spawnp.patch
# fetch https://security.FreeBSD.org/patches/SA-20:18/posix_spawnp.patch.asc
# gpg --verify posix_spawnp.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>.

Restart all daemons that use the library, or reboot the system.

VI.  Correction details

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

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r362281
stable/11/                                                        r362281
releng/11.4/                                                      r363025
- -------------------------------------------------------------------------

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-2020-7458>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:18.posix_spawnp.asc>
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