tech

Mitigating GHOST with Salt

Using SaltStack to recover from CVE-2015–0235 (Qualys Security Advisory, GHOST: glibc gethostbyname buffer overflow)

Most of us sysadmin types were pounded with this announcement this morning. The GHOST vulnerability is worth patching against—most Linux distros have already released patches—but it’s useful to know if your machines are vulnerable, or if after patching, the patch was successful.

The canonical way to test for the vulnerability is with a short C program:

/* ghost.c */
/* Code taken from CVE announcement */
/* See
http://www.openwall.com/lists/oss-security/2015/01/27/9
*/
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#define CANARY "in_the_coal_mine"

struct {
    char buffer[1024];
    char canary[sizeof(CANARY)];
} temp = { "buffer", CANARY };

int main(void) {
    struct hostent resbuf;
    struct hostent *result;
    int herrno;
    int retval;

/*** strlen (name) = size_needed — sizeof (*host_addr) — sizeof (*h_addr_ptrs) — 1; ***/

    size_t len = sizeof(temp.buffer)
                 - 16*sizeof(unsigned char)
                 — 2*sizeof(char *) — 1;
    char name[sizeof(temp.buffer)];

    memset(name, '0', len);
    name[len] = '\0';
    retval = gethostbyname_r(name, &resbuf, temp.buffer,
                 sizeof(temp.buffer), &result, &herrno);
    if (strcmp(temp.canary, CANARY) != 0) {
        puts("vulnerable");
        exit(EXIT_FAILURE);
    }

    if (retval == ERANGE) {
        puts("not vulnerable");
        exit(EXIT_SUCCESS);
    }
    puts("test aborted: should not happen");
    exit(EXIT_FAILURE);
}

Which can then be saved to a file “ghost.c” and compiled on most Linux machines with

gcc ghost.c -o ghost

Running it with ./ghost should produce either “not vulnerable” with an exit code of 0, or “vulnerable” with an exit code of 1.

But let’s say you have 1000 machines, all with running salt-minions. How can we test for this on all of them?

We’ll assume first that they are all the same distro as your Salt master. Yes, I know that’s a degenerate case, but to start with let’s just consider the easy route.

First, save ghost.c to a directory on your master and compile it as describe above. Then put the executable in your /srv/salt directory (or wherever your file_roots points). Put this sls file in the same directory:

# /srv/salt/ghosttest.sls

/tmp/ghost:
  file.managed:
    - source: salt://ghost
    - owner: root
    - mode: '0644'

runghost:
  cmd.run:
    - name: /tmp/ghost

Now you can fire off this on all your minions with

salt \* state.sls ghosttest

Because Salt will treat the result of cmd.run as a failure if the executed command returns a non-zero exit status, all vulnerable minions will show “FAILED”. Successfully patched minions will show “SUCCESS.”

Note that all vulnerable services will need to be restarted after a patch (or the affected system will need to be rebooted). Salt can help with this if, in fact, you need to restart individual services rather than restart an entire box.

There are a couple of odd results you can get back from this. First, on one of my machines I got

w01:
—————
       ID: /tmp/ghost
 Function: file.managed
   Result: True
  Comment: File /tmp/ghost is in the correct state
  Started: 11:06:30.632664
 Duration: 779.398 ms
  Changes:
—————
       ID: runghost
 Function: cmd.run
     Name: /tmp/ghost
   Result: False
  Comment: Command “/tmp/ghost” run
  Started: 11:06:31.412444
 Duration: 60.247 ms
  Changes:
           —————
           pid:
               28508
           retcode:
               127
           stderr:
               /bin/bash: /tmp/ghost: No such file or directory
           stdout:

Salt told me the file was present and in the correct state, but bash said “No such file or directory.” Bug in Salt, right? I mean, that’s happened before.

No, not today! If I logged into the machine and ran the executable by hand I got the same message. In this case it was because all my other machines are 64-bit, but this one is 32-bit, and the test executable was linked against the 64-bit glibc. So the message was correct, but confusing since the missing file is not the executable but the library.

Let’s fix this. I happen to have development tools installed on that box, so let’s build a 32-bit compiled version there, put it back on the master, and also modify the sls file so the correct executable will get copied to 64 or 32 bit machines.

/tmp/ghost.c:
  file.managed:
    - source: salt://ghost.c

gcc ghost.c -o ghost:
  cmd.run:
    - user: root
    - cwd: /tmp

# Note this will not work unless file_recv is 'True' in the
# salt-master config
cp.push:
  module.run:
    - path: /tmp/ghost

Then, run this sls and copy the file out of the cache directory (see cp.push documentation)

# salt <32bitminion> state.sls ghostbuild
# cp /var/cache/salt/master/minions/<32bitminion> /tmp/ghost \
     /srv/salt/ghost32

(replace 32bitminion with the minion_id where you did the build)

Now change your ghostcheck.sls to look like this

/tmp/ghost:
  file.managed:
{% if grains['osarch'] == 'i386' %}
  — source: salt://ghost32
{% else %}
  — source: salt://ghost
{% endif %}
  — owner: root
  — mode: '0700'

runghost:
  cmd.run:
    — name: /tmp/ghost
    — cwd: /tmp
    — user: root
    — require:
      — file: /tmp/ghost

Now I get accurate results from all my minions, 32-bit or 64-bit.

Obviously the simpler way to do this would be to build and run ghost.c on all minions, but many folks don’t keep gcc and friends on things like webservers.

Finally, if you don’t want to reboot all your machines, you just want to restart affected services, you can do the following (props to the hackernews discussion for this snippet)

salt \* cmd.run 'netstat -lnp | grep -e "\(tcp.*LISTEN\|udp\)" | cut -d / -f 2- | sort -u'

which will tell you which services on which machines need to be restarted. Then for each of these services and machines you can say

salt <affectedminion> service.restart <affectedservice>

Finally, shameless plug for the awesome company I work for—if you want to learn more about Salt, SaltConf would be a great place to do it! March 3–5, 2015, Grand America Hotel, Salt Lake City.

tech

Installing macOS X 10.9.2 with Salt

Several weeks ago I installed Salt on all my Macs. I have 7 currently, two of which cannot run Mavericks and are stuck at Lion (10.7). I know you can configure them to install updates automatically, but a couple of these are development machines and one is a server, and I just don’t like the idea of having them install updates and reboot whenever they feel like it.

Furthermore, the 10.9.2 release contains an important fix—the so called ‘gotofail’ security vulnerablity, fully documented here: https://www.imperialviolet.org/2014/02/22/applebug.html. You can check to see if you are vulnerable with http://gotofail.com.

I was dreading manually going to each of these machines and running Software Update, waiting for it to figure out if there were really packages to install (why does that take so long, anyway?), and doing the click dance to get it installed.

Enter Salt.

(full disclaimer—I do work for SaltStack, the company behind open source Salt)

Using Salt turned probably an hour of updating into 3 commands executed at my leisure. Note, I run my salt-master on Ubuntu in a Fusion VM on my Mac Mini server. After downloading the combo updater from Apple’s support site, I mounted it and extracted the .pkg file from it, then copied that file to my Salt master’s /srv directory (/srv/salt/OSXUpd10.9.2.pkg).

Then:

salt-master# salt -C 'G@os:MacOS and G@osrelease:10.9.1' cp.get_file \ OSXUpd10.9.2.pkg /tmp/OSXUpd10.9.2.pkg salt-master# salt -C 'G@os:MacOS and G@osrelease:10.9.1' cmd.run \ 'installer -pkg /tmp/OSXUpd10.9.2.pkg -target /' salt-master# salt -C 'G@os:MacOS and G@osrelease:10.9.1' cmd.run \ 'shutdown -r now'

So what the above says is

  1. For all MacOS machines that are on 10.9.1, copy the package file to the /tmp directory on the machine (thus avoiding my Lion machines). The -C says this is a compound target, and the command will match against both the os grain (to be “MacOS”) and the osrelease grain (to be “10.9.1”).
  2. For those same machines, run Apple’s package utility in unattended mode on the package file, and install that to the boot volume.
  3. Finally, reboot the machine.

The response I got back was identical for each machine, and looks like

mini-server:
   installer: Package name is OS X Update
   installer: Installing at base path /
   installer: The install was successful.
   installer: The install requires restarting now.

So, did it work? After waiting for the machines to come back up (use salt-run manage.status on the Salt master to see when they are all online again), the following will show the OS release number for all my Macs.

salt-master# salt -C 'G@os:MacOS' grains.item osrelease

mini-server:
   osrelease:
     10.9.2
imac-01:
  osrelease:
     10.9.2
air-01:
  osrelease:
     10.9.2
mini-01:
  osrelease:
     10.7.5
macbookpro-01:
  osrelease:
     10.9.2
macbookpro-02:
  osrelease:
     10.9.2
white-macbook:
  osrelease:
     10.7.5

(Just to be clear, names sanitized)

Voila!

tech

Removing WireLurker with Salt

Claud Xiao from Palo Alto Networks has been in touch with me and I updated this script with his recommendations.

Please note I don’t plan to add Windows support, the anti-malware vendors do a great job maintaining signatures and removing stuff like this.

The news hit the fan early yesterday morning—lots of Apple haters were giddy with excitement at the revelation of the WireLurker trojan that infects iOS devices via their host Macintosh when the devices are plugged in via USB.

Publicized by Palo Alto Networks, details on WireLurker can be found at their website. Helpfully, Palo Alto also published a Python script that can detect the infection. Removing the infection from an iOS device is a matter of backing up the device, erasing it completely by restoring it to factory defaults, and then restoring the backup. Props to Topher Kessler of MacIssues for documenting this process.

I took Palo Alto’s script and modified it so it can either be run from the command line or as a Salt execution module. From the command line:

python wireunlurk.py

will scan your Mac for signs of WireLurker. -h for help (not much there) or -c for “clean”.

wireunlurk.py will move any infected files to a dynamically-created directory in /tmp that starts with wireunlurk_bk.

If you want to run this in your Salt infrastructure, put wireunlurk.py in /srv/salt/_modules (or equivalent directory if you have customized it) and run the following on your Salt master:

salt -G 'os:MacOS' saltutil.sync_modules salt -G'os:MacOS' wireunlurk.scan

Add clean=True if you want to clean up the infection as well.

This saved me a significant amount of time scanning my Macs just at home—we have 7 Macs on my home network and rather than ssh’ing to each one, or using a tool like csshX, as soon as I got the script running and ‘saltified’ I executed the above command and could sleep with peace of mind knowing none of our devices were infected.

You can find my modified script here: https://github.com/saltstack/salt-contrib/tree/master/modules/wireunlurk