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SUID Disabler and Permission Hardener

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Really bad bug disabling sudo. Unexplained. Non-reproducible. Not a full disk issue, plenty of free space.

Dec 29 04:17:11 debian-buster-test permission-hardening[413]: INFO: START parsing config_file: ‘/etc/permission-hardening.d/30_default.conf’
Dec 29 04:17:12 debian-buster-test permission-hardening[413]: /usr/lib/security-misc/permission-hardening: line 255: cannot create temp file for here-document: No such file or directory
Dec 29 04:17:12 debian-buster-test permission-hardening[413]: ERROR: cannot parse line: /usr/bin/sudo exactwhitelist

Probably better to abort processing the config file and immediately exit with error when that happens. Will implement that.

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Did that. And probably found the cause:

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Found bug.

/usr/bin/torbrowser: line 252: /bin/mount: Permission denied
/usr/bin/torbrowser: line 270: /bin/mount: Permission denied

/usr/bin/torbrowser (by Whonix developers) uses mount to check if Tor Browser folder is mounted noexec.

/bin/mount is SUID by Debian default.

Therefore /bin/mount makes sense as executable but with SUID removed.

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Fixed.

Not an ideal solution. First add /bin/mount to exactwhitelist and then below set /bin/mount 745 root root. A unintuitive config. But good enough for now?

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Anyone got time to read?

Needs a call for testers.

Polishing a few things just now.

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I think there are two security issues with SUID.

  • A) As I’ve added to the wiki just now Some SUID binaries have a history of privilege escalation security vulnerabilities..
  • B) General attack surface such as kernel attack surface.

What SUID Disabler and Permission Hardener is currently doing is disable as many SUID binaries as reasonable without breaking a Linux desktop operating system. Improving the situation for A)

To however have the full benefit, to do B) we would have to eliminate all SUID binaries. This might be reasonable and doable for CLI environments such as servers ( also think Kicksecure).

What do you think?

If you agree, I guess the configuration file of SUID Disabler and Permission Hardener should be split. The whitelist should be in a separate file. Then a system administrator could easily nuke the whitelist. Alternatively or additional perhaps a ignore_whitelist=true configuration option would be useful? Then we could document this and some users could benefit from a completely SUID free system.

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Improved documentation:

Moved to dedicated wiki page:

Please review.

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More documentation enhancements:
https://www.whonix.org/w/index.php?title=SUID_Disabler_and_Permission_Hardener&type=revision&diff=61041&oldid=61017

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Should we add

  • /opt/ nosuid
  • /usr/local/opt nosuid

?

Rationale for /opt is that some manually installed software installs itself to /opt. Some lesser important functionality might require suid or sgid. The suid/sgid bit might have been accidentally set by a developer. (Or part of legacy install scripts. Useful in past, then forgotten, now obsolete.) Removal of suid / sgid might in many cases go unnoticed by the user. (In cases where that software is run as root anyhow.)

On the other hand, the /opt folder is empty on a default Debian (based) installation. One could argue if the (super) admin installs files there it should be honored by the system and kept unobstructed.

I guess it depends if (re-)mount home [and other?] with noexec (and nosuid [among other useful mount options]) for better security? would re-mount /opt with nosuid anyhow?


Other folders where suid binaries might end up?

  • /home?

But adding that I guess would be excessive because:

Also /root? Depends on outcome of this post Disable SUID Binaries - A) vs B).

Others:

  • /mnt?
  • /media?
  • Any others?
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2 Likes

It can be done for GUI environments also. Nothing really requires setuid. We can replace them with capabilities.

The admin could whitelist their binaries.

Unlikely. We could cover /root since that’s a bit more likely to contain suid binaries and it wouldn’t increase scan time much since most users would be storing their files in an unprivileged user’s home directory.

That would break a lot of things. For example, if I mounted a drive containing another Linux system to /mnt in order to debug an issue, permission hardener would kill all suid binaries in it and become extremely slow. A better solution would be mounting those filesystems with the nosuid option since it’s much easier to revert (mount -o remount,suid /mnt vs. resetting all file permissions).

2 Likes

Nice changes by @madaidan:
https://www.whonix.org/w/index.php?title=SUID_Disabler_and_Permission_Hardener&type=revision&diff=61183&oldid=61082

Inspired me to improve a bit more:
https://www.whonix.org/w/index.php?title=SUID_Disabler_and_Permission_Hardener&action=historysubmit&type=revision&diff=61190&oldid=61183



Great idea!

dpkg-statoverride does not support that yet. feature request: dpkg-statoverride: support for capabilities

Meaning there might be no standard way in which packages implement setting capabilities yet. Therefore I wanted to look at a package that sets capabilities. For example package iputils-ping contains capability enabled binary ping. From the package postinst script:

/var/lib/dpkg/info/iputils-ping.postinst

#!/bin/sh

set -e

if [ "$1" = configure ]; then
    # If we have setcap is installed, try setting cap_net_raw+ep,
    # which allows us to install our binaries without the setuid
    # bit.
    if command -v setcap > /dev/null; then
        if setcap cap_net_raw+ep /bin/ping; then
            chmod u-s /bin/ping
        else
            echo "Setcap failed on /bin/ping, falling back to setuid" >&2
            chmod u+s /bin/ping
        fi
    else
        echo "Setcap is not installed, falling back to setuid" >&2
        chmod u+s /bin/ping
    fi
fi



exit 0

# Local variables:
# mode: shell-script
# tab-width: 4
# indent-tabs-mode: nil
# end:

dpkg -S /sbin/setcap

libcap2-bin: /sbin/setcap

cat debian/control | grep cap

Recommends: libcap2-bin

So if libcap2-bin does not get installed early enough (before iputils-ping) is installed, the Debian maintainer script will set SUID instead of using capabilities. Assuming that other packages might be implemented in a similar way, we should find a way to make sure package libcap2-bin is installed during the build process as early as possible.

Other random examples having a similar use of setcap:

grep -rl setcap /var/lib/dpkg/info

/var/lib/dpkg/info/iproute2.config
/var/lib/dpkg/info/iproute2.postinst
/var/lib/dpkg/info/libcap2-bin.md5sums
/var/lib/dpkg/info/libcap2-bin.list
/var/lib/dpkg/info/libgstreamer1.0-0:amd64.postinst
/var/lib/dpkg/info/gnome-keyring.postinst
/var/lib/dpkg/info/wireshark-common.postinst
/var/lib/dpkg/info/wireshark-common.templates
/var/lib/dpkg/info/iproute2.templates
/var/lib/dpkg/info/kinit.postinst
/var/lib/dpkg/info/iputils-ping.postinst

Meanwhile for packages by Whonix or Kicksecure we can just add Depends: libcap2-bin in debian/control and run setcap during postinst.

Happy to work towards a completely SUID / SGID free by default system. Worthwhile development goal to even get rid of Whonix / Kicksecure requiring sudo / any /etc/sudoers.d exceptions?

Would be specifically interesting in context of:

Now this list of sudoers exceptions needs to be ported to capabilities.

./sdwdate/etc/sudoers.d/sdwdate
./dist-base-files/etc/sudoers.d/30_default-password-lecture
./anon-gw-anonymizer-config/etc/sudoers.d/anonymizer-config-gateway
./tor-control-panel/etc/sudoers.d/tor-control-panel
./tor-control-panel/etc/sudoers.d/restart-tor-gui
./uwt/etc/sudoers.d/uwt
./security-misc/etc/sudoers.d/xfce-security-misc
./security-misc/etc/sudoers.d/security-misc
./security-misc/etc/sudoers.d/pkexec-security-misc
./tb-updater/etc/sudoers.d/tpo-downloader
./whonix-setup-wizard/etc/sudoers.d/whonix-setup-wizard
./usability-misc/etc/sudoers.d/tunnel_unpriv
./usability-misc/etc/sudoers.d/sudo-lecture-disable
./usability-misc/etc/sudoers.d/pwfeedback
./usability-misc/etc/sudoers.d/user-passwordless
./usability-misc/etc/sudoers.d/upgrade-passwordless
./live-config-dist/etc/sudoers.d/live-config-dist
./qubes-whonix/etc/sudoers.d/qubes-whonix
./whonixsetup/etc/sudoers.d/whonixsetup
./sdwdate-gui/etc/sudoers.d/sdwdate-gui
./whonix-xfce-desktop-config/etc/sudoers.d/whonix-xfce-desktop-config
./vm-config-dist/etc/sudoers.d/power-savings-disable-in-vms
./whonixcheck/etc/sudoers.d/whonixcheck
./msgcollector/etc/sudoers.d/msgcollector
./tb-starter/etc/sudoers.d/tb-starter

Help welcome!

Would we need a wrapper around setcap? Because https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=502580 mentions several conditions under which setcap can fail.

  • Take into account systems no supporting fcaps, this includes:
    • (for now) non-Linux systems,
    • Linux w/ old kernels or kernels w/o fcap compiled in,
    • file systems not supporting fcaps, and/or w/o mount time enabled
      xattr.

Or don’t care much about this and just ignore errors using setcap [...] || true?

Added to wiki:

  • It does not search folders /root because no SUID binaries should be there by default. That folder is by default readable only by root. If root was to create a custom SUID and move it there, then root should be able to execute it.

Good point. Now mentioned in wiki.

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Permission Hardener does already support setting capabilities: https://github.com/Whonix/security-misc/blob/master/usr/lib/security-misc/permission-hardening#L458

This is used by anon-apps-config to remove capabilities from ping: https://github.com/Whonix/anon-apps-config/blob/master/etc/permission-hardening.d/30_ping.conf

The lack of dpkg-statoverride support is an issue however. Maybe we can implement something ourselves via an apt / dpkg hook? I.e. on package upgrade, it checks /etc/permission-hardening.d/ for any needed capabilities and if so, sets them.

Capabilities don’t have the same configurability as sudoers exceptions. We cannot restrict which user can execute the program with higher privileges or what arguments they can use.

It may even be better to replace capabilities or setuid with sudoers exceptions as we can restrict execution of those binaries to specific users. For example, if /bin/example is setuid, it can be executed by any user and potentially exploited whereas if a sudoers exception was made, it will only be able to be executed by user user, preventing a compromised sdwdate user from exploiting that program.

Those conditions seem rather niche so yes.

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We should add support for basic bash/AppArmor-style regex so we can, for example, shorten:

/bin/ nosuid
/usr/local/bin/ nosuid
/usr/bin/ nosuid
/usr/local/usr/bin/ nosuid
/sbin/ nosuid
/usr/local/sbin/ nosuid
/usr/sbin/ nosuid
/usr/local/usr/sbin/ nosuid
/lib/ nosuid
/usr/local/lib/ nosuid
/lib32/ nosuid
/usr/local/lib32/ nosuid
/lib64/ nosuid
/usr/local/lib64/ nosuid
/usr/lib/ nosuid
/usr/local/usr/lib/ nosuid
/usr/lib32/ nosuid
/usr/local/usr/lib32/ nosuid
/usr/lib64/ nosuid
/usr/local/usr/lib64/ nosuid

to:

/{,usr/,usr/local/}{,s}bin/ nosuid
/{,usr/,usr/local/}lib{,32,64}/ nosuid

like in apparmor-profile-everything.

Also, do directories like /usr/local/usr/bin/ even exist?

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Issue indeed… The essential issue is this…
(You probably know but I am writing this down as reminder to self and everyone else reading.)

sudo getcap /bin/ping

(Capabilities of /bin/ping already removed.)

sudo apt install --reinstall iputils-ping 
sudo getcap /bin/ping

/bin/ping = cap_net_raw+ep

Even if we could, there would a race condition. A time window of vulnerability. Malware with the ability to abuse a capability could use inotifywait (or some other mechanism (perhaps brute force trying)) to wait until the binary is updated and the capability reinstated.

Perhaps once [1] https://www.whonix.org/wiki/Dev/boot_modes existed, malware lurking under user user should be deactivated once rebooted into admin mode. Then malware running under user user couldn’t exploit the vulnerable time window of the capability briefly being re-introduced. If we go for that, we should probably remove upgrade-nonroot command. It would still be an incomplete solution until… Even if rebooted into admin mode, there might be daemons running under for example user www-data. Once [1] is implemented, probbly when booting into admin mode only a limited amount of systemd services should be executed.

Nothing really is as great as a proper implementation of dpkg-statoverride. In case of permissions, dpkg never changes back to the different/original/weaker permissions (won’t re-enable SUID even briefly).

Which kind of configurablity do we need? Can we re-implement these?

Can the output of whoami or some alternative command be trusted? Or can that be fooled with some LD_PRELOAD trick or something?

Let’s pick livecheck.sh [archive] as an example. The script currently uses sudo --non-interactive /bin/lsblk --noheadings --all --raw --output RO. This could probably be translated to some capability.

  1. You’re right. We don’t want all users to have that capability. If output of whoami or similar could be trusted, the script itself could check if it’s running under user user and exit if not so?
  2. The capability might give access to a lot more than sudo --non-interactive /bin/lsblk --noheadings --all --raw --output RO.

Am I re-inventing SUID or sudo here?

My conclusion for now: An SUID free desktop system (including free of sudo) is impractical.

Now on a second thought… Perhaps livecheck.sh could be implemented in a more sophisticated way. A systemd unit running as root [2] could run sblk --noheadings --all --raw --output RO, then write the output to a world readable file. [3] livecheck.sh could then read from that file instead of running a command with SUID (meaning sudo).

Perhaps similar solutions could be invented for other cases currently dependent on a /etc/sudoers.d exception.

Seems difficult. I would not know how to implement this without over complicating the code. I don’t think many people will read that config, let alone edit it or view the logs.

Currently config doesn’t even support white spaces in folder names. (Untested.) Such as basic feature should be implemented before going fancy with bash/AppArmor-style regex?

Config parsing, the script looks complex enough already for my taste.

/usr/local seems to hold anything that also the root / could hold. I found mentions of all newly added entries on Google when putting the serach term into quotes such as.

“/usr/local/usr/bin/”

Also for consistency, thought good to add. At worst in costs a second or so and a log entry.


[2] Or limited user with sudoers exception or capability? Too complex, fancy?
[3] Or only user user? Too complex, fancy?

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For example, https://github.com/Whonix/whonixcheck/blob/master/etc/sudoers.d/whonixcheck restricts a lot of commands to the whonixcheck user or specific arguments. When switching to capabilities, we can’t really do this.

But we cannot add whoami checks to every program. For example, if instead of the following sudoers exception:

whonixcheck ALL=NOPASSWD: /bin/dmesg

, we added the CAP_SYSLOG capability to the /bin/dmesg binary, any user would now be able to execute /bin/dmesg with that capability. Only by shipping our own /bin/dmesg program would we be able to implement user checks which is infeasible when considering the large amount of programs we’d need to do this for.

Also, LD_PRELOAD wouldn’t work on binaries with capabilities. It’s sanitized to prevent trivial privilege escalation, similar to setuid binaries.

Bash supports this type of regex already.

echo /{,usr/,usr/local/}{,s}bin/

/bin/ /sbin/ /usr/bin/ /usr/sbin/ /usr/local/bin/ /usr/local/sbin/

All we’d need to do on our end is to not handle the strings literally and some basic sanitizing.

This feature wouldn’t only be useful for readability purposes. Permission Hardener currently restricts the permissions of /home/user/:

/home/user/ 0700 user user

But this misses any other user’s home directory whereas if it supported regex, we could change this to:

/home/*/ 0700
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Why not? Not every program indeed. Just to every script by Whonix currently using sudo. I guess Whonix using /etc/sudoers.d is rather uncommon anyhow, and should be avoided? Not a big deal now. But also not great? Uncommon…

dpkg -S /etc/sudoers.d/*

msgcollector: /etc/sudoers.d/msgcollector
security-misc: /etc/sudoers.d/pkexec-security-misc
usability-misc: /etc/sudoers.d/pwfeedback
qubes-core-agent: /etc/sudoers.d/qt_x11_no_mitshm
qubes-core-agent-passwordless-root: /etc/sudoers.d/qubes
qubes-input-proxy-sender: /etc/sudoers.d/qubes-input-trigger
sudo: /etc/sudoers.d/README
security-misc: /etc/sudoers.d/security-misc
usability-misc: /etc/sudoers.d/sudo-lecture-disable
tb-starter: /etc/sudoers.d/tb-starter
tb-updater: /etc/sudoers.d/tpo-downloader
usability-misc: /etc/sudoers.d/tunnel_unpriv
qubes-core-agent: /etc/sudoers.d/umask
usability-misc: /etc/sudoers.d/upgrade-passwordless
usability-misc: /etc/sudoers.d/user-passwordless
security-misc: /etc/sudoers.d/xfce-security-misc

On my system only Whonix and Qubes dropping files into /etc/sudoers.d.

Could be a configurable wrapper somewhere in /etc, perhaps even .d, to allow extending the list of permitted users per program. A script function to be sourceed so it doesn’t have to be re-invented for every script.

Yes, we certainly shouldn’t add capabilities to /bin/dmesg. What I had in mind…

That’s what I had in mind. Creating wrapper scripts for that. The wrapper script would:

  • check whoami (based on sourceed shell function and config folder)
  • have required capability
  • run command with all parameters (example: /bin/journalctl --boot --no-pager -u whonix-firewall)

Simplified example:

#!/bin/bash

if [ ! “$(whoami)” = “user” ]; then
exit 1
fi

/bin/journalctl --boot --no-pager -u whonix-firewall

In result:

  • Only the whitelisted user(s) would be permitted to run it.
  • Using capabilities.
  • Still only a very specific command.

There would be the risk of malicious input in the script being able to run other commands with the given capability but since these wrapper scripts should be similarly simple as above and not parse any command line parameters I think that risk is worth the gain. In other words, I think it’s very doable to get these scripts right and benefit from a fully SUID free system that doesn’t depend on sudo.

I don’t think we have such a massive amount of /etc/sudoers.d reliance that porting to capability enabled wrapper scripts is infeasible?

Nice, that’s something.

What do you think?

But then my design proposal might fail because capabilities cannot be set on scripts. Only on programs. As per:

Any solution for that? We’d have to invent C(++) based wrappers? Perhaps a thin, perhaps even generic wrapper that sets the capability and then runs the helper script?

I guess the config parser could run the function doing the actual work (which runs find) several times per config entry. But I don’t see much benefit. I don’t foresee a lot users or even packages dropping config snippets there, changing expanding the default configuration. This is not going to be the new /etc/apparmor.d with lots of contributions which really deserves a sophisticated configuration language.

vs

Second one lacks user names.

How to auto detect the user name then? If we want to harden permissions for different user names in /home, wouldn’t it be better to have a special function in the script? In that case would be easier to have a function (feature) parsing /home rather than inventing a complex config parsing just for /home folder permission hardening.

(That feature would then be enabled in config using harden_home_folder_permissions=true or so. Or default buit-in enabled and disabled with harden_home_folder_permissions=false.)

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This may be a good idea then.

Wrappers in C should be easy to implement.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pwd.h>
#include <string.h>

int main() {
  const char *permitted_user = "user";
  struct passwd *current_user = getpwuid(geteuid());

  if (strcmp(current_user->pw_name, permitted_user) == 0) {
    system("/bin/journalctl --boot --no-pager -u whonix-firewall"); 
    exit(0);
  }

  puts("ERROR: You are not the correct user.");
  exit(1);
}

stat -c "%U %G"

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