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Better Secure Remount #152

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Better Secure Remount #152

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c019c2f
remount-secure.service
monsieuremre Nov 4, 2023
85c450f
secure-remount
monsieuremre Nov 4, 2023
3763235
generate remount config on install
monsieuremre Nov 4, 2023
fa61337
generate-secure-remount-config
monsieuremre Nov 4, 2023
8ce90bc
/boot/efi super duper hardening and file system correction
monsieuremre Nov 4, 2023
3284b5a
before
monsieuremre Nov 4, 2023
c42e6b2
secure-remount
monsieuremre Nov 4, 2023
c0a5fab
don't force ext4
monsieuremre Nov 4, 2023
9959688
config isnt executable
monsieuremre Nov 4, 2023
7a5e745
Merge branch 'master' into better-secure-remount
adrelanos Nov 6, 2023
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2 changes: 2 additions & 0 deletions debian/security-misc.postinst
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Expand Up @@ -61,6 +61,8 @@ pam-auth-update --package
/usr/libexec/security-misc/permission-lockdown
permission_hardening

/usr/libexec/security-misc/generate-secure-remount-config
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Why (only) at package install time and not a boot time?

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Because it is not expected for the partitioning scheme of a system to change. This would normally only happen with a fresh install or a reinstall. And that would guarantee the package being reinstalled.

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A system administrator might change /etc/fstab at any time.

For example, Qubes has a package that ships /etc/fstab and there have been updated to it.

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Doing this at boot time as discussed. (But does not hurt also doing this at package install time - makes this easier to debug. But then the systemd unit will take care of it anyway.)


## https://phabricator.whonix.org/T377
## Debian has no update-grub trigger yet:
## https://bugs.debian.org/481542
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12 changes: 12 additions & 0 deletions lib/systemd/system/remount-secure.service
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@@ -0,0 +1,12 @@
[Unit]
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Description=Remount partitions with hardened mount options.
After=sysinit.target
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Something similar to this also applies:
https://github.com/Kicksecure/security-misc/pull/148/files#r1380569447

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To do this as early as possible (for better security, avoid race conditions) something like this would be desirable:

Before=sysinit.target
After=?

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Ideally something like...

[Unit]
After=sysinit.target
Before=basic.target

[Install]
WantedBy=basic.target

But this isn't possible.

basic.target: Found ordering cycle on testcmd.service/start

Looking for a solution.

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There is a lot important stuff happening during sysinit.target. To see:
systemctl list-dependencies sysinit.target

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Hmm. I can try, but might not work. I don't know when we do all the mounting the first time. We don't want to be before that.

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Ok so those won't work. I still took it a little earlier. We do the remounting before networking.service and dbus.service. Before those started, malware doesn't have much of a chance probably? I think. We can identify more 'sensitive' services like these to put them in the before condition.

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This is the crux. There was "no answer" to my question how to have a systemd unit do the mount options hardening without race conditions:

But maybe a new systemd target could be invented one that runs after sysinit.target but before basic.target. systemd is probably flexible enough to support such a use case. The question is how.

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This is the critical point. Can it be done without race conditions (which broke the very old implementation and made me even port to dracut).

## so that these services don't fail to start
Before=systemd-logind.service power-profiles-daemon.service switcheroo-control.service networking.service dbus.service

[Service]
Type=oneshot
ExecStart=/usr/libexec/security-misc/secure-remount

[Install]
WantedBy=sysinit.target
66 changes: 66 additions & 0 deletions usr/libexec/security-misc/generate-secure-remount-config
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#!/bin/bash

config_file="/usr/lib/security-misc_secure-remount.conf"

if [ -f $config_file ]
then
rm $config_file
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Could also just unconditionally delete it using rm -f?

fi
touch $config_file

echo "/ / none defaults,remount 0 2" >> $config_file
echo "/dev/shm /dev/shm none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file
echo "/dev /dev none defaults,nosuid,noexec,remount 0 2" >> $config_file
echo "/run /run none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file

if grep --quiet " /boot " /etc/fstab
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then
echo "/boot /boot none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file
else
echo "/boot /boot none defaults,nosuid,nodev,noexec,bind 0 2" >> $config_file
fi

if grep --quiet " /boot/efi " /etc/fstab
then
echo "/boot/efi /boot/efi vfat defaults,nosuid,nodev,noexec,umask=0077,remount 0 2" >> $config_file
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Could be avoided by doing the same for /boot which then would cover /boot/efi as well?

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On efi systems /boot/efi is automatically a seperate partition than /boot. This is the debian default installer behavior, and is also recommended when installing manually, for backwards compatibility with legacy boot. So /boot/efi is mounted seperately, on a different literal partition. That's why it has to be seperately remounted. If the system isn't uefi, we don't do the remounting.

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Actually correction, it is not only recommended, it is mandatory that /boot/efi is a seperate partition. Because the file system has to be vfat apparently. So yes, mounting /boot won't cover /boot/efi in any case if it exists.

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I hope we won't run into the "double mount" issues we had before.

fi

if grep --quiet " /home " /etc/fstab
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then
echo "/home /home none defaults,nosuid,nodev,remount 0 2" >> $config_file
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defaults could undo what the user customized in /etc/fstab?

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If that's the case, we can easily get rid of the 'defaults' for remounts and just stick to it when doing 'bind'. I think that might even be better regardless. Will push with changes.

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Yes.

else
echo "/home /home none defaults,nosuid,nodev,bind 0 2" >> $config_file
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Does this undo potential user customizations in /etc/fstab such as noatime?

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Will test and see.

fi

if grep --quiet " /tmp " /etc/fstab
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then
echo "/tmp /tmp none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file
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Also stuff at the end 0 2 shouldn't be unnecessarily touched.

else
echo "tmpfs /tmp tmpfs defaults,nosuid,nodev,noexec 0 0" >> $config_file
fi

if grep --quiet " /var/log/audit " /etc/fstab
then
echo "/var/log/audit /log/audit none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file
fi

if grep --quiet " /var/tmp " /etc/fstab
then
echo "/var/tmp /var/tmp none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file
else
echo "/tmp /var/tmp none defaults,nosuid,nodev,noexec,bind 0 0" >> $config_file
fi

if grep --quiet " /var/log " /etc/fstab
then
echo "/var/log /var/log none defaults,nosuid,nodev,noexec,remount 0 2" >> $config_file
else
echo "/var/log /var/log none defaults,nosuid,nodev,noexec,bind 0 2" >> $config_file
fi

if grep --quiet " /var " /etc/fstab
then
echo "/var /var none defaults,nosuid,nodev,remount 0 2" >> $config_file
else
echo "/var /var none defaults,nosuid,nodev,bind 0 2" >> $config_file
fi
7 changes: 7 additions & 0 deletions usr/libexec/security-misc/secure-remount
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@@ -0,0 +1,7 @@
#!/bin/bash

config_file="/usr/lib/security-misc_secure-remount.conf"

mount --fstab $config_file --all
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echo "Partitions securely remounted by security-misc"