(lets forget about X11 breaking all this assumptions
Indeed.
While experimenting with module loading disabling, I experienced that broken X can block switching to virtual console. Needless to say (for other readers), if X can do, also malware could do. “SysRq + r” can take away control from X. After that, switching to another virtual console was possible.
Yes, X (or other process with access to input device) can grab it for exclusive access, disabling Alt+Ctrl+F1 or similar combos. This still is independent of what is happening on other terminals. Especially, input devices grabbed in this mode are handled by X server (or other process that grabbed them). As long as X server doesn’t have access to other terminals, it still can’t influence them.
Quote https://www.kernel.org/doc/Documentation/SAK.txt
=========================================
Linux Secure Attention Key (SAK) handling
=========================================
:Date: 18 March 2001
:Author: Andrew Morton
An operating system's Secure Attention Key is a security tool which is
provided as protection against trojan password capturing programs. It
is an undefeatable way of killing all programs which could be
masquerading as login applications. Users need to be taught to enter
this key sequence before they log in to the system.
From the PC keyboard, Linux has two similar but different ways of
providing SAK. One is the ALT-SYSRQ-K sequence. You shouldn't use
this sequence. It is only available if the kernel was compiled with
sysrq support.
The proper way of generating a SAK is to define the key sequence using
``loadkeys``. This will work whether or not sysrq support is compiled
into the kernel.
SAK works correctly when the keyboard is in raw mode. This means that
once defined, SAK will kill a running X server. If the system is in
run level 5, the X server will restart. This is what you want to
happen.
What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
the machine. CTRL-ALT-BACKSPACE is magical to the X server. We'll
choose CTRL-ALT-PAUSE.
In your rc.sysinit (or rc.local) file, add the command::
echo "control alt keycode 101 = SAK" | /bin/loadkeys
And that's it! Only the superuser may reprogram the SAK key.
.. note::
1. Linux SAK is said to be not a "true SAK" as is required by
systems which implement C2 level security. This author does not
know why.
2. On the PC keyboard, SAK kills all applications which have
/dev/console opened.
Unfortunately this includes a number of things which you don't
actually want killed. This is because these applications are
incorrectly holding /dev/console open. Be sure to complain to your
Linux distributor about this!
You can identify processes which will be killed by SAK with the
command::
# ls -l /proc/[0-9]*/fd/* | grep console
l-wx------ 1 root root 64 Mar 18 00:46 /proc/579/fd/0 -> /dev/console
Then::
# ps aux|grep 579
root 579 0.0 0.1 1088 436 ? S 00:43 0:00 gpm -t ps/2
So ``gpm`` will be killed by SAK. This is a bug in gpm. It should
be closing standard input. You can work around this by finding the
initscript which launches gpm and changing it thusly:
Old::
daemon gpm
New::
daemon gpm < /dev/null
Vixie cron also seems to have this problem, and needs the same treatment.
Also, one prominent Linux distribution has the following three
lines in its rc.sysinit and rc scripts::
exec 3<&0
exec 4>&1
exec 5>&2
These commands cause **all** daemons which are launched by the
initscripts to have file descriptors 3, 4 and 5 attached to
/dev/console. So SAK kills them all. A workaround is to simply
delete these lines, but this may cause system management
applications to malfunction - test everything well.
^^ Pretty badass.
Quote Screen Locker (In)Security - Can we disable these at least 4 backdoors?
What does 176 value for /proc/sys/kernel/sysrq mean? Anyone can find documentation for that?
Why use wayback?
The docs are still available. They’re just in the admin-guide directory.
https://www.kernel.org/doc/Documentation/admin-guide/sysrq.rst
It allows only rebooting, syncing and remounting all drives read-only with the sysrq key. Sysrq uses a bitmask to control which features to allow.
According to the kernel docs, 16 is sync, 32 is remount read-only and 128 is reboot/poweroff. 16 + 32 + 128 = 176
Against link rot and couldn’t find the current version.
4 posts were split to a new topic: Send SysRq commands to VirtualBox usability helper - virtualbox-send-sysrq
Now that untrusted root is in our threat model, we might want to look at disabling SysRq again as abusing it can be done remotely by root (/proc/sysrq-trigger or /dev/uinput).
CLIPOS also recommends to disable this.
Would it make sense to add this to apparmor-profile-everything? Since only apparmor-profile-everything implements untrusted root? security-misc alone doesn’t implement untrusted root?
Can’t find. Any reference?
apparmor-profile-everything doesn’t really have anything to do with sysrq or other kernel hardening.
I don’t think we should make this related to apparmor.
It does a bit.
It disables kexec.
https://github.com/Whonix/security-misc/blob/master/etc/sysctl.d/kexec.conf
Hides kernel symbols from root.
https://github.com/Whonix/security-misc/blob/master/etc/sysctl.d/kptr_restrict.conf
Prevents root processes from creating coredumps.
https://github.com/Whonix/security-misc/blob/master/etc/sysctl.d/suid_dumpable.conf
Hardens the BPF JIT compiler for root (that’s what bpf_jit_harden=2 does).
https://github.com/Whonix/security-misc/blob/master/etc/sysctl.d/harden_bpf.conf
Without apparmor-profile-everything, a malicious root can just easyly enable sysrq as per instructions. There for users who only use security-misc don’t benefit from this. Only hinders debugging for these. Therefore I think this fits better into apparmor-profile-everything. Only with apparmor-profile-everything we can have a complete untrusted root implementation.
Not necessarily. An attacker running as root without read-write access to /proc/sys/kernel/sysrq (as is the case for many systemd services due to ProtectKernelTunables for example) can still be limited by this.
madaidan via Whonix Forum:
Not necessarily. An attacker running as root without read-write access to /proc/sys/kernel/sysrq (as is the case for many systemd services due to
ProtectKernelTunablesfor example) can still be limited by this.
In that situation could a compromised process simulate pressing sysrq key?
Yes, should be possible via /dev/uinput.
Alright. Then unfortunately sysrq restriction is required.
Should CONFIG_MAGIC_SYSRQ be disabled in hardened-kernel to get rid of it entirely or should we just whitelist a few functions like SAK?
The o command can be used to quickly shutdown if in an emergency but this makes it easier for DoS and is much better done on the host.
The f command can be used to kill a process taking up too much memory which might be useful to save the system from crashing.