On 2018-02-14 08:21, Benjamin Drung wrote:
Am Mittwoch, den 14.02.2018, 13:09 +0000 schrieb Ard Biesheuvel:
On 14 February 2018 at 12:52, Benjamin Drung
<benjamin.dr...@profitbricks.com> wrote:
Hi,

I am exploring the possibility to store SSH and other keys in UEFI
variables for systems that do not have persistent storage. These
systems boot via network and need individual SSH keys which ideally
should not be distributed via network.

The plan is to write a small daemon that starts at boot and gets
the
SSH keys from EFI variables to individualize the system with SSH
keys.
I plan to release the code as free software. Simple proof-of-
concept
code:

mount -t efivarfs none /sys/firmware/efi/efivars
for key in ssh_host_dsa_key ssh_host_ecdsa_key ssh_host_rsa_key; do
   dd ibs=1 skip=4 if=/sys/firmware/efi/efivars/${key}-89df11f4-
38e6-473e-ab43-b4406b76fba9 of=/etc/ssh/$key
done

I am not the first person having the idea to use UEFI variables to
store keys:
https://www.usenix.org/conference/srecon17asia/program/presentation
/korgachin

There is one problem: The keys should be readable only by root.
When
mounting efivarfs, all variables have the permission 644 which
makes
them readable by all users. I have different ideas how to solve it:

1) Hard-code a list of GUIDs that should be only readable by root
in
the kernel module. These modules would also be not set to
immutable.

2) Instead of hard-coding GUIDs, add a kernel module parameter to
specify the GUIDs. Maybe have a default list in the kernel module.

3) Add a mount option to specify the protected GUIDs.

Feedback is welcome.


I'd consider a patch that makes the permissions a mount option for
efivarfs, applying to all variables. The reason is that these
variables shouldn't have been world readable in the first place, and
I
am reluctant to make this overly complex.

Having some variables (like the BootXXXX and BootOrder variables) world
readable is useful. This allows normal users to run 'efibootmgr' to
display the boot options.
Some variables maybe (ISTR variables for things like the system time-zone or the firmware locale settings, which _might_ be useful), but I would say the boot variables are not on that list. The only practical application for a regular (non-root) user to read those variables is to gather info for an attack on the system. Anybody who legitimately needs to access them (either for debugging the boot process, or changing the boot options) should have administrative access to the system anyway, and even then they will usually not need to read them.

In fact, most of the UEFI variables fall into the same category, but even more so, userspace has no legitimate reason to need to read them. You can get an absolutely insane amount of info out of them on some systems, most of which is a gold-mine for potential attackers. For the handful that may actually be useful to userspace, most would be needed only during system startup, and thus could safely be made readable by root only.

On the other hand, you should realize that UEFI was never designed to
keep secrets, and so whether it is a good idea to put secrets in UEFI
variables to begin with is dubious IMHO.

If the UEFI is as secure as storing an unencrypted file on a hard
drive, I am satisfied. Or do you have a better idea where to store the
SSH keys for a diskless system that boots via network?

There really isn't any other option unless you're willing to put a small amount of flash storage in the system somehow (maybe a small USB flash drive connected directly to a USB header inside the system?). As far as the security of UEFI variables, the same limitations as storing the data on an unencrypted hard drive apply, with the addition that it's much easier to get at them through stuff like Intel's AMT or IPMI than it is to read data off of the hard drive.
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