Re: Read-protected UEFI variables

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

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.drung@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.