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Re: [RFC PATCH] gdb: Add more support for debugging on EFI platforms

From: Peter Jones
Subject: Re: [RFC PATCH] gdb: Add more support for debugging on EFI platforms
Date: Fri, 10 Mar 2023 15:27:13 -0500

On Thu, Mar 09, 2023 at 06:00:04PM -0500, Robbie Harwood wrote:
> Glenn Washburn <> writes:
> >  2. Why should the load command not be printed when secure boot is enabled?
> >    * This was also requested by Daniel, I assume because of infomation 
> > leakage
> >      that may be a security concern.
> I seem to have also missed Daniel's reply about this earlier, which was:
> >> I think leaking info about the GRUB image addresses on the Secure
> >> Boot enabled systems is not the best idea. Or do you think having
> >> this feature enabled by default overweight potential dangers coming
> >> from its misuse?
> I don't know how these could help an attacker.  They'd need access to
> console out to retrieve the values, and some way to send input... and
> that's basically physical presence: at the very least, if they have
> those, I imagine they'd just edit the menu entries, or drop to the grub
> shell.

I'd like to also point out that we currently don't have anything like
ASLR in the pre-boot space, nor much hope of meaningfully getting it,
and typically have an identity memory mapped.  So, a serious attacker
can take a workflow like:

1) make a rough guess what kind of system they're attacking - vendor,
   product line, minimum memory size, etc.
2) get two or three similar machines on ebay
3) build a certificate chain that's got the exact same topology and cert
   sizes as the real one
4) enroll that the normal way
5) rebuild the grub binary with the "should we print this line" test
   inverted, so it's literally 1 bit different (i.e. jz -> jnz)
6) sign it
7) watch the load patterns on each of those machines and build a pretty
   simple model for candidate addresses
8) try them all when trying to exploit whatever bug they've found,
   either on individual attempts or possibly multiple at once if
   they're clever

If they're also using the shotgun method of identifying targets - i.e.
they've already used some other technique to get root on a bunch of
machines and now they're trying to escalate however many they can to
have Secure Boot breakouts - they're going to have a fair degree of
success with this method, and hiding the addresses will be an incredibly
minor speed bump for them.

I think worrying about exposing load addresses on the console with open
source firmware and no ASLR is not worth the trouble.


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