Re: [coreboot] question on SMM

[Stefan Reinauer]

On 30-Jun-17 10:46, ron minnich wrote:
> Thanks for the good explanations.
>
> So I have a question for you all. We've been doing some testing of 
> linux-as-ramstage. We've done a proof of concept that linux can set up 
> the SMM handler at 0xa0000, the relocate stub at 0x38000, run the 
> relocate stub, and have a working smm handler. The smm handler can 
> trampoline to 64-bit mode and call the kernel, using existing 
> mechanisms. So our SMM handler, in this scenario, is a set of 
> functions provided by the kernel, not a binary blob. The result is a 
> teeny tiny SMM handler and complete elimination of the vendor-supplied 
> SMM code.

How do you do the relocation on a system where the vendor bios has 
already relocated the SMM_BASE (i.e. when not running on a machine you 
have the bios source code for)?
> There are lots of benefits. The SMM is no longer at a fixed location 
> -- it's kind of ASLR for SMM code; there is very little code that runs 
> in SMM; and the SMM handlers we implement run in 64-bit mode with full 
> memory protections. The big one for me is that persistent firmware 
> blobs are reduced by one -- it's part of a goal to create an air gap 
> between firmware and kernel. Another part of this work is that we're 
> going to discard firmware-supplied ACPI tables and use ones supplied 
> by the kernel.

SMM was never a blob on coreboot.
> I realize this is not a general approach. But for small, limited 
> configurations, such as OCP servers which come in a small number of 
> flavors, it's quite doable.
>
> The only question that has been raised: are we losing an essential 
> security guarantee since flash is writeable in this kernel-based 
> "SMM"? The big question is whether we're opening up the possibility of 
> firmware getting changed, once the kernel is our "smm mode". Is there 
> a reasonable mitigation we could use in the SMM handler before we 
> trampoline back up to the kernel?

You could add the security that your bios implemented back into your 
kernel SMM handler ;-) Look at the open source SMM handler in coreboot 
for how that is done.
> Thoughts on this are welcome.
>
> ron
>
> On Fri, Jun 30, 2017 at 6:01 AM Alexander Couzens <[e-mail] 
> > wrote:
> On Fri, 30 Jun 2017 04:25:06 +0000
> ron minnich <[e-mail] <mailto:[e-mail]>> wrote:
>
> > there's something I am certain I don't understand about SMM on intel
> > chipsets.
> >
> > The question is pretty simple. Consider a system with a recent intel
> > chipset and flash. Is there some special secret sauce that disables
> > writing to flash unless in SMM and if so, what is it?
>
> There is also a talk explaining it (without SMM_BWP).
>
> https://media.ccc.de/v/31c3_-_6129_-_en_-_saal_2_-_201412282030_-_attacks_on_uefi_security_inspired_by_darth_venamis_s_misery_and_speed_racer_-_rafal_wojtczuk_-_corey_kallenberg

Stefan