[SeaBIOS] Saving a few bytes across a reboot

Marc-André Lureau marcandre.lureau at gmail.com
Thu Feb 8 16:52:15 CET 2018


On Wed, Feb 7, 2018 at 6:21 PM, Laszlo Ersek <lersek at redhat.com> wrote:
> On 02/07/18 17:44, Stefan Berger wrote:
>> On 02/07/2018 10:50 AM, Laszlo Ersek wrote:
>>> OK, but if the OS is allowed to modify this set of "queued operations",
>>> then what protection is expected of SMM? Whether you can modify the TPM
>>> directly, or queue random commands for it at libery, what's the
>>> difference?
>> On the OS level it is presumably an operation that is reserved to the
>> admin to queue the operation.
>> I am not that familiar with UEFI and who is allowed to run code there
>> and what code it can execute. But UEFI seems to lock the variable that
>> holds that PPI code that tells it what to do after next reboot. So
>> presumably a UEFI module cannot modify that variable but can only read
>> it (and hopefully not manipulate NVRAM directly). If PPI was implemented
>> through a memory location where the code gets written to it could do
>> that likely easily (unless memory protections are setup by UEFI, which I
>> don't know), cause a reset and have UEFI execute on that code.
> This makes sense... but then it doesn't make sense :)
> Assume that the variable is indeed "locked" (so that random UEFI drivers
> / apps cannot rewrite it using the UEFI variable service). Then,
> - if the lock is enforced in SMM, then the variable will be locked from
>   the OS as well, not just from 3rd party UEFI apps, so no PPI
>   operations can ever be queued,
> - if the lock is "simulated" in ACPI or in non-SMM firmare code (= in
>   the "runtime DXE driver" layer), then the lock can be circumvented by
>   both 3rd party UEFI apps and the OS.

Regarding security of PPI pending operations, the spec clearly says
"The location for tracking the pending PPI
operation, including the tracking of necessary PLATFORM RESET
operations, does not need to be a secure or trusted location." (9.9

I assume this is because the user has to confirm the pending operation
in pre-os console, so if some attacker wanted to clear the TPM, the
user would have to confirm it (same for other operation of flags
manipulation). That may not be the best security design, but at least,
the user could be in control.

How does the UEFI runtime variable services verify the authenticity of
the requests? Or does it only check a request validity?


Marc-André Lureau

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