On 7/12/2023 3:05 AM, Gerd Hoffmann wrote:
> On Tue, Jul 11, 2023 at 04:52:37PM -0700, Taylor Beebe wrote:
>> In the past, memory protection settings were configured via FixedAtBuild PCDs,
>> which resulted in a build-time configuration of memory mitigations. This
>> approach limited the flexibility of applying mitigations to the
>> system and made it difficult to update or adjust the settings post-build.
>
> Can we have both?
>
> Being able to adjust settings at runtime is great. But being able to
> set them at compile time on the command line (via build --pcd), without
> patching code, is very useful too.
>
> I'd suggest to keep the PCDs, create a profile from PCD settings and use
> that profile by default. At least for the transition phase and while we
> don't have good support (yet) to actually manage profiles.
>
Hey, Gerd.
The idea to keep PCDs around as another method of configuring
protections is good, but I don't think there would be a way to tell if a
zero-ed PCD value was an explicit setting or just the default without
adding another PCD to indicate which value should be used. I think if
the HOB is produced by the platform those settings should be used by
default. Is that what you're envisioning as well?
The flow in this case would be: DxeIpl before handoff will search for
the memory protection HOB entry. If it exists, do nothing. If the HOB
was not produced, create a HOB entry using the PCD values.
I suppose we could also do some sort of hybrid where the logic always
uses the PCD values if they are non-zero, but that may be confusing for
platform developers.
> Speaking of profile management: What is the longer-term vision here?
>
> Have a configuration form in the uefi firmware setup (aka UiApp)?
>
> Try adapt settings automatically, for example pick a less strict profile
> in case an old/broken bootloader triggers a page fault due to using the
> wrong memory type for allocations?
>
Overall, the idea is to empower platform developers with more
configuration and compatibility options to encourage the use of memory
protections in debug and production scenarios.
On Surface products, we've made certain memory protections a necessary
feature of the trusted boot path with the ability to fall back to an
unverified boot path if a page fault occurs. We also added a
configuration feature to only allow the trusted boot path which can be
managed via enterprise policy or through the UEFI menu.
There are lots of ways OEMs might want to configure their platform
security and I think it's an open question what sorts of profile
management tools would be useful to add to EDK2.
> For virtual machine firmware it a good idea to allow picking up the
> profile configuration from the host. For qemu that can use fw_cfg,
> similar to the PcdSetNxForStack option we have today.
I don't have much experience using the fw_cfg so I'd need to look into
the details, but would it make sense to expand the options which can be
passed via fw_cfg to be the gamut of memory protection configuration
settings? I can add it to the v2 if you think so.
>> This patch series also increases the memory protection level for OvmfPkg and
>> ArmVirtPkg.
>
> Not a good idea, especially not using the 'debug' profile (which turns
> on all guard bits) because that slows down firmware boot alot.
>
I haven't done performance testing, but I don't notice much slowdown.
Isn't development the primary use case for these virtual platforms? Is
there another profile you think would work better?
Thanks for your feedback :)
> take care,
> Gerd
>
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