Re: [WIP RFC PATCH 0/6] Generic Firmware Variable Filesystem

[Daniel Axtens <dja@axtens.net>]

Hi Greg,

>> >> As PowerNV moves towards secure boot, we need a place to put secure
>> >> variables. One option that has been canvassed is to make our secure
>> >> variables look like EFI variables. This is an early sketch of another
>> >> approach where we create a generic firmware variable file system,
>> >> fwvarfs, and an OPAL Secure Variable backend for it.
>> >
>> > Is there a need of new filesystem ? I am wondering why can't these be 
>> > exposed via sysfs / securityfs ?
>> > Probably, something like... /sys/firmware/secureboot or 
>> > /sys/kernel/security/secureboot/  ?
>> 
>> I suppose we could put secure variables in sysfs, but I'm not sure
>> that's what sysfs was intended for. I understand sysfs as "a
>> filesystem-based view of kernel objects" (from
>> Documentation/filesystems/configfs/configfs.txt), and I don't think a
>> secure variable is really a kernel object in the same way most other
>> things in sysfs are... but I'm open to being convinced.
>
> What makes them more "secure" than anything else that is in sysfs today?
> I didn't see anything in this patchset that provided "additional
> security", did I miss it?

You're right, there's no additional security. What I should have said
was that I didn't think that _firmware_ variables were kernel objects in
the same way that other things in sysfs are. Having read the rest of
your reply it seems I'm mistaken on this.

> I would just recommend putting this in sysfs.  Make a new subsystem
> (i.e. class) and away you go.
>
>> My hope with fwvarfs is to provide a generic place for firmware
>> variables so that we don't need to expand the list of firmware-specific
>> filesystems beyond efivarfs. I am also aiming to make things simple to
>> use so that people familiar with firmware don't also have to become
>> familiar with filesystem code in order to expose firmware variables to
>> userspace.

>> fwvarfs can also be used for variables that are not security relevant as
>> well. For example, with the EFI backend (patch 3), both secure and
>> insecure variables can be read.
>
> I don't remember why efi variables were not put in sysfs, I think there
> was some reasoning behind it originally.  Perhaps look in the linux-efi
> archives.

I'll have a look: I suspect the appeal of efivarfs is that it allows for
things like non-case-sensitive matching on the GUID part of the filename
while retaining case-sensitivity on the part of the filename
representing the variable name.

As suggested, I'll try a sysfs class. I think that will allow me to
kill off most of the abstraction layer too. Thanks for the input.

Regards,
Daniel

>
> thanks,
>
> greg k-h