Re: [PATCH 2/4] fs: define a firmware security filesystem named fwsecurityfs

Nayna
2022-11-14T23:07:14Z

On 11/10/22 04:58, Greg Kroah-Hartman wrote:
> On Wed, Nov 09, 2022 at 03:10:37PM -0500, Nayna wrote:
>> On 11/9/22 08:46, Greg Kroah-Hartman wrote:
>>> On Sun, Nov 06, 2022 at 04:07:42PM -0500, Nayna Jain wrote:
>>>> securityfs is meant for Linux security subsystems to expose policies/logs
>>>> or any other information. However, there are various firmware security
>>>> features which expose their variables for user management via the kernel.
>>>> There is currently no single place to expose these variables. Different
>>>> platforms use sysfs/platform specific filesystem(efivarfs)/securityfs
>>>> interface as they find it appropriate. Thus, there is a gap in kernel
>>>> interfaces to expose variables for security features.
>>>>
>>>> Define a firmware security filesystem (fwsecurityfs) to be used by
>>>> security features enabled by the firmware. These variables are platform
>>>> specific. This filesystem provides platforms a way to implement their
>>>> own underlying semantics by defining own inode and file operations.
>>>>
>>>> Similar to securityfs, the firmware security filesystem is recommended
>>>> to be exposed on a well known mount point /sys/firmware/security.
>>>> Platforms can define their own directory or file structure under this path.
>>>>
>>>> Example:
>>>>
>>>> # mount -t fwsecurityfs fwsecurityfs /sys/firmware/security
>>> Why not juset use securityfs in /sys/security/firmware/ instead? Then
>>> you don't have to create a new filesystem and convince userspace to
>>> mount it in a specific location?
>> From man 5 sysfs page:
>>
>> /sys/firmware: This subdirectory contains interfaces for viewing and
>> manipulating firmware-specific objects and attributes.
>>
>> /sys/kernel: This subdirectory contains various files and subdirectories
>> that provide information about the running kernel.
>>
>> The security variables which are being exposed via fwsecurityfs are managed
>> by firmware, stored in firmware managed space and also often consumed by
>> firmware for enabling various security features.
> Ok, then just use the normal sysfs interface for /sys/firmware, why do
> you need a whole new filesystem type?
>
>> From git commit b67dbf9d4c1987c370fd18fdc4cf9d8aaea604c2, the purpose of
>> securityfs(/sys/kernel/security) is to provide a common place for all kernel
>> LSMs. The idea of
>> fwsecurityfs(/sys/firmware/security) is to similarly provide a common place
>> for all firmware security objects.
>>
>> /sys/firmware already exists. The patch now defines a new /security
>> directory in it for firmware security features. Using /sys/kernel/security
>> would mean scattering firmware objects in multiple places and confusing the
>> purpose of /sys/kernel and /sys/firmware.
> sysfs is confusing already, no problem with making it more confusing :)
>
> Just document where you add things and all should be fine.
>
>> Even though fwsecurityfs code is based on securityfs, since the two
>> filesystems expose different types of objects and have different
>> requirements, there are distinctions:
>>
>> 1. fwsecurityfs lets users create files in userspace, securityfs only allows
>> kernel subsystems to create files.
> Wait, why would a user ever create a file in this filesystem? If you
> need that, why not use configfs? That's what that is for, right?

The purpose of fwsecurityfs is not to expose configuration items but
rather security objects used for firmware security features. I think
these are more comparable to EFI variables, which are exposed via an
EFI-specific filesystem, efivarfs, rather than configfs.

>
>> 2. firmware and kernel objects may have different requirements. For example,
>> consideration of namespacing. As per my understanding, namespacing is
>> applied to kernel resources and not firmware resources. That's why it makes
>> sense to add support for namespacing in securityfs, but we concluded that
>> fwsecurityfs currently doesn't need it. Another but similar example of it
>> is: TPM space, which is exposed from hardware. For containers, the TPM would
>> be made as virtual/software TPM. Similarly for firmware space for
>> containers, it would have to be something virtualized/software version of
>> it.
> I do not understand, sorry. What does namespaces have to do with this?
> sysfs can already handle namespaces just fine, why not use that?

Firmware objects are not namespaced. I mentioned it here as an example
of the difference between firmware and kernel objects. It is also in
response to the feedback from James Bottomley in RFC v2
[https://lore.kernel.org/linuxppc-dev/41ca51e8db9907d9060cc38adb59a66dcae4c59b.camel@HansenPartnership.com/].

>
>> 3. firmware objects are persistent and read at boot time by interaction with
>> firmware, unlike kernel objects which are not persistent.
> That doesn't matter, sysfs exports what the hardware provides, and that
> might persist over boot.
>
> So I don't see why a new filesystem is needed.
>
> You didn't explain why sysfs, or securitfs (except for the location in
> the tree) does not work at all for your needs. The location really
> doesn't matter all that much as you are creating a brand new location
> anyway so we can just declare "this is where this stuff goes" and be ok.

For rest of the questions, here is the summarized response.

Based on mailing list previous discussions [1][2][3] and considering
various firmware security use cases, our fwsecurityfs proposal seemed to
be a reasonable and acceptable approach based on the feedback [4].

[1] https://lore.kernel.org/linuxppc-dev/YeuyUVVdFADCuDr4@kroah.com/#t
[2] https://lore.kernel.org/linuxppc-dev/Yfk6gucNmJuR%2Fegi@kroah.com/
[3]
https://lore.kernel.org/all/Yfo%2F5gYgb9Sv24YB@kroah.com/t/#m40250fdb3fddaafe502ab06e329e63381b00582d
[4] https://lore.kernel.org/linuxppc-dev/YrQqPhi4+jHZ1WJc@kroah.com/

RFC v1 was using sysfs. After considering feedback[1][2][3], the
following are design considerations for unification via fwsecurityfs:

1. Unify the location: Defining a security directory under /sys/firmware
facilitates exposing objects related to firmware security features in a
single place. Different platforms can create their respective directory
structures within /sys/firmware/security.

2. Unify the code:  To support unification, having the fwsecurityfs
filesystem API allows different platforms to define the inode and file
operations they need. fwsecurityfs provides a common API that can be
used by each platform-specific implementation to support its particular
requirements and interaction with firmware. Initializing
platform-specific functions is the purpose of the
fwsecurityfs_arch_init() function that is called on mount. Patch 3/4
implements fwsecurityfs_arch_init() for powerpc.

Similar to the common place securityfs provides for LSMs to interact
with kernel security objects, fwsecurityfs would provide a common place
for all firmware security objects, which interact with the firmware
rather than the kernel. Although at the API level, the two filesystem
look similar, the requirements for firmware and kernel objects are
different. Therefore, reusing securityfs wasn't a good fit for the
firmware use case and we are proposing a similar but different
filesystem -  fwsecurityfs - focused for firmware security.

>
> And again, how are you going to get all Linux distros to now mount your
> new filesystem?

It would be analogous to the way securityfs is mounted.

Thanks & Regards,

- Nayna

>
> thanks,
>
> greg k-h