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

Nayna
2022-11-09T20:11:20Z

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.

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.

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.

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.

3. firmware objects are persistent and read at boot time by interaction
with firmware, unlike kernel objects which are not persistent.

For a more detailed explanation refer to the LSS-NA 2022 "PowerVM
Platform Keystore - Securing Linux Credentials Locally" talk and
slides[1]. The link to previously posted RFC version is [2].

[1]
https://static.sched.com/hosted_files/lssna2022/25/NaynaJain_PowerVM_PlatformKeyStore_SecuringLinuxCredentialsLocally.pdf
[2] https://lore.kernel.org/linuxppc-dev/YrQqPhi4+jHZ1WJc@kroah.com/

Thanks & Regards,

- Nayna

>
> thanks,
>
> greg k-h