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

Nayna
2022-11-22T23:22:05Z

On 11/19/22 06:48, Ritesh Harjani (IBM) wrote:
> Hello Nayna,

Hi Ritesh,

>
> On 22/11/09 03:10PM, 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?
> I am also curious to know on why not use securityfs, given the similarity
> between the two. :)
> More specifics on that below...
>
>> 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.
> That's ok. As I see it users of securityfs can define their own fileops
> (like how you are doing in fwsecurityfs).
> See securityfs_create_file() & securityfs_create_symlink(), can accept the fops
> & iops. Except maybe securityfs_create_dir(), that could be since there might
> not be a usecase for it. But do you also need it in your case is the question to
> ask.

Please refer to the function plpks_secvars_init() in Patch 4/4.

>
>> From git commit b67dbf9d4c1987c370fd18fdc4cf9d8aaea604c2, the purpose of
>> securityfs(/sys/kernel/security) is to provide a common place for all kernel
>> LSMs. The idea of
> Which was then seperated out by commit,
> da31894ed7b654e2 ("securityfs: do not depend on CONFIG_SECURITY").
>
> securityfs now has a seperate CONFIG_SECURITYFS config option. In fact I was even
> thinking of why shouldn't we move security/inode.c into fs/securityfs/inode.c .
> fs/* is a common place for all filesystems. Users of securityfs can call it's
> exported kernel APIs to create files/dirs/symlinks.
>
> If we move security/inode.c to fs/security/inode.c, then...
> ...below call within securityfs_init() should be moved into some lsm sepecific
> file.
>
> #ifdef CONFIG_SECURITY
> static struct dentry *lsm_dentry;
> static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,
> loff_t *ppos)
> {
> return simple_read_from_buffer(buf, count, ppos, lsm_names,
> strlen(lsm_names));
> }
>
> static const struct file_operations lsm_ops = {
> .read = lsm_read,
> .llseek = generic_file_llseek,
> };
> #endif
>
> securityfs_init()
>
> #ifdef CONFIG_SECURITY
> lsm_dentry = securityfs_create_file("lsm", 0444, NULL, NULL,
> &lsm_ops);
> #endif
>
> So why not move it? Maybe others, can comment more on whether it's a good idea
> to move security/inode.c into fs/security/inode.c?
> This should then help others identify securityfs filesystem in fs/security/
> for everyone to notice and utilize for their use?
>> 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.
> We can also think of it this way that, all security related exports should
> happen via /sys/kernel/security/. Then /sys/kernel/security/firmware/ becomes
> the security related firmware exports.
>
> If you see find /sys -iname firmware, I am sure you will find other firmware
> specifics directories related to other specific subsystems
> (e.g.
> root@qemu:/home/qemu# find /sys -iname firmware
> /sys/devices/ndbus0/nmem0/firmware
> /sys/devices/ndbus0/firmware
> /sys/firmware
> )
>
> But it could be, I am not an expert here, although I was thinking a good
> Documentation might solve this problem.

Documentation on
sysfs(https://man7.org/linux/man-pages/man5/sysfs.5.html) already
differentiates /sys/firmware and /sys/kernel as I responded earlier.
The objects we are exposing are firmware objects and not kernel objects.

>
>> 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.
> Sorry could you please elaborate how? both securityfs & fwsecurityfs
> calls simple_fill_super() which uses the same inode (i_op) and inode file
> operations (i_fop) from fs/libfs.c for their root inode. So how it is enabling
> user (as in userspace) to create a file in this filesystem?
>
> So am I missing anything?

The ability to let user(as in userspace) to create a file in a
filesystem comes by allowing to define inode operations.

Please look at the implementation differences for functions
xxx_create_dir() and xxx_create_dentry() of securityfs vs fwsecurityfs.
Also refer to Patch 4/4 for use of fwsecurityfs_create_dir() where inode
operations are defined.

>
>> 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
> It "currently" doesn't need it. But can it in future? Then why not go with
> securityfs which has an additional namespacing feature available?
> That's actually also the point of utilizing an existing FS which can get
> features like this in future. As long as it doesn't affect the functionality
> of your use case, we simply need not reject securityfs, no?

Thanks for your review and feedback. To summarize:

From the perspective of our use case, we need to expose firmware
security objects to userspace for management. Not all of the objects
pre-exist and we would like to allow root to create them from userspace.

From a unification perspective, I have considered a common location at
/sys/firmware/security for managing any platform's security objects. And
I've proposed a generic filesystem, which could be used by any platform
to represent firmware security objects via /sys/firmware/security.

Here are some alternatives to generic filesystem in discussion:

1. Start with a platform-specific filesystem. If more platforms would
like to use the approach, it can be made generic. We would still have a
common location of /sys/firmware/security and new code would live in
arch. This is my preference and would be the best fit for our use case.

2. Use securityfs.  This would mean modifying it to satisfy other use
cases, including supporting userspace file creation. I don't know if the
securityfs maintainer would find that acceptable. I would also still
want some way to expose variables at /sys/firmware/security.

3. Use a sysfs-based approach. This would be a platform-specific
implementation. However, sysfs has a similar issue to securityfs for
file creation. When I tried it in RFC v1[1], I had to implement a
workaround to achieve that.

[1]
https://lore.kernel.org/linuxppc-dev/20220122005637.28199-3-nayna@linux.ibm.com/

Thanks & Regards,

- Nayna