On 18.12.23 17:18, Alexander Mikhalitsyn wrote:

On Fri, 15 Dec 2023 14:26:53 +0100
Michael Weiß [off-list ref] wrote:

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On 15.12.23 13:31, Christian Brauner wrote:

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On Wed, Dec 13, 2023 at 03:38:13PM +0100, Michael Weiß wrote:

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devguard is a simple LSM to allow CAP_MKNOD in non-initial user
namespace in cooperation of an attached cgroup device program. We
just need to implement the security_inode_mknod() hook for this.
In the hook, we check if the current task is guarded by a device
cgroup using the lately introduced cgroup_bpf_current_enabled()
helper. If so, we strip out SB_I_NODEV from the super block.

Access decisions to those device nodes are then guarded by existing
device cgroups mechanism.

Signed-off-by: Michael Weiß <redacted>
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I think you misunderstood me... My point was that I believe you don't
need an additional LSM at all and no additional LSM hook. But I might be
wrong. Only a POC would show.

Yeah sorry, I got your point now.

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Just write a bpf lsm program that strips SB_I_NODEV in the existing
security_sb_set_mnt_opts() call which is guranteed to be called when a
new superblock is created.

This does not work since SB_I_NODEV is a required_iflag in
mount_too_revealing(). This I have already tested when writing the
simple LSM here. So maybe we need to drop SB_I_NODEV from required_flags
there, too. Would that be safe?

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Store your device access rules in a bpf map or in the sb->s_security
blob (This is where I'm fuzzy and could use a bpf LSM expert's input.).

Then make that bpf lsm program kick in everytime a
security_inode_mknod() and security_file_open() is called and do device
access management in there. Actually, you might need to add one hook
when the actual device that's about to be opened is know. 
This should be where today the device access hooks are called.

And then you should already be done with this. The only thing that you
need is the capable check patch.

You don't need that cgroup_bpf_current_enabled() per se. Device
management could now be done per superblock, and not per task. IOW, you
allowlist a bunch of devices that can be created and opened. Any task
that passes basic permission checks and that passes the bpf lsm program
may create device nodes.

That's a way more natural device management model than making this a per
cgroup thing. Though that could be implemented as well with this.

I would try to write a bpf lsm program that does device access
management with your capable() sysctl patch applied and see how far I
get.

I don't have the time otherwise I'd do it.

I'll give it a try but no promises how fast this will go.

Hi Michael,

thanks for your work on this!

If you don't mind I'm ready to help you with writing the PoC for this bpf-based approach,
as I have touched eBPF earlier I guess I can save some your time. (I'll post it here and you will incude it
in your patch series.)

Yeah for sure. This would be very helpful thanks.
I'll start to sort Christians patches of this thread and get the missing security
hook for the remaining checks lined up from v2 then.

Kind regards,
Alex

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