On Fri, Dec 15, 2023 at 10:08:08AM -0800, Alexei Starovoitov wrote:

On Fri, Dec 15, 2023 at 6:15 AM Christian Brauner [off-list ref] wrote:

quoted

On Fri, Dec 15, 2023 at 02:26:53PM +0100, Michael Weiß wrote:

quoted

On 15.12.23 13:31, Christian Brauner wrote:

quoted

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

quoted

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>
---

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.

I think I might have had a misconception about how this works.
A bpf LSM program can't easily alter a kernel object such as struct
super_block I've been told.

Right. bpf cannot change arbitrary kernel objects,
but we can add a kfunc that will change a specific bit in a specific
data structure.
Adding a new lsm hook that does:
    rc = call_int_hook(sb_device_access, 0, sb);
    switch (rc) {
    case 0: do X
    case 1: do Y

is the same thing, but uglier, since return code will be used
to do this action.
The 'do X' can be one kfunc
and 'do Y' can be another.
If later we find out that 'do X' is not a good idea we can remove
that kfunc.

The reason I moved the SB_I_MANAGED_DEVICES here is that I want a single
central place where that is done for any possible LSM that wants to
implement device management. So we don't have to go chasing where that
bit is set for each LSM. I also don't want to have LSMs raise bits in
sb->s_iflags directly as that's VFS property.