Re: [PATCH RFC 1/4] bpf: add cgroup device guard to flag a cgroup device prog

[Christian Brauner <brauner@kernel.org>]

On Mon, Aug 14, 2023 at 04:26:09PM +0200, Michael Weiß wrote:
> Introduce the BPF_F_CGROUP_DEVICE_GUARD flag for BPF_PROG_LOAD
> which allows to set a cgroup device program to be a device guard.

Currently we block access to devices unconditionally in may_open_dev().
Anything that's mounted by an unprivileged containers will get
SB_I_NODEV set in s_i_flags.

Then we currently mediate device access in:

* inode_permission()
  -> devcgroup_inode_permission()
* vfs_mknod()
  -> devcgroup_inode_mknod()
* blkdev_get_by_dev() // sget()/sget_fc(), other ways to open block devices and friends
  -> devcgroup_check_permission()
* drivers/gpu/drm/amd/amdkfd // weird restrictions on showing gpu info afaict
  -> devcgroup_check_permission()

All your new flag does is to bypass that SB_I_NODEV check afaict and let
it proceed to the devcgroup_*() checks for the vfs layer.

But I don't get the semantics yet.
Is that a flag which is set on BPF_PROG_TYPE_CGROUP_DEVICE programs or
is that a flag on random bpf programs? It looks like it would be the
latter but design-wise I would expect this to be a property of the
device program itself.