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

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Date:   Tue, 15 Aug 2023 10:59:22 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Michael Weiß <michael.weiss@...ec.fraunhofer.de>
Cc:     Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        Alexander Viro <viro@...iv.linux.org.uk>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        gyroidos@...ec.fraunhofer.de
Subject: Re: [PATCH RFC 1/4] bpf: add cgroup device guard to flag a cgroup
 device prog

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.