+CC Stéphane Graber <stgraber@xxxxxxxxxx>
On Mon, Aug 14, 2023 at 4:26 PM Michael Weiß
<michael.weiss@xxxxxxxxxxxxxxxxxxx> wrote:
>
> If a container manager restricts its unprivileged (user namespaced)
> children by a device cgroup, it is not necessary to deny mknod
> anymore. Thus, user space applications may map devices on different
> locations in the file system by using mknod() inside the container.
>
> A use case for this, we also use in GyroidOS, is to run virsh for
> VMs inside an unprivileged container. virsh creates device nodes,
> e.g., "/var/run/libvirt/qemu/11-fgfg.dev/null" which currently fails
> in a non-initial userns, even if a cgroup device white list with the
> corresponding major, minor of /dev/null exists. Thus, in this case
> the usual bind mounts or pre populated device nodes under /dev are
> not sufficient.
>
> To circumvent this limitation, we allow mknod() in fs/namei.c if a
> bpf cgroup device guard is enabeld for the current task using
> devcgroup_task_is_guarded() and check CAP_MKNOD for the current user
> namespace by ns_capable() instead of the global CAP_MKNOD.
>
> To avoid unusable device nodes on file systems mounted in
> non-initial user namespace, may_open_dev() ignores the SB_I_NODEV
> for cgroup device guarded tasks.
>
> Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
> ---
> fs/namei.c | 19 ++++++++++++++++---
> 1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index e56ff39a79bc..ef4f22b9575c 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3221,6 +3221,9 @@ EXPORT_SYMBOL(vfs_mkobj);
>
> bool may_open_dev(const struct path *path)
> {
> + if (devcgroup_task_is_guarded(current))
> + return !(path->mnt->mnt_flags & MNT_NODEV);
> +
> return !(path->mnt->mnt_flags & MNT_NODEV) &&
> !(path->mnt->mnt_sb->s_iflags & SB_I_NODEV);
> }
> @@ -3976,9 +3979,19 @@ int vfs_mknod(struct mnt_idmap *idmap, struct inode *dir,
> if (error)
> return error;
>
> - if ((S_ISCHR(mode) || S_ISBLK(mode)) && !is_whiteout &&
> - !capable(CAP_MKNOD))
> - return -EPERM;
> + /*
> + * In case of a device cgroup restirction allow mknod in user
> + * namespace. Otherwise just check global capability; thus,
> + * mknod is also disabled for user namespace other than the
> + * initial one.
> + */
> + if ((S_ISCHR(mode) || S_ISBLK(mode)) && !is_whiteout) {
> + if (devcgroup_task_is_guarded(current)) {
> + if (!ns_capable(current_user_ns(), CAP_MKNOD))
> + return -EPERM;
> + } else if (!capable(CAP_MKNOD))
> + return -EPERM;
> + }
>
> if (!dir->i_op->mknod)
> return -EPERM;
>
> --
> 2.30.2
>
