Re: [PATCH RFC 4/4] fs: allow mknod in non-initial userns using cgroup device guard

[Alexander Mikhalitsyn]

+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
>