Subject: Re: [kernel-hardening] [RFC v4 03/18] bpf,landlock: Add a new arraymap type to deal with (Landlock) handles

From: Jann Horn <[email-masked]>
Date: Wed, 26 Oct 2016 21:01:47 +0200

On Wed, Oct 26, 2016 at 08:56:39AM +0200, Mickaël Salaün wrote:
> This new arraymap looks like a set and brings new properties:
> * strong typing of entries: the eBPF functions get the array type of
>   elements instead of CONST_PTR_TO_MAP (e.g.
>   CONST_PTR_TO_LANDLOCK_HANDLE_FS);
> * force sequential filling (i.e. replace or append-only update), which
>   allow quick browsing of all entries.
> 
> This strong typing is useful to statically check if the content of a map
> can be passed to an eBPF function. For example, Landlock use it to store
> and manage kernel objects (e.g. struct file) instead of dealing with
> userland raw data. This improve efficiency and ensure that an eBPF
> program can only call functions with the right high-level arguments.
> 
> The enum bpf_map_handle_type list low-level types (e.g.
> BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) which are identified when
> updating a map entry (handle). This handle types are used to infer a
> high-level arraymap type which are listed in enum bpf_map_array_type
> (e.g. BPF_MAP_ARRAY_TYPE_LANDLOCK_FS).
> 
> For now, this new arraymap is only used by Landlock LSM (cf. next
> commits) but it could be useful for other needs.
> 
> Changes since v3:
> * make handle arraymap safe (RCU) and remove buggy synchronize_rcu()
> * factor out the arraymay walk
> 
> Changes since v2:
> * add a RLIMIT_NOFILE-based limit to the maximum number of arraymap
>   handle entries (suggested by Andy Lutomirski)
> * remove useless checks
> 
> Changes since v1:
> * arraymap of handles replace custom checker groups
> * simpler userland API
[...]
> +	case BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD:
> +		handle_file = fget(handle->fd);
> +		if (IS_ERR(handle_file))
> +			return ERR_CAST(handle_file);
> +		/* check if the FD is tied to a user mount point */
> +		if (unlikely(handle_file->f_path.mnt->mnt_flags & MNT_INTERNAL)) {
> +			fput(handle_file);
> +			return ERR_PTR(-EINVAL);
> +		}
> +		path_get(&handle_file->f_path);
> +		ret = kmalloc(sizeof(*ret), GFP_KERNEL);
> +		ret->path = handle_file->f_path;
> +		fput(handle_file);

You can use fdget() and fdput() here because the reference to
handle_file is dropped before the end of the syscall.

> +		break;
> +	case BPF_MAP_HANDLE_TYPE_UNSPEC:
> +	default:
> +		return ERR_PTR(-EINVAL);
> +	}
> +	ret->type = handle_type;
> +	return ret;
> +}
> +
> +static void *nop_map_lookup_elem(struct bpf_map *map, void *key)
> +{
> +	return ERR_PTR(-EINVAL);
> +}
> +
> +/* called from syscall or from eBPF program */
> +static int landlock_array_map_update_elem(struct bpf_map *map, void *key,
> +		void *value, u64 map_flags)
> +{

This being callable from eBPF context is IMO pretty surprising and should
at least be well-documented. Also: What is the usecase here?