devmap: Allow map lookups from eBPF

We don't currently allow lookups into a devmap from eBPF, because the map
lookup returns a pointer directly to the dev->ifindex, which shouldn't be
modifiable from eBPF.

However, being able to do lookups in devmaps is useful to know (e.g.)
whether forwarding to a specific interface is enabled. Currently, programs
work around this by keeping a shadow map of another type which indicates
whether a map index is valid.

Since we now have a flag to make maps read-only from the eBPF side, we can
simply lift the lookup restriction if we make sure this flag is always set.

Signed-off-by: Toke Høiland-Jørgensen <[email protected]>
Acked-by: Jonathan Lemon <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

kernel/bpf/devmap.c

@@ -89,6 +89,11 @@ static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
 	    attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
 		return ERR_PTR(-EINVAL);
 
+	/* Lookup returns a pointer straight to dev->ifindex, so make sure the
+	 * verifier prevents writes from the BPF side
+	 */
+	attr->map_flags |= BPF_F_RDONLY_PROG;
+
 	dtab = kzalloc(sizeof(*dtab), GFP_USER);
 	if (!dtab)
 		return ERR_PTR(-ENOMEM);

kernel/bpf/verifier.c

@@ -3414,12 +3414,9 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
 		if (func_id != BPF_FUNC_get_local_storage)
 			goto error;
 		break;
-	/* devmap returns a pointer to a live net_device ifindex that we cannot
-	 * allow to be modified from bpf side. So do not allow lookup elements
-	 * for now.
-	 */
 	case BPF_MAP_TYPE_DEVMAP:
-		if (func_id != BPF_FUNC_redirect_map)
+		if (func_id != BPF_FUNC_redirect_map &&
+		    func_id != BPF_FUNC_map_lookup_elem)
 			goto error;
 		break;
 	/* Restrict bpf side of cpumap and xskmap, open when use-cases