[PATCH net-next 2/2] devmap: Allow map lookups from eBPF

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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.

To allow lookups, simply copy the ifindex into a scratch variable and
return a pointer to this. If an eBPF program does modify it, this doesn't
matter since it will be overridden on the next lookup anyway. While this
does add a write to every lookup, the overhead of this is negligible
because the cache line is hot when both the write and the subsequent read
happens.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 kernel/bpf/devmap.c   |    8 +++++++-
 kernel/bpf/verifier.c |    7 ++-----
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 5ae7cce5ef16..830650300ea4 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -65,6 +65,7 @@ struct xdp_bulk_queue {
 struct bpf_dtab_netdev {
 	struct net_device *dev; /* must be first member, due to tracepoint */
 	struct bpf_dtab *dtab;
+	int ifindex_scratch;
 	unsigned int bit;
 	struct xdp_bulk_queue __percpu *bulkq;
 	struct rcu_head rcu;
@@ -375,7 +376,12 @@ static void *dev_map_lookup_elem(struct bpf_map *map, void *key)
 	struct bpf_dtab_netdev *obj = __dev_map_lookup_elem(map, *(u32 *)key);
 	struct net_device *dev = obj ? obj->dev : NULL;
 
-	return dev ? &dev->ifindex : NULL;
+	if (dev) {
+		obj->ifindex_scratch = dev->ifindex;
+		return &obj->ifindex_scratch;
+	}
+
+	return NULL;
 }
 
 static void dev_map_flush_old(struct bpf_dtab_netdev *dev)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5c2cb5bd84ce..7128a9821481 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2893,12 +2893,9 @@ 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