bpf: fix direct packet access for flow dissector progs

Commit d58e468 ("flow_dissector: implements flow dissector BPF
hook") added direct packet access for skbs in may_access_direct_pkt_data()
function where this enables read and write access to the skb->data. This
is buggy because without a prologue generator such as bpf_unclone_prologue()
we would allow for writing into cloned skbs. Original intention might have
been to only allow read access where this is not needed (similar as the
flow_dissector_func_proto() indicates which enables only bpf_skb_load_bytes()
as well), therefore this patch fixes it to restrict to read-only.

Fixes: d58e468 ("flow_dissector: implements flow dissector BPF hook")
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Cc: Petar Penkov <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>

kernel/bpf/verifier.c

@@ -1387,21 +1387,23 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
 				       enum bpf_access_type t)
 {
 	switch (env->prog->type) {
+	/* Program types only with direct read access go here! */
 	case BPF_PROG_TYPE_LWT_IN:
 	case BPF_PROG_TYPE_LWT_OUT:
 	case BPF_PROG_TYPE_LWT_SEG6LOCAL:
 	case BPF_PROG_TYPE_SK_REUSEPORT:
-		/* dst_input() and dst_output() can't write for now */
+	case BPF_PROG_TYPE_FLOW_DISSECTOR:
 		if (t == BPF_WRITE)
 			return false;
 		/* fallthrough */
+
+	/* Program types with direct read + write access go here! */
 	case BPF_PROG_TYPE_SCHED_CLS:
 	case BPF_PROG_TYPE_SCHED_ACT:
 	case BPF_PROG_TYPE_XDP:
 	case BPF_PROG_TYPE_LWT_XMIT:
 	case BPF_PROG_TYPE_SK_SKB:
 	case BPF_PROG_TYPE_SK_MSG:
-	case BPF_PROG_TYPE_FLOW_DISSECTOR:
 		if (meta)
 			return meta->pkt_access;