cls_flower: call nla_ok() before nla_next()

fl_set_enc_opt() simply checks if there are still bytes left to parse,
but this is not sufficent as syzbot seems to be able to generate
malformatted netlink messages. nla_ok() is more strict so should be
used to validate the next nlattr here.

And nla_validate_nested_deprecated() has less strict check too, it is
probably too late to switch to the strict version, but we can just
call nla_ok() too after it.

Reported-and-tested-by: syzbot+2624e3778b18fc497c92@syzkaller.appspotmail.com
Fixes: 0a6e7778 ("net/sched: allow flower to match tunnel options")
Fixes: 79b1011c ("net: sched: allow flower to match erspan options")
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Link: https://lore.kernel.org/r/20210115185024.72298-1-xiyou.wangcong@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/cls_flower.c

@@ -1272,6 +1272,10 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
 
 		nla_opt_msk = nla_data(tb[TCA_FLOWER_KEY_ENC_OPTS_MASK]);
 		msk_depth = nla_len(tb[TCA_FLOWER_KEY_ENC_OPTS_MASK]);
+		if (!nla_ok(nla_opt_msk, msk_depth)) {
+			NL_SET_ERR_MSG(extack, "Invalid nested attribute for masks");
+			return -EINVAL;
+		}
 	}
 
 	nla_for_each_attr(nla_opt_key, nla_enc_key,
@@ -1307,9 +1311,6 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
 				NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
 				return -EINVAL;
 			}
-
-			if (msk_depth)
-				nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
 			break;
 		case TCA_FLOWER_KEY_ENC_OPTS_VXLAN:
 			if (key->enc_opts.dst_opt_type) {
@@ -1340,9 +1341,6 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
 				NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
 				return -EINVAL;
 			}
-
-			if (msk_depth)
-				nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
 			break;
 		case TCA_FLOWER_KEY_ENC_OPTS_ERSPAN:
 			if (key->enc_opts.dst_opt_type) {
@@ -1373,14 +1371,20 @@ static int fl_set_enc_opt(struct nlattr **tb, struct fl_flow_key *key,
 				NL_SET_ERR_MSG(extack, "Key and mask miss aligned");
 				return -EINVAL;
 			}
-
-			if (msk_depth)
-				nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
 			break;
 		default:
 			NL_SET_ERR_MSG(extack, "Unknown tunnel option type");
 			return -EINVAL;
 		}
+
+		if (!msk_depth)
+			continue;
+
+		if (!nla_ok(nla_opt_msk, msk_depth)) {
+			NL_SET_ERR_MSG(extack, "A mask attribute is invalid");
+			return -EINVAL;
+		}
+		nla_opt_msk = nla_next(nla_opt_msk, &msk_depth);
 	}
 
 	return 0;