net/sched: flower: fix possible OOB write in fl_set_geneve_opt()

If we send two TCA_FLOWER_KEY_ENC_OPTS_GENEVE packets and their total
size is 252 bytes(key->enc_opts.len = 252) then
key->enc_opts.len = opt->length = data_len / 4 = 0 when the third
TCA_FLOWER_KEY_ENC_OPTS_GENEVE packet enters fl_set_geneve_opt. This
bypasses the next bounds check and results in an out-of-bounds.

Fixes: 0a6e7778 ("net/sched: allow flower to match tunnel options")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansen-van-vuuren@amd.com>
Link: https://lore.kernel.org/r/20230531102805.27090-1-hbh25y@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/sched/cls_flower.c

@@ -1153,6 +1153,9 @@ static int fl_set_geneve_opt(const struct nlattr *nla, struct fl_flow_key *key,
 	if (option_len > sizeof(struct geneve_opt))
 		data_len = option_len - sizeof(struct geneve_opt);
 
+	if (key->enc_opts.len > FLOW_DIS_TUN_OPTS_MAX - 4)
+		return -ERANGE;
+
 	opt = (struct geneve_opt *)&key->enc_opts.data[key->enc_opts.len];
 	memset(opt, 0xff, option_len);
 	opt->length = data_len / 4;