ipv6: sr: validate HMAC algorithm ID in seg6_hmac_info_add

The seg6_genl_sethmac() directly uses the algorithm ID provided by the
userspace without verifying whether it is an HMAC algorithm supported
by the system.
If an unsupported HMAC algorithm ID is configured, packets using SRv6 HMAC
will be dropped during encapsulation or decapsulation.

Fixes: 4f4853d ("ipv6: sr: implement API to control SR HMAC structure")
Signed-off-by: Minhong He <heminhong@kylinos.cn>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250815063845.85426-1-heminhong@kylinos.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Minhong He

net/ipv6/seg6_hmac.c

@@ -304,6 +304,9 @@ int seg6_hmac_info_add(struct net *net, u32 key, struct seg6_hmac_info *hinfo)
 	struct seg6_pernet_data *sdata = seg6_pernet(net);
 	int err;
 
+	if (!__hmac_get_algo(hinfo->alg_id))
+		return -EINVAL;
+
 	err = rhashtable_lookup_insert_fast(&sdata->hmac_infos, &hinfo->node,
 					    rht_params);