KEYS: Add a key type op to permit the key description to be vetted

Add a key type operation to permit the key type to vet the description of a new
key that key_alloc() is about to allocate. The operation may reject the
description if it wishes with an error of its choosing. If it does this, the
key will not be allocated.

Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Mimi Zohar <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

Documentation/keys.txt

···
      viable.
 
 
+ (*) int (*vet_description)(const char *description);
+
+     This optional method is called to vet a key description.  If the key type
+     doesn't approve of the key description, it may return an error, otherwise
+     it should return 0.
+
+
  (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
 
      This method is called to attach a payload to a key during construction.

include/linux/key-type.h

···
 	 */
 	size_t def_datalen;
 
+	/* vet a description */
+	int (*vet_description)(const char *description);
+
 	/* instantiate a key of this type
 	 * - this method should call key_payload_reserve() to determine if the
 	 *   user's quota will hold the payload

net/rxrpc/ar-key.c

···
 #include <keys/user-type.h>
 #include "ar-internal.h"
 
+static int rxrpc_vet_description_s(const char *);
 static int rxrpc_instantiate(struct key *, const void *, size_t);
 static int rxrpc_instantiate_s(struct key *, const void *, size_t);
 static void rxrpc_destroy(struct key *);
···
  */
 struct key_type key_type_rxrpc_s = {
 	.name		= "rxrpc_s",
+	.vet_description = rxrpc_vet_description_s,
 	.instantiate	= rxrpc_instantiate_s,
 	.match		= user_match,
 	.destroy	= rxrpc_destroy_s,
 	.describe	= rxrpc_describe,
 };
+
+/*
+ * Vet the description for an RxRPC server key
+ */
+static int rxrpc_vet_description_s(const char *desc)
+{
+	unsigned long num;
+	char *p;
+
+	num = simple_strtoul(desc, &p, 10);
+	if (*p != ':' || num > 65535)
+		return -EINVAL;
+	num = simple_strtoul(p + 1, &p, 10);
+	if (*p || num < 1 || num > 255)
+		return -EINVAL;
+	return 0;
+}
 
 /*
  * parse an RxKAD type XDR format token

security/keys/key.c

···
 	if (!desc || !*desc)
 		goto error;
 
+	if (type->vet_description) {
+		ret = type->vet_description(desc);
+		if (ret < 0) {
+			key = ERR_PTR(ret);
+			goto error;
+		}
+	}
+
 	desclen = strlen(desc) + 1;
 	quotalen = desclen + type->def_datalen;