iscsi-target; Enforce 1024 byte maximum for CHAP_C key value

This patch adds a check in chap_server_compute_md5() to enforce a
1024 byte maximum for the CHAP_C key value following the requirement
in RFC-3720 Section 11.1.4:

   "..., C and R are large-binary-values and their binary length (not
   the length of the character string that represents them in encoded
   form) MUST not exceed 1024 bytes."
Reported-by: rahul.rane <rahul.rane@calsoftinc.com>
Tested-by: rahul.rane <rahul.rane@calsoftinc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_auth.c

@@ -355,6 +355,10 @@ static int chap_server_compute_md5(
 		pr_err("Unable to convert incoming challenge\n");
 		goto out;
 	}
+	if (challenge_len > 1024) {
+		pr_err("CHAP_C exceeds maximum binary size of 1024 bytes\n");
+		goto out;
+	}
 	/*
 	 * During mutual authentication, the CHAP_C generated by the
 	 * initiator must not match the original CHAP_C generated by