support SSH2_AGENTC_ADD_ID_CONSTRAINED for sk-provider@openssh.com (#612)

* support SSH2_AGENTC_ADD_ID_CONSTRAINED by treating it as SSH2_AGENTC_ADD_IDENTITY

This ignores the requested constraints:
- SSH_AGENT_CONSTRAIN_LIFETIME
- SSH_AGENT_CONSTRAIN_CONFIRM
- SSH_AGENT_CONSTRAIN_MAXSIGN
- SSH_AGENT_CONSTRAIN_EXTENSION

SSH2_AGENTC_ADD_ID_CONSTRAINED is needed to support add U2F/Fido2 ssh keys to the agent from WSL ssh-add and KeePassXC
ref PowerShell/Win32-OpenSSH#1961

* update buffer pointer to after comment string

sshbuf_peek_string_direct doesn't update request offset pointer

* parse agent constraint messages

returns SSH_AGENT_FAILURE on unsupported constraint types, such as:
* SSH_AGENT_CONSTRAIN_LIFETIME
* SSH_AGENT_CONSTRAIN_CONFIRM
* SSH_AGENT_CONSTRAIN_MAXSIGN

returns SSH_AGENT_FAILURE on unsupported constrain extensions, such as:
"restrict-destination-v00@openssh.com"

accepts and ignores constrain extension "sk-provider@openssh.com"

* reject non-internal skproviders & log

contrib/win32/win32compat/ssh-agent/connection.c

@@ -143,6 +143,7 @@ process_request(struct agent_connection* con)
 		r = process_unsupported_request(request, response, con);
 		break;
 	case SSH2_AGENTC_ADD_IDENTITY:
+	case SSH2_AGENTC_ADD_ID_CONSTRAINED:
 		r =  process_add_identity(request, response, con);
 		break;
 	case SSH2_AGENTC_REQUEST_IDENTITIES:

contrib/win32/win32compat/ssh-agent/keyagent-request.c

@@ -224,6 +224,64 @@ process_unsupported_request(struct sshbuf* request, struct sshbuf* response, str
 	return r;
 }
 
+static int
+parse_key_constraint_extension(struct sshbuf *m)
+{
+	char *ext_name = NULL, *skprovider = NULL;
+	int r;
+
+	if ((r = sshbuf_get_cstring(m, &ext_name, NULL)) != 0) {
+		error_fr(r, "parse constraint extension");
+		goto out;
+	}
+	debug_f("constraint ext %s", ext_name);
+	if (strcmp(ext_name, "sk-provider@openssh.com") == 0) {
+		if ((r = sshbuf_get_cstring(m, &skprovider, NULL)) != 0) {
+			error_fr(r, "parse %s", ext_name);
+			goto out;
+		}
+		if (strcmp(skprovider, "internal") != 0) {
+			error_f("unsupported sk-provider: %s", skprovider);
+			r = SSH_ERR_FEATURE_UNSUPPORTED;
+			goto out;
+		}
+	} else {
+		error_f("unsupported constraint \"%s\"", ext_name);
+		r = SSH_ERR_FEATURE_UNSUPPORTED;
+		goto out;
+	}
+	/* success */
+	r = 0;
+ out:
+	free(ext_name);
+	return r;
+}
+
+static int
+parse_key_constraints(struct sshbuf *m)
+{
+	int r;
+	u_char ctype;
+
+	while (sshbuf_len(m)) {
+		if ((r = sshbuf_get_u8(m, &ctype)) != 0) {
+			error("get constraint type returned %d", r);
+			return r;
+		}
+		switch (ctype) {
+		case SSH_AGENT_CONSTRAIN_EXTENSION:
+			if ((r = parse_key_constraint_extension(m)) != 0)
+				return r;
+			break;
+		default:
+			error("Unknown constraint %d", ctype);
+			return SSH_ERR_FEATURE_UNSUPPORTED;
+		}
+	}
+
+	return 0;
+}
+
 int
 process_add_identity(struct sshbuf* request, struct sshbuf* response, struct agent_connection* con) 
 {
@@ -242,12 +300,18 @@ process_add_identity(struct sshbuf* request, struct sshbuf* response, struct age
 	blob = sshbuf_ptr(request);
 	if (sshkey_private_deserialize(request, &key) != 0 ||
 	   (blob_len = (sshbuf_ptr(request) - blob) & 0xffffffff) == 0 ||
-	    sshbuf_peek_string_direct(request, &comment, &comment_len) != 0) {
+	    sshbuf_get_cstring(request, &comment, &comment_len) != 0) {
 		debug("key add request is invalid");
 		request_invalid = 1;
 		goto done;
 	}
 
+	if ((r = parse_key_constraints(request)) != 0) {
+		if (r != SSH_ERR_FEATURE_UNSUPPORTED)
+			request_invalid = 1;
+		goto done;
+	}
+
 	memset(&sa, 0, sizeof(SECURITY_ATTRIBUTES));
 	sa.nLength = sizeof(sa);
 	if ((!ConvertStringSecurityDescriptorToSecurityDescriptorW(REG_KEY_SDDL, SDDL_REVISION_1, &sa.lpSecurityDescriptor, &sa.nLength)) ||