cifs: allow disabling insecure dialects in the config

allow disabling cifs (SMB1 ie vers=1.0) and vers=2.0 in the
config for the build of cifs.ko if want to always prevent mounting
with these less secure dialects.
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>

fs/cifs/Kconfig

@@ -66,9 +66,24 @@ config CIFS_STATS2
 	  Unless you are a developer or are doing network performance analysis
 	  or tuning, say N.
 
+config CIFS_ALLOW_INSECURE_LEGACY
+	bool "Support legacy servers which use less secure dialects"
+	depends on CIFS
+	default y
+	help
+	  Modern dialects, SMB2.1 and later (including SMB3 and 3.1.1), have
+	  additional security features, including protection against
+	  man-in-the-middle attacks and stronger crypto hashes, so the use
+	  of legacy dialects (SMB1/CIFS and SMB2.0) is discouraged.
+
+	  Disabling this option prevents users from using vers=1.0 or vers=2.0
+	  on mounts with cifs.ko
+
+	  If unsure, say Y.
+
 config CIFS_WEAK_PW_HASH
 	bool "Support legacy servers which use weaker LANMAN security"
-	depends on CIFS
+	depends on CIFS && CIFS_ALLOW_INSECURE_LEGACY
 	help
 	  Modern CIFS servers including Samba and most Windows versions
 	  (since 1997) support stronger NTLM (and even NTLMv2 and Kerberos)

fs/cifs/connect.c

@@ -1174,6 +1174,7 @@ cifs_parse_smb_version(char *value, struct smb_vol *vol, bool is_smb3)
 	substring_t args[MAX_OPT_ARGS];
 
 	switch (match_token(value, cifs_smb_version_tokens, args)) {
+#ifdef CONFIG_CIFS_ALLOW_INSECURE_LEGACY
 	case Smb_1:
 		if (disable_legacy_dialects) {
 			cifs_dbg(VFS, "mount with legacy dialect disabled\n");
@@ -1198,6 +1199,14 @@ cifs_parse_smb_version(char *value, struct smb_vol *vol, bool is_smb3)
 		vol->ops = &smb20_operations;
 		vol->vals = &smb20_values;
 		break;
+#else
+	case Smb_1:
+		cifs_dbg(VFS, "vers=1.0 (cifs) mount not permitted when legacy dialects disabled\n");
+		return 1;
+	case Smb_20:
+		cifs_dbg(VFS, "vers=2.0 mount not permitted when legacy dialects disabled\n");
+		return 1;
+#endif /* CIFS_ALLOW_INSECURE_LEGACY */
 	case Smb_21:
 		vol->ops = &smb21_operations;
 		vol->vals = &smb21_values;