cifs: allow disabling less secure legacy dialects

To improve security it may be helpful to have additional ways to restrict the
ability to override the default dialects (SMB2.1, SMB3 and SMB3.02) on mount
with old dialects (CIFS/SMB1 and SMB2) since vers=1.0 (CIFS/SMB1) and vers=2.0
are weaker and less secure.

Add a module parameter "disable_legacy_dialects"
(/sys/module/cifs/parameters/disable_legacy_dialects) which can be set to
1 (or equivalently Y) to forbid use of vers=1.0 or vers=2.0 on mount.

Also cleans up a few build warnings about globals for various module parms.

Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/cifsfs.c

@@ -58,6 +58,7 @@ bool traceSMB;
 bool enable_oplocks = true;
 bool linuxExtEnabled = true;
 bool lookupCacheEnabled = true;
+bool disable_legacy_dialects; /* false by default */
 unsigned int global_secflags = CIFSSEC_DEF;
 /* unsigned int ntlmv2_support = 0; */
 unsigned int sign_CIFS_PDUs = 1;
@@ -83,6 +84,15 @@ MODULE_PARM_DESC(cifs_max_pending, "Simultaneous requests to server for "
 module_param(enable_oplocks, bool, 0644);
 MODULE_PARM_DESC(enable_oplocks, "Enable or disable oplocks. Default: y/Y/1");
 
+module_param(disable_legacy_dialects, bool, 0644);
+MODULE_PARM_DESC(disable_legacy_dialects, "To improve security it may be "
+				  "helpful to restrict the ability to "
+				  "override the default dialects (SMB2.1, "
+				  "SMB3 and SMB3.02) on mount with old "
+				  "dialects (CIFS/SMB1 and SMB2) since "
+				  "vers=1.0 (CIFS/SMB1) and vers=2.0 are weaker"
+				  " and less secure. Default: n/N/0");
+
 extern mempool_t *cifs_sm_req_poolp;
 extern mempool_t *cifs_req_poolp;
 extern mempool_t *cifs_mid_poolp;

fs/cifs/cifsglob.h

@@ -1700,16 +1700,17 @@ GLOBAL_EXTERN atomic_t smBufAllocCount;
 GLOBAL_EXTERN atomic_t midCount;
 
 /* Misc globals */
-GLOBAL_EXTERN bool enable_oplocks; /* enable or disable oplocks */
-GLOBAL_EXTERN bool lookupCacheEnabled;
-GLOBAL_EXTERN unsigned int global_secflags;	/* if on, session setup sent
+extern bool enable_oplocks; /* enable or disable oplocks */
+extern bool lookupCacheEnabled;
+extern unsigned int global_secflags;	/* if on, session setup sent
 				with more secure ntlmssp2 challenge/resp */
-GLOBAL_EXTERN unsigned int sign_CIFS_PDUs;  /* enable smb packet signing */
-GLOBAL_EXTERN bool linuxExtEnabled;/*enable Linux/Unix CIFS extensions*/
-GLOBAL_EXTERN unsigned int CIFSMaxBufSize;  /* max size not including hdr */
-GLOBAL_EXTERN unsigned int cifs_min_rcv;    /* min size of big ntwrk buf pool */
-GLOBAL_EXTERN unsigned int cifs_min_small;  /* min size of small buf pool */
-GLOBAL_EXTERN unsigned int cifs_max_pending; /* MAX requests at once to server*/
+extern unsigned int sign_CIFS_PDUs;  /* enable smb packet signing */
+extern bool linuxExtEnabled;/*enable Linux/Unix CIFS extensions*/
+extern unsigned int CIFSMaxBufSize;  /* max size not including hdr */
+extern unsigned int cifs_min_rcv;    /* min size of big ntwrk buf pool */
+extern unsigned int cifs_min_small;  /* min size of small buf pool */
+extern unsigned int cifs_max_pending; /* MAX requests at once to server*/
+extern bool disable_legacy_dialects;  /* forbid vers=1.0 and vers=2.0 mounts */
 
 #ifdef CONFIG_CIFS_ACL
 GLOBAL_EXTERN struct rb_root uidtree;

fs/cifs/connect.c

@@ -61,6 +61,7 @@
 #define RFC1001_PORT 139
 
 extern mempool_t *cifs_req_poolp;
+extern bool disable_legacy_dialects;
 
 /* FIXME: should these be tunable? */
 #define TLINK_ERROR_EXPIRE	(1 * HZ)
@@ -1146,10 +1147,18 @@ cifs_parse_smb_version(char *value, struct smb_vol *vol)
 
 	switch (match_token(value, cifs_smb_version_tokens, args)) {
 	case Smb_1:
+		if (disable_legacy_dialects) {
+			cifs_dbg(VFS, "mount with legacy dialect disabled\n");
+			return 1;
+		}
 		vol->ops = &smb1_operations;
 		vol->vals = &smb1_values;
 		break;
 	case Smb_20:
+		if (disable_legacy_dialects) {
+			cifs_dbg(VFS, "mount with legacy dialect disabled\n");
+			return 1;
+		}
 		vol->ops = &smb20_operations;
 		vol->vals = &smb20_values;
 		break;