smb3.1.1: add new module load parm require_gcm_256

Add new module load parameter require_gcm_256. If set, then only
request AES-256-GCM (strongest encryption type).
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsfs.c

@@ -71,6 +71,7 @@ bool enable_oplocks = true;
 bool linuxExtEnabled = true;
 bool lookupCacheEnabled = true;
 bool disable_legacy_dialects; /* false by default */
+bool require_gcm_256; /* false by default */
 unsigned int global_secflags = CIFSSEC_DEF;
 /* unsigned int ntlmv2_support = 0; */
 unsigned int sign_CIFS_PDUs = 1;
@@ -104,6 +105,9 @@ MODULE_PARM_DESC(slow_rsp_threshold, "Amount of time (in seconds) to wait "
 module_param(enable_oplocks, bool, 0644);
 MODULE_PARM_DESC(enable_oplocks, "Enable or disable oplocks. Default: y/Y/1");
 
+module_param(require_gcm_256, bool, 0644);
+MODULE_PARM_DESC(require_gcm_256, "Require strongest (256 bit) GCM encryption. Default: n/N/0");
+
 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 "

fs/cifs/cifsglob.h

@@ -1956,6 +1956,7 @@ extern bool lookupCacheEnabled;
 extern unsigned int global_secflags;	/* if on, session setup sent
 				with more secure ntlmssp2 challenge/resp */
 extern unsigned int sign_CIFS_PDUs;  /* enable smb packet signing */
+extern bool require_gcm_256; /* require use of strongest signing (aes-gcm-256) */
 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 */

fs/cifs/smb2pdu.c

@@ -449,10 +449,16 @@ static void
 build_encrypt_ctxt(struct smb2_encryption_neg_context *pneg_ctxt)
 {
 	pneg_ctxt->ContextType = SMB2_ENCRYPTION_CAPABILITIES;
-	pneg_ctxt->DataLength = cpu_to_le16(6); /* Cipher Count + two ciphers */
+	if (require_gcm_256) {
+		pneg_ctxt->DataLength = cpu_to_le16(4); /* Cipher Count + 1 cipher */
+		pneg_ctxt->CipherCount = cpu_to_le16(1);
+		pneg_ctxt->Ciphers[0] = SMB2_ENCRYPTION_AES256_GCM;
+	} else {
+		pneg_ctxt->DataLength = cpu_to_le16(6); /* Cipher Count + 2 ciphers */
 		pneg_ctxt->CipherCount = cpu_to_le16(2);
 		pneg_ctxt->Ciphers[0] = SMB2_ENCRYPTION_AES128_GCM;
 		pneg_ctxt->Ciphers[1] = SMB2_ENCRYPTION_AES128_CCM;
+	}
 }
 
 static unsigned int