SMB3.1.1: do not log warning message if server doesn't populate salt

In the negotiate protocol preauth context, the server is not required
to populate the salt (although it is done by most servers) so do
not warn on mount.

We retain the checks (warn) that the preauth context is the minimum
size and that the salt does not exceed DataLength of the SMB response.
Although we use the defaults in the case that the preauth context
response is invalid, these checks may be useful in the future
as servers add support for additional mechanisms.

CC: Stable <stable@vger.kernel.org>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -427,8 +427,8 @@ build_preauth_ctxt(struct smb2_preauth_neg_context *pneg_ctxt)
 	pneg_ctxt->ContextType = SMB2_PREAUTH_INTEGRITY_CAPABILITIES;
 	pneg_ctxt->DataLength = cpu_to_le16(38);
 	pneg_ctxt->HashAlgorithmCount = cpu_to_le16(1);
-	pneg_ctxt->SaltLength = cpu_to_le16(SMB311_SALT_SIZE);
-	get_random_bytes(pneg_ctxt->Salt, SMB311_SALT_SIZE);
+	pneg_ctxt->SaltLength = cpu_to_le16(SMB311_LINUX_CLIENT_SALT_SIZE);
+	get_random_bytes(pneg_ctxt->Salt, SMB311_LINUX_CLIENT_SALT_SIZE);
 	pneg_ctxt->HashAlgorithms = SMB2_PREAUTH_INTEGRITY_SHA512;
 }
 
@@ -566,6 +566,9 @@ static void decode_preauth_context(struct smb2_preauth_neg_context *ctxt)
 	if (len < MIN_PREAUTH_CTXT_DATA_LEN) {
 		pr_warn_once("server sent bad preauth context\n");
 		return;
+	} else if (len < MIN_PREAUTH_CTXT_DATA_LEN + le16_to_cpu(ctxt->SaltLength)) {
+		pr_warn_once("server sent invalid SaltLength\n");
+		return;
 	}
 	if (le16_to_cpu(ctxt->HashAlgorithmCount) != 1)
 		pr_warn_once("Invalid SMB3 hash algorithm count\n");

fs/cifs/smb2pdu.h

@@ -333,12 +333,20 @@ struct smb2_neg_context {
 	/* Followed by array of data */
 } __packed;
 
-#define SMB311_SALT_SIZE			32
+#define SMB311_LINUX_CLIENT_SALT_SIZE			32
 /* Hash Algorithm Types */
 #define SMB2_PREAUTH_INTEGRITY_SHA512	cpu_to_le16(0x0001)
 #define SMB2_PREAUTH_HASH_SIZE 64
 
-#define MIN_PREAUTH_CTXT_DATA_LEN	(SMB311_SALT_SIZE + 6)
+/*
+ * SaltLength that the server send can be zero, so the only three required
+ * fields (all __le16) end up six bytes total, so the minimum context data len
+ * in the response is six bytes which accounts for
+ *
+ *      HashAlgorithmCount, SaltLength, and 1 HashAlgorithm.
+ */
+#define MIN_PREAUTH_CTXT_DATA_LEN 6
+
 struct smb2_preauth_neg_context {
 	__le16	ContextType; /* 1 */
 	__le16	DataLength;
@@ -346,7 +354,7 @@ struct smb2_preauth_neg_context {
 	__le16	HashAlgorithmCount; /* 1 */
 	__le16	SaltLength;
 	__le16	HashAlgorithms; /* HashAlgorithms[0] since only one defined */
-	__u8	Salt[SMB311_SALT_SIZE];
+	__u8	Salt[SMB311_LINUX_CLIENT_SALT_SIZE];
 } __packed;
 
 /* Encryption Algorithms Ciphers */