ksmbd: add support for key exchange

When mounting cifs client, can see the following warning message.

CIFS: decode_ntlmssp_challenge: authentication has been weakened as server
does not support key exchange

To remove this warning message, Add support for key exchange feature to
ksmbd. This patch decrypts 16-byte ciphertext value sent by the client
using RC4 with session key. The decrypted value is the recovered secondary
key that will use instead of the session key for signing and sealing.
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/Kconfig

@@ -369,8 +369,8 @@ source "fs/ksmbd/Kconfig"
 
 config SMBFS_COMMON
 	tristate
-	default y if CIFS=y
-	default m if CIFS=m
+	default y if CIFS=y || SMB_SERVER=y
+	default m if CIFS=m || SMB_SERVER=m
 
 source "fs/coda/Kconfig"
 source "fs/afs/Kconfig"

fs/ksmbd/auth.c

@@ -29,6 +29,7 @@
 #include "mgmt/user_config.h"
 #include "crypto_ctx.h"
 #include "transport_ipc.h"
+#include "../smbfs_common/arc4.h"
 
 /*
  * Fixed format data defining GSS header and fixed string
@@ -336,6 +337,29 @@ int ksmbd_decode_ntlmssp_auth_blob(struct authenticate_message *authblob,
 				nt_len - CIFS_ENCPWD_SIZE,
 				domain_name, conn->ntlmssp.cryptkey);
 	kfree(domain_name);
+
+	/* The recovered secondary session key */
+	if (conn->ntlmssp.client_flags & NTLMSSP_NEGOTIATE_KEY_XCH) {
+		struct arc4_ctx *ctx_arc4;
+		unsigned int sess_key_off, sess_key_len;
+
+		sess_key_off = le32_to_cpu(authblob->SessionKey.BufferOffset);
+		sess_key_len = le16_to_cpu(authblob->SessionKey.Length);
+
+		if (blob_len < (u64)sess_key_off + sess_key_len)
+			return -EINVAL;
+
+		ctx_arc4 = kmalloc(sizeof(*ctx_arc4), GFP_KERNEL);
+		if (!ctx_arc4)
+			return -ENOMEM;
+
+		cifs_arc4_setkey(ctx_arc4, sess->sess_key,
+				 SMB2_NTLMV2_SESSKEY_SIZE);
+		cifs_arc4_crypt(ctx_arc4, sess->sess_key,
+				(char *)authblob + sess_key_off, sess_key_len);
+		kfree_sensitive(ctx_arc4);
+	}
+
 	return ret;
 }
 
@@ -408,6 +432,9 @@ ksmbd_build_ntlmssp_challenge_blob(struct challenge_message *chgblob,
 	    (cflags & NTLMSSP_NEGOTIATE_EXTENDED_SEC))
 		flags |= NTLMSSP_NEGOTIATE_EXTENDED_SEC;
 
+	if (cflags & NTLMSSP_NEGOTIATE_KEY_XCH)
+		flags |= NTLMSSP_NEGOTIATE_KEY_XCH;
+
 	chgblob->NegotiateFlags = cpu_to_le32(flags);
 	len = strlen(ksmbd_netbios_name());
 	name = kmalloc(2 + UNICODE_LEN(len), GFP_KERNEL);