ksmbd: fix null pointer dereference error in generate_encryptionkey

If client send two session setups with krb5 authenticate to ksmbd,
null pointer dereference error in generate_encryptionkey could happen.
sess->Preauth_HashValue is set to NULL if session is valid.
So this patch skip generate encryption key if session is valid.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-27654
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/server/smb2pdu.c

@@ -1621,11 +1621,24 @@ static int krb5_authenticate(struct ksmbd_work *work,
 
 	rsp->SecurityBufferLength = cpu_to_le16(out_len);
 
-	if ((conn->sign || server_conf.enforced_signing) ||
+	/*
+	 * If session state is SMB2_SESSION_VALID, We can assume
+	 * that it is reauthentication. And the user/password
+	 * has been verified, so return it here.
+	 */
+	if (sess->state == SMB2_SESSION_VALID) {
+		if (conn->binding)
+			goto binding_session;
+		return 0;
+	}
+
+	if ((rsp->SessionFlags != SMB2_SESSION_FLAG_IS_GUEST_LE &&
+	    (conn->sign || server_conf.enforced_signing)) ||
 	    (req->SecurityMode & SMB2_NEGOTIATE_SIGNING_REQUIRED))
 		sess->sign = true;
 
-	if (smb3_encryption_negotiated(conn)) {
+	if (smb3_encryption_negotiated(conn) &&
+	    !(req->Flags & SMB2_SESSION_REQ_FLAG_BINDING)) {
 		retval = conn->ops->generate_encryptionkey(conn, sess);
 		if (retval) {
 			ksmbd_debug(SMB,
@@ -1638,6 +1651,7 @@ static int krb5_authenticate(struct ksmbd_work *work,
 		sess->sign = false;
 	}
 
+binding_session:
 	if (conn->dialect >= SMB30_PROT_ID) {
 		chann = lookup_chann_list(sess, conn);
 		if (!chann) {