lib/net: Limit the maximum size of TCP or SSL input buffers

This commit is the 2/3 part of the fix for the following security
vulnerability:
 - CVE-2021-42076 DoS via excess length messages

The issue has been reported by Matthias Gerstner <mgerstner@suse.de>.

src/lib/net/SecureSocket.cpp

@@ -43,6 +43,7 @@
 
 #define MAX_ERROR_SIZE 65535
 
+static const std::size_t MAX_INPUT_BUFFER_SIZE = 1024 * 1024;
 static const float s_retryDelay = 0.01f;
 
 enum {
@@ -178,6 +179,10 @@ SecureSocket::doRead()
         do {
             m_inputBuffer.write(buffer, bytesRead);
 
+            if (m_inputBuffer.getSize() > MAX_INPUT_BUFFER_SIZE) {
+                break;
+            }
+
             status = secureRead(buffer, sizeof(buffer), bytesRead);
             if (status < 0) {
                 return kBreak;