Reject frames larger than amqpprox max frame size

amqpprox supports frames up to a max size of 150000 bytes but frames
larger than this are not rejected. This patch ensures we reject larger
frames as soon as their length can be read.

libamqpprox/amqpprox_frame.cpp

@@ -54,6 +54,14 @@ bool Frame::decode(Frame       *frame,
     memcpy(&frame->length, &buffer[3], sizeof frame->length);
 
     if ((frameOverhead() + frame->length) > bufferLen) {
+        if ((frameOverhead() + frame->length) > getMaxFrameSize()) {
+            LOG_ERROR << "Frame: " << (int)frame->type << " " << frame->channel
+                      << " was declared to be longer than max frame size ("
+                      << frame->length << " vs " << getMaxFrameSize()
+                      << "). Cannot decode.";
+            throw std::runtime_error(
+                "Cannot decode huge frame. See log for details");
+        }
         return false;
     }
 

tests/amqpprox_frame.t.cpp

@@ -206,3 +206,22 @@ TEST(Frame, Cant_Encode_Payload_Too_Large) {
     EXPECT_FALSE(Frame::encode(output, &sz, f1));
 }
 
+TEST(Frame, BadFrameLength)
+{
+    std::vector<uint8_t> buffer(Frame::getMaxFrameSize());
+    const void          *endOfFrame = nullptr;
+    std::size_t          remaining  = 11111;
+
+    // This frame claims to have a length of 0xff000002 which is bigger than
+    // amqpprox's max supported frame size. We expect a decode error here.
+    const char *hugeFrame = "\x08\x00\x01\x0FF\x00\x00\x02\xFF\xFF\xCD\xFF";
+
+    buffer.assign(hugeFrame, hugeFrame + 11);
+    Frame f1;
+    EXPECT_THROW(
+        Frame::decode(&f1, &endOfFrame, &remaining, buffer.data(), 11),
+        std::runtime_error);
+
+    EXPECT_EQ(remaining, 11111);
+    EXPECT_EQ(endOfFrame, nullptr);
+}