GH-891: Fix format of injected SSH_MSG_IGNORE

The message was constructed wrongly as SSH_MSG_IGNORE + (random data)
instead of as SSH_MSG_IGNORE + (length of random data) + (random data).

This is a bug only in the 3.0 branch; in the 2.x branch the packet is
constructed correctly.

Our regression tests failed to catch this because neither Apache MINA
SSHD nor openSSH look at the body of an SSH_MSG_IGNORE packet. Some
other servers do.

Tighten the InjectIgnoreFilterTest to also verify the full packet
format.

Author: tomaswolf

CHANGES.md

@@ -33,6 +33,7 @@ newer Java versions the already built-in cryptographic algorithms for ChaCha20,
 ## Bug Fixes
 
 * [GH-852](https://github.com/apache/mina-sshd/issues/852) Fix wrong import
+* [GH-891](https://github.com/apache/mina-sshd/issues/891) (Regression in 3.0.0-M1) Fix format of injected SSH_MSG_IGNORE
 
 ## Major Code Re-factoring
 

sshd-core/src/main/java/org/apache/sshd/common/session/filters/InjectIgnoreFilter.java

@@ -141,10 +141,12 @@ private int shouldSendIgnore(int cmd) {
 
         private Buffer createIgnoreBuffer(int length) {
             int size = length + random.random(length + 1);
-            Buffer buffer = new ByteArrayBuffer(SshConstants.SSH_PACKET_HEADER_LEN + 1 + size + CryptFilter.MAX_PADDING + 64);
+            Buffer buffer = new ByteArrayBuffer(
+                    SshConstants.SSH_PACKET_HEADER_LEN + 1 + Integer.BYTES + size + CryptFilter.MAX_PADDING + 64);
             buffer.rpos(SshConstants.SSH_PACKET_HEADER_LEN);
             buffer.wpos(SshConstants.SSH_PACKET_HEADER_LEN);
             buffer.putByte(SshConstants.SSH_MSG_IGNORE);
+            buffer.putUInt(size);
             int start = buffer.wpos();
             buffer.wpos(buffer.wpos() + size);
             random.fill(buffer.array(), start, size);

sshd-core/src/test/java/org/apache/sshd/common/session/filters/InjectIgnoreFilterTest.java

@@ -76,15 +76,20 @@ void expectIgnores() throws Exception {
         }
         assertEquals((frequency + 1) * rounds, outputs.outputs.size());
         List<IoWriteFutureWithData> out = outputs.outputs;
+        int foundIgnore = 0;
         for (int i = 0; i < outputs.outputs.size();) {
             for (int j = 0; j < frequency - 1; j++) {
                 Buffer data = out.get(i++).data;
                 assertEquals(-1, data.rawByte(data.rpos()));
             }
             Buffer data = out.get(i++).data;
-            assertEquals(SshConstants.SSH_MSG_IGNORE, data.rawByte(data.rpos()));
+            assertEquals(SshConstants.SSH_MSG_IGNORE, data.getByte());
+            foundIgnore++;
+            long dataLength = data.getUInt();
+            assertEquals(data.available(), dataLength);
             data = out.get(i++).data;
             assertEquals(-1, data.rawByte(data.rpos()));
         }
+        assertEquals(rounds, foundIgnore);
     }
 }