Merge commit from fork

Bound both the Vec allocation and loop iteration count by the remaining
packet data when parsing SSH_MSG_USERAUTH_INFO_RESPONSE. Previously, the
response count was read as a u32 from the client and passed directly to
Vec::with_capacity(), allowing a malicious client to trigger a multi-GB
allocation with a single small packet, crashing the server via OOM.

The fix caps the count to the maximum number of responses that could
fit in the remaining packet data (each needs at least 4 bytes for a
length prefix), bounding allocation to at most the packet size (~256KB).

Author: coreyleavitt

russh/src/server/encrypted.rs

@@ -522,7 +522,13 @@ async fn read_userauth_info_response<H: Handler + Send, R: Reader>(
     if let Some(CurrentRequest::KeyboardInteractive { ref submethods }) = auth_request.current {
         let n = map_err!(u32::decode(r))?;
 
-        let mut responses = Vec::with_capacity(n as usize);
+        // Bound both allocation and iteration by remaining packet data to
+        // prevent a malicious client from causing a multi-GB allocation or
+        // billions of loop iterations with a crafted count.
+        // Each response needs at least 4 bytes (length prefix).
+        let max_responses = r.remaining_len().saturating_add(3) / 4;
+        let n = (n as usize).min(max_responses);
+        let mut responses = Vec::with_capacity(n);
         for _ in 0..n {
             responses.push(Bytes::decode(r).ok())
         }