fix(host_metrics source): add defensive check to prevent panics (#22604)

* fix(host_metrics source): avoid panic when reading from buffer

* tweaks

* add docs because these functions are complex

* changelog

changelog.d/host_metrics_tcp_panic.fix.md

@@ -0,0 +1,3 @@
+Fix potential panic in the `host_metrics` source when collecting TCP metrics.
+
+authors: pront

src/sources/host_metrics/tcp.rs

@@ -71,6 +71,8 @@ enum TcpError {
     InvalidTcpState { state: u8 },
     #[snafu(display("Received an error message from netlink; code: {code}"))]
     NetlinkMsgError { code: i32 },
+    #[snafu(display("Invalid message length: {length}"))]
+    InvalidLength { length: usize },
 }
 
 #[repr(u8)]
@@ -134,7 +136,84 @@ struct TcpStats {
     tx_queued_bytes: f64,
 }
 
-async fn fetch_nl_inet_hdrs(addr_family: u8) -> Result<Vec<InetResponseHeader>, TcpError> {
+/// Parses Netlink messages from a buffer, extracting [`InetResponseHeader`]s.
+///
+/// # Arguments
+/// * `buffer` - Raw byte slice containing Netlink messages.
+/// * `headers` - Mutable vector to store parsed [`InetResponseHeader`]s.
+///
+/// # Returns
+/// * `Ok(true)` if parsing is complete (Done message received).
+/// * `Ok(false)` if more data is expected. In this case, this function can be called again.
+/// * `Err(TcpError)` on invalid length, deserialization failure, or Netlink error.
+///
+/// # Errors
+/// Returns [`TcpError`] variants for invalid message lengths or Netlink errors.
+fn parse_netlink_messages(
+    buffer: &[u8],
+    headers: &mut Vec<InetResponseHeader>,
+) -> Result<bool, TcpError> {
+    let mut offset = 0;
+    let mut done = false;
+
+    while offset < buffer.len() {
+        let remaining_bytes = &buffer[offset..];
+        if remaining_bytes.len() < 4 {
+            // Still treat this as an error since we can't even read the length
+            return Err(TcpError::InvalidLength {
+                length: remaining_bytes.len(),
+            });
+        }
+        // This function panics if the buffer length is less than 4.
+        let length = NativeEndian::read_u32(&remaining_bytes[0..4]) as usize;
+        if length == 0 {
+            break;
+        }
+        if length > remaining_bytes.len() {
+            return Err(TcpError::InvalidLength { length });
+        }
+
+        let msg_bytes = &remaining_bytes[..length];
+        let rx_packet =
+            <NetlinkMessage<SockDiagMessage>>::deserialize(msg_bytes).context(NetlinkParseSnafu)?;
+
+        match rx_packet.payload {
+            NetlinkPayload::InnerMessage(SockDiagMessage::InetResponse(response)) => {
+                headers.push(response.header);
+            }
+            NetlinkPayload::Done(_) => {
+                done = true;
+                break;
+            }
+            NetlinkPayload::Error(error) => {
+                if let Some(code) = error.code {
+                    return Err(TcpError::NetlinkMsgError { code: code.get() });
+                }
+            }
+            _ => {}
+        }
+
+        offset += length;
+    }
+
+    Ok(done)
+}
+
+/// Fetches [`InetResponseHeader`]s for TCP sockets using Netlink.
+///
+/// # Arguments
+/// * `addr_family` - Address family (`AF_INET` for IPv4, `AF_INET6` for IPv6).
+///
+/// # Returns
+/// * `Ok(Vec<InetResponseHeader>)` containing headers for active TCP sockets.
+/// * `Err(TcpError)` on socket creation, send, receive, or parsing errors.
+///
+/// # Errors
+/// Returns [`TcpError`] for socket-related or message parsing failures.
+///
+/// # Notes
+/// Asynchronously queries the kernel via a Netlink socket for TCP socket info.
+async fn fetch_netlink_inet_headers(addr_family: u8) -> Result<Vec<InetResponseHeader>, TcpError> {
     let unicast_socket: SocketAddr = SocketAddr::new(0, 0);
     let mut socket = TokioSocket::new(NETLINK_SOCK_DIAG).context(NetlinkSocketSnafu)?;
 
@@ -163,34 +242,12 @@ async fn fetch_nl_inet_hdrs(addr_family: u8) -> Result<Vec<InetResponseHeader>,
         .context(NetlinkSendSnafu)?;
 
     let mut receive_buffer = vec![0; 4096];
-    let mut inet_resp_hdrs: Vec<InetResponseHeader> = Vec::new();
-    'outer: while let Ok(()) = socket.recv(&mut &mut receive_buffer[..]).await {
-        let mut offset = 0;
-        'inner: loop {
-            let bytes = &receive_buffer[offset..];
-            let length = NativeEndian::read_u32(&bytes[0..4]) as usize;
-            if length == 0 {
-                break 'inner;
-            }
-            let rx_packet =
-                <NetlinkMessage<SockDiagMessage>>::deserialize(bytes).context(NetlinkParseSnafu)?;
-
-            match rx_packet.payload {
-                NetlinkPayload::InnerMessage(SockDiagMessage::InetResponse(response)) => {
-                    inet_resp_hdrs.push(response.header);
-                }
-                NetlinkPayload::Done(_) => {
-                    break 'outer;
-                }
-                NetlinkPayload::Error(error) => {
-                    if let Some(code) = error.code {
-                        return Err(TcpError::NetlinkMsgError { code: code.get() });
-                    }
-                }
-                _ => {}
-            }
+    let mut inet_resp_hdrs = Vec::with_capacity(32); // Pre-allocate with an estimate
 
-            offset += rx_packet.header.length as usize;
+    while let Ok(()) = socket.recv(&mut &mut receive_buffer[..]).await {
+        let done = parse_netlink_messages(&receive_buffer, &mut inet_resp_hdrs)?;
+        if done {
+            break;
         }
     }
 
@@ -214,11 +271,11 @@ fn parse_nl_inet_hdrs(
 
 async fn build_tcp_stats() -> Result<TcpStats, TcpError> {
     let mut tcp_stats = TcpStats::default();
-    let resp = fetch_nl_inet_hdrs(AF_INET).await?;
+    let resp = fetch_netlink_inet_headers(AF_INET).await?;
     parse_nl_inet_hdrs(resp, &mut tcp_stats)?;
 
     if is_ipv6_enabled() {
-        let resp = fetch_nl_inet_hdrs(AF_INET6).await?;
+        let resp = fetch_netlink_inet_headers(AF_INET6).await?;
         parse_nl_inet_hdrs(resp, &mut tcp_stats)?;
     }
 
@@ -239,7 +296,7 @@ mod tests {
     };
 
     use super::{
-        fetch_nl_inet_hdrs, parse_nl_inet_hdrs, TcpStats, STATE, TCP_CONNS_TOTAL,
+        fetch_netlink_inet_headers, parse_nl_inet_hdrs, TcpStats, STATE, TCP_CONNS_TOTAL,
         TCP_RX_QUEUED_BYTES_TOTAL, TCP_TX_QUEUED_BYTES_TOTAL,
     };
     use crate::sources::host_metrics::{HostMetrics, HostMetricsConfig, MetricsBuffer};
@@ -296,7 +353,7 @@ mod tests {
         // initiate a connection
         let _stream = TcpStream::connect(addr).await.unwrap();
 
-        let hdrs = fetch_nl_inet_hdrs(AF_INET).await.unwrap();
+        let hdrs = fetch_netlink_inet_headers(AF_INET).await.unwrap();
         // there should be at least two connections, one for the server and one for the client
         assert!(hdrs.len() >= 2);