Add dynamic Softnet policy control (#181)

* Add dynamic Softnet policy control

* Simplify Softnet policy RPC methods

* Use jsonrpsee types for Softnet policy RPC

* Reject superseded Softnet policy revisions

* Apply Softnet policy updates only after enqueue

* Flush final Softnet response after input EOF

* Signal EOF when disabling Softnet control

* Normalize Softnet default-route isolation

* Keep policy rules on Proxy (#182)

* Simplify Softnet policy protocol

* Bound pipelined Softnet policy responses

* Keep Softnet port forwarding active on control wakes

---------

Co-authored-by: edi-oai <edi@openai.com>

Cargo.lock

@@ -1414,6 +1414,18 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "jsonrpsee-types"
+version = "0.26.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc88ff4688e43cc3fa9883a8a95c6fa27aa2e76c96e610b737b6554d650d7fd5"
+dependencies = [
+ "http 1.1.0",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.12",
+]
+
 [[package]]
 name = "language-tags"
 version = "0.3.2"
@@ -2417,14 +2429,15 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.128"
+version = "1.0.151"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6ff5456707a1de34e7e37f2a6fd3d3f808c318259cbd01ab6377795054b483d8"
+checksum = "c841b55ecdae098c80dcae9cf767f6f8a0c2cdb3416bbef72181df4d0fe73f14"
 dependencies = [
  "itoa",
  "memchr",
- "ryu",
  "serde",
+ "serde_core",
+ "zmij",
 ]
 
 [[package]]
@@ -2568,6 +2581,7 @@ dependencies = [
  "dhcproto",
  "ip_network",
  "ipnet",
+ "jsonrpsee-types",
  "libc",
  "log",
  "mac_address",
@@ -2579,6 +2593,8 @@ dependencies = [
  "privdrop",
  "sentry",
  "sentry-anyhow",
+ "serde",
+ "serde_json",
  "serial_test",
  "smoltcp",
  "system-configuration",
@@ -3747,3 +3763,9 @@ dependencies = [
  "quote",
  "syn 2.0.117",
 ]
+
+[[package]]
+name = "zmij"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "29666d0abbfad1e3dc4dcf6144730dd3a3ab225bbbdac83319345b1b44ccfc1b"

Cargo.toml

@@ -34,6 +34,9 @@ oslog = "0.2.0"
 log = "0.4.29"
 serial_test = "3"
 coarsetime = "0.1.37"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+jsonrpsee-types = "0.26"
 
 [profile.release]
 debug = true

README.md

@@ -43,3 +43,18 @@ For proper functioning, Softnet binary requires two things:
 ## Running
 
 Softnet is started and managed automatically by Tart if `--net-softnet` flag is provided when calling `tart run`.
+
+### Dynamic network policy
+
+Softnet can update the running VM's IPv4 egress policy without restarting the VM. Pass a connected Unix stream socket as `--control-fd` to enable a newline-delimited [JSON-RPC 2.0](https://www.jsonrpc.org/specification) control channel. The socket is duplex and must be separate from `--vm-fd`, which carries VM packets.
+
+The supported methods are `softnet.policy.get` and `softnet.policy.set`. A complete policy update looks like this (each request and response occupies one line):
+
+```json
+{"jsonrpc":"2.0","id":"42","method":"softnet.policy.set","params":{"allow":["@host","10.0.0.0/8"],"block":["0.0.0.0/0"]}}
+{"jsonrpc":"2.0","id":"42","result":{"allow":["10.0.0.0/8","@host"],"block":["0.0.0.0/0"],"ruleCount":3}}
+```
+
+Every request must include a non-null string (at most 256 bytes) or non-negative integer `id`; notifications are rejected so policy changes always have an acknowledgment. Policy updates are atomic: all targets are parsed and a new prefix map is built before the active policy changes. Longest-prefix matching and block precedence for identical prefixes are preserved. Targets are normalized and deduplicated. A policy update may contain at most 4096 combined allow/block targets, and a request frame may not exceed 1 MiB.
+
+Use `block=["0.0.0.0/0"]` with specific allow targets for a default-deny policy. Closing the control socket leaves the last accepted policy active.

lib/poller.rs

@@ -12,6 +12,7 @@ pub struct Poller<'poller> {
     timeout: Duration,
     vm_fd: BorrowedFd<'poller>,
     host_fd: BorrowedFd<'poller>,
+    control_fd: Option<BorrowedFd<'poller>>,
 }
 
 #[derive(IntoPrimitive)]
@@ -19,6 +20,7 @@ pub struct Poller<'poller> {
 enum EventKey {
     VM,
     Host,
+    Control,
     Interrupt,
 }
 
@@ -26,6 +28,7 @@ impl Poller<'_> {
     pub fn new<'poller>(
         vm_fd: RawFd,
         host_fd: RawFd,
+        control_fd: Option<RawFd>,
         timeout: Duration,
     ) -> Result<Poller<'poller>> {
         let poller = polling::Poller::new()?;
@@ -36,6 +39,7 @@ impl Poller<'_> {
             timeout,
             vm_fd: unsafe { BorrowedFd::borrow_raw(vm_fd) },
             host_fd: unsafe { BorrowedFd::borrow_raw(host_fd) },
+            control_fd: control_fd.map(|fd| unsafe { BorrowedFd::borrow_raw(fd) }),
         })
     }
 
@@ -46,6 +50,14 @@ impl Poller<'_> {
                 self.vm_interest(),
                 PollMode::Edge,
             )?;
+
+            if let Some(control_fd) = self.control_fd {
+                self.poller.add_with_mode(
+                    control_fd.as_raw_fd(),
+                    polling::Event::all(EventKey::Control.into()),
+                    PollMode::Edge,
+                )?;
+            }
             self.poller.add_with_mode(
                 self.host_fd.as_raw_fd(),
                 self.host_interest(),
@@ -79,10 +91,17 @@ impl Poller<'_> {
             .events
             .iter()
             .any(|ev| ev.key == Into::<usize>::into(EventKey::Interrupt));
-
         Ok((vm_readable, host_readable, interrupt))
     }
 
+    pub fn remove_control(&mut self) -> Result<()> {
+        if let Some(control_fd) = self.control_fd.take() {
+            self.poller.delete(control_fd)?;
+        }
+
+        Ok(())
+    }
+
     fn vm_interest(&self) -> polling::Event {
         polling::Event::readable(EventKey::VM.into())
     }

lib/proxy/mod.rs

@@ -1,3 +1,4 @@
+mod control;
 mod exposed_port;
 mod host;
 mod port_forwarder;
@@ -10,6 +11,7 @@ use crate::host::NetType;
 use crate::poller::Poller;
 use crate::vm::VM;
 use anyhow::Result;
+use control::Control;
 pub use exposed_port::ExposedPort;
 use ipnet::Ipv4Net;
 use mac_address::MacAddress;
@@ -29,6 +31,7 @@ pub struct Proxy<'proxy> {
     vm_mac_address: smoltcp::wire::EthernetAddress,
     dhcp_snooper: DhcpSnooper,
     rules: PrefixMap<Ipv4Net, Action>,
+    control: Option<Control>,
     enobufs_encountered: bool,
     port_forwarder: PortForwarder,
 }
@@ -65,6 +68,7 @@ impl Proxy<'_> {
         allow: Vec<Target>,
         block: Vec<Target>,
         exposed_ports: Vec<ExposedPort>,
+        control_fd: Option<RawFd>,
     ) -> Result<Proxy<'proxy>> {
         let vm = VM::new(vm_fd)?;
         let host = Host::new(
@@ -72,7 +76,17 @@ impl Proxy<'_> {
             !allow.contains(&Target::Prefix(Ipv4Net::zero())),
         )?;
         let poller_timeout = Duration::from_millis(100);
-        let poller = Poller::new(vm.as_raw_fd(), host.as_raw_fd(), poller_timeout)?;
+        let control = control_fd
+            .map(|control_fd| {
+                Control::new(control_fd, host.gateway_ip, allow.clone(), block.clone())
+            })
+            .transpose()?;
+        let poller = Poller::new(
+            vm.as_raw_fd(),
+            host.as_raw_fd(),
+            control.as_ref().map(AsRawFd::as_raw_fd),
+            poller_timeout,
+        )?;
 
         // Craft packet filter rules
         //
@@ -105,6 +119,7 @@ impl Proxy<'_> {
             vm_mac_address: smoltcp::wire::EthernetAddress(vm_mac_address.bytes()),
             dhcp_snooper: DhcpSnooper::new(poller_timeout),
             rules,
+            control,
             enobufs_encountered: false,
             port_forwarder: PortForwarder::new(exposed_ports),
         })
@@ -129,6 +144,10 @@ impl Proxy<'_> {
             // Update coarse time for the DHCP snooper
             coarsetime::Instant::update();
 
+            // Service control on every wake (including timeouts) so a bounded read or a pending
+            // response continues making progress even when no new edge is generated.
+            self.service_control();
+
             if vm_readable {
                 self.read_from_vm(buf.as_mut_slice())?;
             }
@@ -153,6 +172,8 @@ impl Proxy<'_> {
     }
 
     fn read_from_vm(&mut self, buf: &mut [u8]) -> Result<()> {
+        let mut packets_read = 0;
+
         loop {
             match self.vm.read(buf) {
                 Ok(n) => {
@@ -162,6 +183,12 @@ impl Proxy<'_> {
                     if let Ok(frame) = EthernetFrame::new_checked(&buf[..n]) {
                         self.process_frame_from_vm(frame)?;
                     }
+
+                    packets_read += 1;
+                    if packets_read == 128 {
+                        self.service_control();
+                        packets_read = 0;
+                    }
                 }
                 Err(err) => {
                     if err.kind() == ErrorKind::WouldBlock {
@@ -186,6 +213,8 @@ impl Proxy<'_> {
                             self.process_frame_from_host(&pkt)?;
                         }
                     }
+
+                    self.service_control();
                 }
                 Err(err) => {
                     if let vmnet::Error::VmnetReadNothing = err {
@@ -197,6 +226,34 @@ impl Proxy<'_> {
             }
         }
     }
+
+    fn service_control(&mut self) {
+        let Some(control) = self.control.as_mut() else {
+            return;
+        };
+
+        let keep_open = match control.service(&mut self.rules) {
+            Ok(keep_open) => keep_open,
+            Err(err) => {
+                log::warn!("disabling Softnet control socket: {err:#}");
+                false
+            }
+        };
+
+        if keep_open {
+            return;
+        }
+
+        if let Err(err) = self.poller.remove_control() {
+            log::warn!("failed to remove Softnet control socket from the poller: {err:#}");
+        }
+
+        if let Some(control) = self.control.take()
+            && let Err(err) = control.shutdown()
+        {
+            log::warn!("failed to shut down Softnet control socket: {err:#}");
+        }
+    }
 }
 
 #[cfg(test)]
@@ -295,6 +352,7 @@ mod tests {
                 .map(|cidr| cidr.parse().unwrap())
                 .collect(),
             Vec::default(),
+            None,
         )
         .unwrap();
 

src/main.rs

@@ -31,6 +31,13 @@ struct Args {
     )]
     vm_fd: c_int,
 
+    #[clap(
+        long,
+        value_parser = parse_vm_fd,
+        help = "connected Unix stream FD for newline-delimited JSON-RPC policy control"
+    )]
+    control_fd: Option<c_int>,
+
     #[clap(long, help = "MAC address to enforce for the VM")]
     vm_mac_address: mac_address::MacAddress,
 
@@ -203,6 +210,7 @@ fn try_main() -> anyhow::Result<()> {
         args.allow,
         args.block,
         args.expose,
+        args.control_fd.map(|fd| fd as RawFd),
     )
     .context("failed to initialize proxy")?;
 
@@ -286,4 +294,21 @@ mod tests {
                 .contains("file descriptor must be non-negative")
         );
     }
+
+    #[test]
+    fn test_cli_rejects_negative_control_fd_before_startup() {
+        let error = Args::try_parse_from([
+            "softnet",
+            "--vm-fd=0",
+            "--control-fd=-1",
+            "--vm-mac-address=02:00:00:00:00:01",
+        ])
+        .unwrap_err();
+
+        assert!(
+            error
+                .to_string()
+                .contains("file descriptor must be non-negative")
+        );
+    }
 }