Validate VM file descriptors before ownership transfer

lib/vm.rs

@@ -1,5 +1,7 @@
-use anyhow::Result;
-use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
+use anyhow::{Context, Result, bail};
+use std::io;
+use std::mem::{size_of, zeroed};
+use std::os::fd::{AsRawFd, FromRawFd, RawFd};
 use std::os::unix::net::UnixDatagram;
 
 pub struct VM {
@@ -8,6 +10,9 @@ pub struct VM {
 
 impl VM {
     pub fn new(vm_fd: RawFd) -> Result<VM> {
+        let vm_fd = duplicate_vm_fd(vm_fd)?;
+
+        // SAFETY: duplicate_vm_fd only returns a valid descriptor that it owns.
         let sock = unsafe { UnixDatagram::from_raw_fd(vm_fd) };
         sock.set_nonblocking(true)?;
 
@@ -23,8 +28,150 @@ impl VM {
     }
 }
 
+fn duplicate_vm_fd(vm_fd: RawFd) -> Result<RawFd> {
+    if vm_fd < 0 {
+        bail!("invalid VM file descriptor {vm_fd}: value must be non-negative");
+    }
+
+    // SAFETY: fcntl duplicates the descriptor without transferring ownership of vm_fd.
+    let duplicated_fd = unsafe { libc::fcntl(vm_fd, libc::F_DUPFD_CLOEXEC, 0) };
+    if duplicated_fd == -1 {
+        return Err(io::Error::last_os_error())
+            .with_context(|| format!("failed to duplicate VM file descriptor {vm_fd}"));
+    }
+
+    if let Err(error) = validate_vm_fd(duplicated_fd) {
+        // SAFETY: duplicated_fd is an open descriptor owned by this function.
+        unsafe { libc::close(duplicated_fd) };
+        return Err(error);
+    }
+
+    Ok(duplicated_fd)
+}
+
+fn validate_vm_fd(vm_fd: RawFd) -> Result<()> {
+    // SAFETY: fcntl only reads descriptor state and does not take ownership.
+    if unsafe { libc::fcntl(vm_fd, libc::F_GETFD) } == -1 {
+        return Err(io::Error::last_os_error())
+            .with_context(|| format!("failed to inspect VM file descriptor {vm_fd}"));
+    }
+
+    let mut socket_type = 0;
+    let mut socket_type_len = size_of::<libc::c_int>() as libc::socklen_t;
+
+    // SAFETY: socket_type and socket_type_len are valid writable buffers of the sizes given.
+    if unsafe {
+        libc::getsockopt(
+            vm_fd,
+            libc::SOL_SOCKET,
+            libc::SO_TYPE,
+            (&mut socket_type as *mut libc::c_int).cast(),
+            &mut socket_type_len,
+        )
+    } == -1
+    {
+        return Err(io::Error::last_os_error())
+            .with_context(|| format!("VM file descriptor {vm_fd} is not a socket"));
+    }
+
+    if socket_type != libc::SOCK_DGRAM {
+        bail!("VM file descriptor {vm_fd} is not a Unix datagram socket");
+    }
+
+    let mut address: libc::sockaddr_storage = unsafe { zeroed() };
+    let mut address_len = size_of::<libc::sockaddr_storage>() as libc::socklen_t;
+
+    // SAFETY: address and address_len are valid writable buffers of the sizes given.
+    if unsafe {
+        libc::getsockname(
+            vm_fd,
+            (&mut address as *mut libc::sockaddr_storage).cast(),
+            &mut address_len,
+        )
+    } == -1
+    {
+        return Err(io::Error::last_os_error()).with_context(|| {
+            format!("failed to inspect the address family of VM file descriptor {vm_fd}")
+        });
+    }
+
+    if address.ss_family as libc::c_int != libc::AF_UNIX {
+        bail!("VM file descriptor {vm_fd} is not a Unix socket");
+    }
+
+    Ok(())
+}
+
 impl AsRawFd for VM {
     fn as_raw_fd(&self) -> RawFd {
         self.sock.as_raw_fd()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::VM;
+    use std::fs::File;
+    use std::os::fd::AsRawFd;
+    use std::os::unix::net::{UnixDatagram, UnixStream};
+
+    #[test]
+    fn test_new_rejects_negative_fd() {
+        let error = VM::new(-1).err().unwrap();
+
+        assert_eq!(
+            error.to_string(),
+            "invalid VM file descriptor -1: value must be non-negative"
+        );
+    }
+
+    #[test]
+    fn test_new_rejects_non_socket_fd_without_taking_ownership() {
+        let file = File::open("/dev/null").unwrap();
+        let error = VM::new(file.as_raw_fd()).err().unwrap();
+
+        assert!(error.to_string().contains("is not a socket"));
+        assert!(file.metadata().is_ok());
+    }
+
+    #[test]
+    fn test_new_rejects_closed_fd() {
+        let (socket, _peer) = UnixDatagram::pair().unwrap();
+        let vm_fd = socket.as_raw_fd();
+        drop(socket);
+
+        let error = VM::new(vm_fd).err().unwrap();
+
+        assert!(
+            error
+                .to_string()
+                .contains("failed to duplicate VM file descriptor")
+        );
+    }
+
+    #[test]
+    fn test_new_rejects_non_datagram_socket() {
+        let (stream, _peer) = UnixStream::pair().unwrap();
+        let error = VM::new(stream.as_raw_fd()).err().unwrap();
+
+        assert!(error.to_string().contains("not a Unix datagram socket"));
+    }
+
+    #[test]
+    fn test_new_does_not_close_original_fd_when_vm_is_dropped() {
+        let (socket, _peer) = UnixDatagram::pair().unwrap();
+        let vm = VM::new(socket.as_raw_fd()).unwrap();
+        drop(vm);
+
+        let socket_fd_is_open = unsafe { libc::fcntl(socket.as_raw_fd(), libc::F_GETFD) != -1 };
+
+        if socket_fd_is_open {
+            drop(socket);
+        } else {
+            // Avoid double-closing the descriptor if this test catches an unsafe implementation.
+            std::mem::forget(socket);
+        }
+
+        assert!(socket_fd_is_open);
+    }
+}

src/main.rs

@@ -26,6 +26,7 @@ use uzers::{get_current_groupname, get_current_username, get_effective_uid};
 struct Args {
     #[clap(
         long,
+        value_parser = parse_vm_fd,
         help = "FD number to use for communicating with the VM's networking stack"
     )]
     vm_fd: c_int,
@@ -217,6 +218,18 @@ fn try_main() -> anyhow::Result<()> {
     proxy.run()
 }
 
+fn parse_vm_fd(value: &str) -> Result<c_int, String> {
+    let vm_fd = value
+        .parse::<c_int>()
+        .map_err(|err| format!("invalid file descriptor: {err}"))?;
+
+    if vm_fd < 0 {
+        return Err("file descriptor must be non-negative".to_string());
+    }
+
+    Ok(vm_fd)
+}
+
 fn sudo_escalation_works() -> bool {
     let exe = std::env::current_exe().unwrap();
     let args = std::env::args().skip(1);
@@ -252,3 +265,25 @@ fn set_bootpd_lease_time(lease_time: u32) {
         SCPreferencesCommitChanges(prefs.as_concrete_TypeRef());
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::Args;
+    use clap::Parser;
+
+    #[test]
+    fn test_cli_rejects_negative_vm_fd_before_startup() {
+        let error = Args::try_parse_from([
+            "softnet",
+            "--vm-fd=-1",
+            "--vm-mac-address=02:00:00:00:00:01",
+        ])
+        .unwrap_err();
+
+        assert!(
+            error
+                .to_string()
+                .contains("file descriptor must be non-negative")
+        );
+    }
+}