vhost: fix panic on update_config call when device is not active

There is a window in time when the vhost device is not yet initialized
by the guest, but the user already sends the request to update its
config. Though this is an unlikely scenario, fix it by actually checking
the `active_state` instead of asserting on it.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

Author: ShadowCurse

src/vmm/src/devices/virtio/block/vhost_user/device.rs

@@ -267,12 +267,11 @@ impl<T: VhostUserHandleBackend> VhostUserBlockImpl<T> {
 
     pub fn update_config(&mut self) -> Result<(), VhostUserBlockError> {
         let start_time = get_time_us(ClockType::Monotonic);
-        let interrupt = self
-            .device_state
-            .active_state()
-            .expect("Device is not initialized")
-            .interrupt
-            .clone();
+
+        let Some(device_state) = self.device_state.active_state() else {
+            return Err(VhostUserBlockError::DeviceNotActive);
+        };
+        let interrupt = device_state.interrupt.clone();
 
         // This buffer is used for config size check in vhost crate.
         let buffer = [0u8; BLOCK_CONFIG_SPACE_SIZE as usize];
@@ -732,6 +731,9 @@ mod tests {
         vhost_block.write_config(0x69, &[0]);
         assert_eq!(vhost_block.config_space, vec![0x69, 0x69, 0x69]);
 
+        // Before device is active, update_config should error out
+        vhost_block.update_config().unwrap_err();
+
         // Testing [`config_update`]
         vhost_block.device_state = DeviceState::Activated(ActiveState {
             mem: default_mem(),

src/vmm/src/devices/virtio/block/vhost_user/mod.rs

@@ -20,6 +20,8 @@ pub const QUEUE_SIZE: u16 = 256;
 pub enum VhostUserBlockError {
     /// Cannot create config
     Config,
+    /// Device is not active
+    DeviceNotActive,
     /// Snapshotting of vhost-user-blk devices is not supported
     SnapshottingNotSupported,
     /// Vhost-user error: {0}