virtio-devices: rng: Use checked descriptor iterator

Replace manual translate_gva call with checked_iter which validates
the descriptor buffer range against guest memory before I/O.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

virtio-devices/src/rng.rs

@@ -19,7 +19,8 @@ use thiserror::Error;
 use virtio_queue::{Queue, QueueT};
 use vm_memory::{Bytes, GuestAddressSpace, GuestMemoryAtomic};
 use vm_migration::{Migratable, MigratableError, Pausable, Snapshot, Snapshottable, Transportable};
-use vm_virtio::{AccessPlatform, Translatable};
+use vm_virtio::AccessPlatform;
+use vm_virtio::checked_descriptor::DescriptorChainExt;
 use vmm_sys_util::eventfd::EventFd;
 
 use super::{
@@ -61,26 +62,25 @@ impl RngEpollHandler {
         let mut used_descs = false;
         while let Some(mut desc_chain) = queue.pop_descriptor_chain(self.mem.memory()) {
             let mut total_len: usize = 0;
-            while let Some(desc) = desc_chain.next() {
+
+            // Validate the entire descriptor chain upfront so we never
+            // partially fill buffers for a chain that turns out to be
+            // invalid.
+            let descs: Vec<_> = desc_chain
+                .checked_iter(self.access_platform.as_deref())
+                .collect::<Result<_, _>>()
+                .unwrap_or_default();
+
+            for desc in &descs {
                 if !desc.is_write_only() {
                     warn!("Skipping device-readable descriptor");
                     continue;
                 }
-                if desc.len() == 0 {
+                if desc.is_empty() {
                     continue;
                 }
-                let addr = match desc
-                    .addr()
-                    .translate_gva(self.access_platform.as_deref(), desc.len() as usize)
-                {
-                    Ok(a) => a,
-                    Err(e) => {
-                        warn!("Failed to translate descriptor address: {e}");
-                        break;
-                    }
-                };
                 match desc_chain.memory().read_volatile_from(
-                    addr,
+                    desc.addr(),
                     &mut self.random_file,
                     desc.len() as usize,
                 ) {