virtio-queue: panic on invalid avail ring index

The number of descriptor chain heads to process should always be smaller
or equal to the queue size, as the driver should never ask the VMM to
process a available ring entry more than once. Checking and reporting
such incorrect driver behavior can prevent potential hanging and
Denial-of-Service from happening on the VMM side.

Issue reported in rust-vmm/vm-virtio and fixed in
rust-vmm/vm-virtio#196.

Signed-off-by: Diana Popa <dpopa@amazon.com>
Co-authored-by: Bo Chen <chen.bo@intel.com>

Author: dianpopa

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

@@ -1477,7 +1477,6 @@ pub(crate) mod tests {
             add_flush_requests_batch(&mut block, &mem, &vq, 1);
             simulate_queue_event(&mut block, Some(false));
             assert_eq!(block.is_io_engine_throttled, true);
-            assert_eq!(block.queues[0].len(&mem), 1);
 
             simulate_async_completion_event(&mut block, true);
             assert_eq!(block.is_io_engine_throttled, false);

src/devices/src/virtio/queue.rs

@@ -49,7 +49,7 @@ impl fmt::Display for QueueError {
     }
 }
 
-/// A virtio descriptor constraints with C representive.
+/// A virtio descriptor constraints with C representative.
 #[repr(C)]
 #[derive(Default, Clone, Copy)]
 struct Descriptor {
@@ -104,9 +104,9 @@ impl<'a> DescriptorChain<'a> {
         // These reads can't fail unless Guest memory is hopelessly broken.
         let desc = match mem.read_obj::<Descriptor>(desc_head) {
             Ok(ret) => ret,
-            Err(_) => {
+            Err(err) => {
                 // TODO log address
-                error!("Failed to read from memory");
+                error!("Failed to read virtio descriptor from memory: {}", err);
                 return None;
             }
         };
@@ -289,7 +289,7 @@ impl Queue {
     }
 
     /// Returns the number of yet-to-be-popped descriptor chains in the avail ring.
-    pub fn len(&self, mem: &GuestMemoryMmap) -> u16 {
+    fn len(&self, mem: &GuestMemoryMmap) -> u16 {
         (self.avail_idx(mem) - self.next_avail).0
     }
 
@@ -300,7 +300,21 @@ impl Queue {
 
     /// Pop the first available descriptor chain from the avail ring.
     pub fn pop<'a, 'b>(&'a mut self, mem: &'b GuestMemoryMmap) -> Option<DescriptorChain<'b>> {
-        if self.len(mem) == 0 {
+        let len = self.len(mem);
+        // The number of descriptor chain heads to process should always
+        // be smaller or equal to the queue size, as the driver should
+        // never ask the VMM to process a available ring entry more than
+        // once. Checking and reporting such incorrect driver behavior
+        // can prevent potential hanging and Denial-of-Service from
+        // happening on the VMM side.
+        if len > self.actual_size() {
+            // We are choosing to interrupt execution since this could be a potential malicious
+            // driver scenario. This way we also eliminate the risk of repeatedly
+            // logging and potentially clogging the microVM through the log system.
+            panic!("The number of available virtio descriptors is greater than queue size!");
+        }
+
+        if len == 0 {
             return None;
         }
 
@@ -425,7 +439,7 @@ impl Queue {
         // via `self.is_valid()`, so it's safe to unwrap and use unchecked offsets here.
         // Note: the `MmioTransport` code ensures that queue addresses cannot be changed by the
         // guest       after device activation, so we can be certain that no change has
-        // occured since       the last `self.is_valid()` check.
+        // occurred since the last `self.is_valid()` check.
         let addr = self.avail_ring.unchecked_add(2);
         Wrapping(mem.read_obj::<u16>(addr).unwrap())
     }
@@ -461,7 +475,17 @@ impl Queue {
             return true;
         }
 
-        if self.len(mem) != 0 {
+        let len = self.len(mem);
+        if len != 0 {
+            // The number of descriptor chain heads to process should always
+            // be smaller or equal to the queue size.
+            if len > self.actual_size() {
+                // We are choosing to interrupt execution since this could be a potential malicious
+                // driver scenario. This way we also eliminate the risk of
+                // repeatedly logging and potentially clogging the microVM through
+                // the log system.
+                panic!("The number of available virtio descriptors is greater than queue size!");
+            }
             return false;
         }
 
@@ -741,6 +765,92 @@ pub(crate) mod tests {
         assert_eq!(q.avail_event(m), 2);
     }
 
+    #[test]
+    #[should_panic(
+        expected = "The number of available virtio descriptors is greater than queue size!"
+    )]
+    fn test_invalid_avail_idx_no_notification() {
+        // This test ensures constructing a descriptor chain succeeds
+        // with valid available ring indexes while it produces an error with invalid
+        // indexes.
+        // No notification suppression enabled.
+        let m = &create_anon_guest_memory(&[(GuestAddress(0), 0x6000)], false).unwrap();
+
+        // We set up a queue of size 4.
+        let vq = VirtQueue::new(GuestAddress(0), m, 4);
+        let mut q = vq.create_queue();
+
+        for j in 0..4 {
+            vq.dtable[j].set(
+                0x1000 * (j + 1) as u64,
+                0x1000,
+                VIRTQ_DESC_F_NEXT,
+                (j + 1) as u16,
+            );
+        }
+
+        // Create 2 descriptor chains.
+        // the chains are (0, 1) and (2, 3)
+        vq.dtable[1].flags.set(0);
+        vq.dtable[3].flags.set(0);
+        vq.avail.ring[0].set(0);
+        vq.avail.ring[1].set(2);
+        vq.avail.idx.set(2);
+
+        // We've just set up two chains.
+        assert_eq!(q.len(m), 2);
+
+        // We process the first descriptor.
+        let d = q.pop(m).unwrap().next_descriptor();
+        assert!(matches!(d, Some(x) if !x.has_next()));
+        // We confuse the device and set the available index as being 6.
+        vq.avail.idx.set(6);
+
+        // We've actually just popped a descriptor so 6 - 1 = 5.
+        assert_eq!(q.len(m), 5);
+
+        // However, since the apparent length set by the driver is more than the queue size,
+        // we would be running the risk of going through some descriptors more than once.
+        // As such, we expect to panic.
+        q.pop(m);
+    }
+
+    #[test]
+    #[should_panic(
+        expected = "The number of available virtio descriptors is greater than queue size!"
+    )]
+    fn test_invalid_avail_idx_with_notification() {
+        // This test ensures constructing a descriptor chain succeeds
+        // with valid available ring indexes while it produces an error with invalid
+        // indexes.
+        // Notification suppression is enabled.
+        let m = &create_anon_guest_memory(&[(GuestAddress(0), 0x6000)], false).unwrap();
+
+        // We set up a queue of size 4.
+        let vq = VirtQueue::new(GuestAddress(0), m, 4);
+        let mut q = vq.create_queue();
+
+        q.uses_notif_suppression = true;
+
+        // Create 1 descriptor chain of 4.
+        for j in 0..4 {
+            vq.dtable[j].set(
+                0x1000 * (j + 1) as u64,
+                0x1000,
+                VIRTQ_DESC_F_NEXT,
+                (j + 1) as u16,
+            );
+        }
+        // We need to clear the VIRTQ_DESC_F_NEXT for the last descriptor.
+        vq.dtable[3].flags.set(0);
+        vq.avail.ring[0].set(0);
+
+        // driver sets available index to suspicious value.
+        vq.avail.idx.set(6);
+
+        q.pop_or_enable_notification(m);
+    }
+
     #[test]
     fn test_add_used() {
         let m = &create_anon_guest_memory(&[(GuestAddress(0), 0x10000)], false).unwrap();