Fix invalid attestation verification condition (#1321)

* Fix bug with attestation target

* Change comment wording

consensus/fork_choice/src/fork_choice.rs

@@ -608,10 +608,20 @@ where
                 beacon_block_root: indexed_attestation.data.beacon_block_root,
             })?;
 
-        if block.target_root != target.root {
+        // If an attestation points to a block that is from an earlier slot than the attestation,
+        // then all slots between the block and attestation must be skipped. Therefore if the block
+        // is from a prior epoch to the attestation, then the target root must be equal to the root
+        // of the block that is being attested to.
+        let expected_target = if target.epoch > block.slot.epoch(E::slots_per_epoch()) {
+            indexed_attestation.data.beacon_block_root
+        } else {
+            block.target_root
+        };
+
+        if expected_target != target.root {
             return Err(InvalidAttestation::InvalidTarget {
                 attestation: target.root,
-                local: block.target_root,
+                local: expected_target,
             });
         }
 

consensus/fork_choice/tests/tests.rs

@@ -106,6 +106,14 @@ impl ForkChoiceTest {
         self
     }
 
+    /// Skips `count` slots, without producing a block.
+    pub fn skip_slots(self, count: usize) -> Self {
+        for _ in 0..count {
+            self.harness.advance_slot();
+        }
+        self
+    }
+
     /// Build the chain whilst `predicate` returns `true`.
     pub fn apply_blocks_while<F>(self, mut predicate: F) -> Self
     where
@@ -810,3 +818,22 @@ fn invalid_attestation_delayed_slot() {
         .skip_slot()
         .inspect_queued_attestations(|queue| assert_eq!(queue.len(), 0));
 }
+
+/// Tests that the correct target root is used when the attested-to block is in a prior epoch to
+/// the attestation.
+#[test]
+fn valid_attestation_skip_across_epoch() {
+    ForkChoiceTest::new()
+        .apply_blocks(E::slots_per_epoch() as usize - 1)
+        .skip_slots(2)
+        .apply_attestation_to_chain(
+            MutationDelay::NoDelay,
+            |attestation, _chain| {
+                assert_eq!(
+                    attestation.data.target.root,
+                    attestation.data.beacon_block_root
+                )
+            },
+            |result| result.unwrap(),
+        );
+}