protocols/kad: Fix right shift overflow panic in record_received

protocols/kad/src/behaviour.rs

@@ -946,8 +946,6 @@ where
             return
         }
 
-        let now = Instant::now();
-
         // Calculate the expiration exponentially inversely proportional to the
         // number of nodes between the local node and the closest node to the key
         // (beyond the replication factor). This ensures avoiding over-caching
@@ -956,9 +954,7 @@ where
         let num_between = self.kbuckets.count_nodes_between(&target);
         let k = self.queries.config().replication_factor.get();
         let num_beyond_k = (usize::max(k, num_between) - k) as u32;
-        let expiration = self.record_ttl.map(|ttl|
-            now + Duration::from_secs(ttl.as_secs() >> num_beyond_k)
-        );
+        let expiration = exp_decr_expiration(self.record_ttl, num_beyond_k);
         // The smaller TTL prevails. Only if neither TTL is set is the record
         // stored "forever".
         record.expires = record.expires.or(expiration).min(expiration);
@@ -1030,6 +1026,15 @@ where
     }
 }
 
+/// Calculate exponentially decreasing expiration from a default time-to-live by a factor.
+fn exp_decr_expiration(default_ttl: Option<Duration>, factor: u32) -> Option<Instant> {
+    default_ttl.map(|ttl| Instant::now() + Duration::from_secs(
+        ttl.as_secs()
+            .checked_shr(factor)
+            .unwrap_or(0),
+    ))
+}
+
 impl<TStore> NetworkBehaviour for Kademlia<TStore>
 where
     for<'a> TStore: RecordStore<'a>,
@@ -1911,4 +1916,3 @@ impl QueryInfo {
         }
     }
 }
-

protocols/kad/src/behaviour/test.rs

@@ -682,3 +682,15 @@ fn exceed_jobs_max_queries() {
         })
     )
 }
+
+#[test]
+fn exp_decr_expiration_overflow() {
+    fn prop_no_panic(ttl: Option<Duration>, factor: u32) {
+        exp_decr_expiration(ttl, factor);
+    }
+
+    // Right shifting a u64 by >63 results in a panic.
+    prop_no_panic(KademliaConfig::default().record_ttl, 64);
+
+    quickcheck(prop_no_panic as fn(_, _))
+}