[child-table] check neighbor is in ChildTable before cast as Child (#8071)

This commit adds a check `Get<ChildTable>().Contains(Neighbor &)`
to ensure that the neighbor is from the child table before casting
the neighbor entry to `Child`. This adds safety and protection against
potential corner-case where we have a neighbor entry which is
`Parent` or `ParentCandidate` that is a REED.

src/core/thread/mesh_forwarder.cpp

@@ -1834,8 +1834,8 @@ uint16_t MeshForwarder::CalcFrameVersion(const Neighbor *aNeighbor, bool aIePres
         version = Mac::Frame::kFcfFrameVersion2015;
     }
 #if OPENTHREAD_FTD && OPENTHREAD_CONFIG_MAC_CSL_TRANSMITTER_ENABLE
-    else if (aNeighbor != nullptr && !Mle::IsActiveRouter(aNeighbor->GetRloc16()) &&
-             Get<Mle::MleRouter>().IsRouterOrLeader() && static_cast<const Child *>(aNeighbor)->IsCslSynchronized())
+    else if ((aNeighbor != nullptr) && Get<ChildTable>().Contains(*aNeighbor) &&
+             static_cast<const Child *>(aNeighbor)->IsCslSynchronized())
     {
         version = Mac::Frame::kFcfFrameVersion2015;
     }

src/core/thread/mesh_forwarder_ftd.cpp

@@ -50,7 +50,6 @@ Error MeshForwarder::SendMessage(Message &aMessage)
 {
     Mle::MleRouter &mle   = Get<Mle::MleRouter>();
     Error           error = kErrorNone;
-    Neighbor *      neighbor;
 
     aMessage.SetOffset(0);
     aMessage.SetDatagramTag(0);
@@ -104,17 +103,20 @@ Error MeshForwarder::SendMessage(Message &aMessage)
                 }
             }
         }
-        else if ((neighbor = Get<NeighborTable>().FindNeighbor(ip6Header.GetDestination())) != nullptr &&
-                 !neighbor->IsRxOnWhenIdle() && !aMessage.IsDirectTransmission())
+        else // Destination is unicast
         {
-            // destined for a sleepy child
-            Child &child = *static_cast<Child *>(neighbor);
-            mIndirectSender.AddMessageForSleepyChild(aMessage, child);
-        }
-        else
-        {
-            // schedule direct transmission
-            aMessage.SetDirectTransmission();
+            Neighbor *neighbor = Get<NeighborTable>().FindNeighbor(ip6Header.GetDestination());
+
+            if ((neighbor != nullptr) && !neighbor->IsRxOnWhenIdle() && !aMessage.IsDirectTransmission() &&
+                Get<ChildTable>().Contains(*neighbor))
+            {
+                // Destined for a sleepy child
+                mIndirectSender.AddMessageForSleepyChild(aMessage, *static_cast<Child *>(neighbor));
+            }
+            else
+            {
+                aMessage.SetDirectTransmission();
+            }
         }
 
         break;
@@ -283,7 +285,7 @@ void MeshForwarder::RemoveMessages(Child &aChild, Message::SubType aSubType)
 
                 IgnoreError(message.Read(0, ip6header));
 
-                if (&aChild == static_cast<Child *>(Get<NeighborTable>().FindNeighbor(ip6header.GetDestination())))
+                if (&aChild == Get<NeighborTable>().FindNeighbor(ip6header.GetDestination()))
                 {
                     message.ClearDirectTransmission();
                 }
@@ -297,7 +299,7 @@ void MeshForwarder::RemoveMessages(Child &aChild, Message::SubType aSubType)
 
                 IgnoreError(meshHeader.ParseFrom(message));
 
-                if (&aChild == static_cast<Child *>(Get<NeighborTable>().FindNeighbor(meshHeader.GetDestination())))
+                if (&aChild == Get<NeighborTable>().FindNeighbor(meshHeader.GetDestination()))
                 {
                     message.ClearDirectTransmission();
                 }

src/core/thread/mle_router.cpp

@@ -2724,7 +2724,8 @@ void MleRouter::HandleChildUpdateResponse(RxInfo &aRxInfo)
     Child *    child;
     uint16_t   addressRegistrationOffset = 0;
 
-    if ((aRxInfo.mNeighbor == nullptr) || IsActiveRouter(aRxInfo.mNeighbor->GetRloc16()))
+    if ((aRxInfo.mNeighbor == nullptr) || IsActiveRouter(aRxInfo.mNeighbor->GetRloc16()) ||
+        !Get<ChildTable>().Contains(*aRxInfo.mNeighbor))
     {
         Log(kMessageReceive, kTypeChildUpdateResponseOfUnknownChild, aRxInfo.mMessageInfo.GetPeerAddr());
         ExitNow(error = kErrorNotFound);
@@ -3529,7 +3530,7 @@ void MleRouter::RemoveNeighbor(Neighbor &aNeighbor)
     }
     else if (!IsActiveRouter(aNeighbor.GetRloc16()))
     {
-        OT_ASSERT(mChildTable.GetChildIndex(static_cast<Child &>(aNeighbor)) < kMaxChildren);
+        OT_ASSERT(mChildTable.Contains(aNeighbor));
 
         if (aNeighbor.IsStateValidOrRestoring())
         {

src/core/thread/neighbor_table.cpp

@@ -271,6 +271,7 @@ void NeighborTable::Signal(Event aEvent, const Neighbor &aNeighbor)
         case kChildRemoved:
         case kChildModeChanged:
 #if OPENTHREAD_FTD
+            OT_ASSERT(Get<ChildTable>().Contains(aNeighbor));
             static_cast<Child::Info &>(info.mInfo.mChild).SetFrom(static_cast<const Child &>(aNeighbor));
 #endif
             break;