Check for buffer overflow in prim_ops::et_copy_index() (#12827)

Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at
bottom):
* __->__ #12697

^
Test on top of D78676341.

Differential Revision:
[D78701418](https://our.internmc.facebook.com/intern/diff/D78701418/)

Co-authored-by: lucylq <[email protected]>

Author: pytorchbot

kernels/prim_ops/et_copy_index.cpp

@@ -86,11 +86,9 @@ void et_copy_index(KernelRuntimeContext& context, EValue** stack) {
     // If we're copying past the first index then the shape of
     // copy_from and copy_to without the leading dimension should be
     // the same. i.e. copy_to.size[1:] == copy_from.size[:].
-    if (index > 0) {
-      ET_CHECK_MSG(
-          copy_to.sizes()[i + 1] == copy_from.sizes()[i],
-          "Mismatch in shape between copy_to and copy_from tensors");
-    }
+    ET_CHECK_MSG(
+        copy_to.sizes()[i + 1] == copy_from.sizes()[i],
+        "Mismatch in shape between copy_to and copy_from tensors");
     expected_output_size[i + 1] = copy_from.sizes()[i];
   }
 
@@ -111,8 +109,17 @@ void et_copy_index(KernelRuntimeContext& context, EValue** stack) {
   // If we've reached here, it means the copy_to tensor has been
   // successfully resized so we can now copy over the data from
   // copy_from into the copy_to tensor.
+
+  // Check that the destination has enough space for the copy.
+  size_t offset = index * size_copy_from;
+  size_t copy_to_size = copy_to.element_size() * copy_to.numel();
+  ET_CHECK_MSG(
+      offset + size_copy_from <= copy_to_size,
+      "Buffer overflow: copy_to tensor is smaller than copy_from tensor.");
+
   memcpy(
-      (void*)((uintptr_t)copy_to_ptr + index * size_copy_from),
+      // NOLINTNEXTLINE(performance-no-int-to-ptr)
+      (void*)((uintptr_t)copy_to_ptr + offset),
       copy_from_ptr,
       size_copy_from);
 }

kernels/prim_ops/test/prim_ops_test.cpp

@@ -131,7 +131,7 @@ TEST_F(RegisterPrimOpsTest, TestETCopyIndex) {
   Tensor copy_to = tf.make({2, 2}, {0, 0, 0, 0});
 #else
   std::vector<int> buf(4);
-  SizesType expected_output_size[2] = {0, 0};
+  SizesType expected_output_size[2] = {0, 2};
   Tensor copy_to =
       tf.make({2, 2}, {0, 0, 0, 0}, {}, TensorShapeDynamism::DYNAMIC_BOUND);
   // Resize the tensor to 0 size for the tests.