Fix write-heap-buffer-overflow in et_copy_index (pytorch#15605)

Summary:

The crash is a write-heap-buffer-overflow that occurs in the `et_copy_index` function. The root cause is the lack of proper validation of the `index` argument, which can lead to an out-of-bounds write when `index` is negative or exceeds the bounds of the `copy_to` tensor.

The patch fixes the crash by adding two checks: `ET_CHECK_MSG(index >= 0, "Index must be non-negative");` and `ET_CHECK_MSG(index < copy_to.sizes()[0], "Index out of bounds");`. These checks ensure that `index` is within the valid range for the `copy_to` tensor, preventing the out-of-bounds write.

Other considerations that reviewers should take into account when validating the patch include verifying that the added checks do not introduce any performance regressions and that they correctly handle edge cases, such as when `index` is equal to `copy_to.sizes()[0] - 1`. Reviewers should also check that the patch does not alter the existing functionality of the `et_copy_index` function and that it is consistent with the surrounding code.

Additionally, reviewers may want to consider testing the patch with various inputs, including negative `index` values, `index` values that exceed the bounds of `copy_to`, and valid `index` values, to ensure that the patch correctly prevents the write-heap-buffer-overflow crash.

Here is the commit message:

```
Fix write-heap-buffer-overflow crash in et_copy_index

The crash is a write-heap-buffer-overflow that occurs in the `et_copy_index` function.
The root cause is the lack of proper validation of the `index` argument, which can lead to an out-of-bounds write when `index` is negative or exceeds the bounds of the `copy_to` tensor.

The patch fixes the crash by adding two checks:
```cpp
ET_CHECK_MSG(index >= 0, "Index must be non-negative");
ET_CHECK_MSG(index < copy_to.sizes()[0], "Index out of bounds");
```
These checks ensure that `index` is within the valid range for the `copy_to` tensor, preventing the out-of-bounds write.

Other considerations that reviewers should take into account when validating the patch include verifying that the added checks do not introduce any performance regressions and that they correctly handle edge cases, such as when `index` is equal to `copy_to.sizes()[0] - 1`. Reviewers should also check that the patch does not alter the existing functionality of the `et_copy_index` function and that it is consistent with the surrounding code.
```

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Differential Revision: D80399111

Author: lucylq

kernels/prim_ops/et_copy_index.cpp

@@ -59,7 +59,7 @@ constexpr size_t kTensorDimensionLimit = 16;
 //        torch.ops.executorch.prim.add.int(iteration_index, 1, iteration_index)
 //        done_bool = torch.ops.executorch.prim.eq.int(iteration_index,
 //        sym_size, done_bool) # Emitter inserts a instruction here, if
-//        done_bool == False jump to selcect_copy op # if not continue. return
+//        done_bool == False jump to select_copy op # if not continue. return
 //        add_tensor
 //
 // The output of each iteration (copy_from) is copied into the copy_to tensor at
@@ -79,6 +79,9 @@ void et_copy_index(KernelRuntimeContext& context, Span<EValue*> stack) {
   auto copy_from = (*stack[1]).toTensor();
   auto index = (*stack[2]).toInt();
 
+  ET_CHECK_MSG(index >= 0, "Index must be non-negative");
+  ET_CHECK_MSG(index < copy_to.sizes()[0], "Index out of bounds");
+
   // Number of bytes we need to copy over from copy_from tensor.
   size_t size_copy_from = (copy_from.element_size()) * (copy_from.numel());
 
@@ -118,8 +121,14 @@ void et_copy_index(KernelRuntimeContext& context, Span<EValue*> stack) {
   // copy_from into the copy_to tensor.
 
   // Check that the destination has enough space for the copy.
+  ET_CHECK_MSG(
+    size_copy_from == 0 || index <= SIZE_MAX / size_copy_from,
+    "Offset multiplication overflow");
   size_t offset = index * size_copy_from;
   size_t copy_to_size = copy_to.element_size() * copy_to.numel();
+  ET_CHECK_MSG(
+      offset <= SIZE_MAX - size_copy_from,
+      "Offset addition overflow");
   ET_CHECK_MSG(
       offset + size_copy_from <= copy_to_size,
       "Buffer overflow: copy_to tensor is smaller than copy_from tensor.");