Fix CPU Attention overflow issue (#27822)

### Description
<!-- Describe your changes. -->

Fix `int` overflow issue in
`ComputeAttentionSoftmaxInplace<MLFloat16>()` by using `size_t` and `SafeInt` instead.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Fix overflow issue.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: edgchen1

onnxruntime/core/providers/cpu/llm/attention.cc

@@ -3,6 +3,7 @@
 
 #include "core/providers/cpu/llm/attention.h"
 #include "core/providers/cpu/llm/attention_helper.h"
+#include "core/providers/cpu/llm/attention_softmax.h"
 
 #include "core/common/common.h"
 #include "core/common/safeint.h"
@@ -77,23 +78,6 @@ void make_copy<MLFloat16, bool>(MLFloat16* mask_data, const bool* mask_index, si
   }
 }
 
-template <typename T>
-inline void ComputeAttentionSoftmaxInplace(T* score, int N, int D, ThreadPool* tp, AllocatorPtr) {
-  MlasComputeSoftmax(score, score, N, D, false, false, 0.0f, tp);
-}
-
-template <>
-inline void ComputeAttentionSoftmaxInplace<MLFloat16>(MLFloat16* score, int N, int D, ThreadPool* tp, AllocatorPtr allocator) {
-  ORT_ENFORCE(tp == nullptr, "No parallelized version of softmax for float16.");
-  // Mlas Lacks kernels for fp16 softmax, we convert into float32 and call the float32 version.
-  void* allocated_ptr = allocator->Alloc(static_cast<size_t>(N * D * sizeof(float)));
-  BufferUniquePtr float_buffer(allocated_ptr, BufferDeleter(allocator));
-  float* ptr = reinterpret_cast<float*>(allocated_ptr);
-  MlasConvertHalfToFloatBuffer(score, ptr, N * D);
-  MlasComputeSoftmax(ptr, ptr, N, D, false, false, 0.0f, tp);
-  MlasConvertFloatToHalfBuffer(ptr, score, N * D);
-}
-
 template <typename T>
 inline void ComputeAttentionSoftcapInplace(T* scores, int sequence_length, T softcap) {
   MlasComputeSoftcap(scores, scores, sequence_length, softcap);

onnxruntime/core/providers/cpu/llm/attention_softmax.h

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#pragma once
+
+#include "core/common/common.h"
+#include "core/common/float16.h"
+#include "core/common/safeint.h"
+#include "core/framework/allocator.h"
+#include "core/framework/buffer_deleter.h"
+#include "core/mlas/inc/mlas.h"
+#include "core/platform/threadpool.h"
+
+namespace onnxruntime {
+
+template <typename T>
+inline void ComputeAttentionSoftmaxInplace(T* score, size_t N, size_t D,
+                                           concurrency::ThreadPool* tp, AllocatorPtr) {
+  MlasComputeSoftmax(score, score, N, D, false, false, 0.0f, tp);
+}
+
+template <>
+inline void ComputeAttentionSoftmaxInplace<MLFloat16>(MLFloat16* score, size_t N, size_t D,
+                                                      concurrency::ThreadPool* tp, AllocatorPtr allocator) {
+  ORT_ENFORCE(tp == nullptr, "No parallelized version of softmax for float16.");
+  // MLAS lacks kernels for fp16 softmax, so we convert to float32 and use the float32 version.
+  auto num_elements = SafeInt<size_t>(N) * D;
+  void* allocated_ptr = allocator->Alloc(num_elements * sizeof(float));
+  BufferUniquePtr float_buffer(allocated_ptr, BufferDeleter(allocator));
+  float* ptr = reinterpret_cast<float*>(allocated_ptr);
+  MlasConvertHalfToFloatBuffer(score, ptr, num_elements);
+  MlasComputeSoftmax(ptr, ptr, N, D, false, false, 0.0f, tp);
+  MlasConvertFloatToHalfBuffer(ptr, score, num_elements);
+}
+
+}  // namespace onnxruntime

onnxruntime/test/providers/cpu/llm/attention_op_test.cc

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 #include <cassert>
+#include <limits>
 #include "gtest/gtest.h"
 #include "core/session/onnxruntime_cxx_api.h"
 #include "test/common/tensor_op_test_utils.h"

onnxruntime/test/providers/cpu/llm/attention_softmax_test.cc

@@ -0,0 +1,86 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#if !defined(ORT_NO_EXCEPTIONS)
+
+#include <exception>
+#include <limits>
+
+#include "gtest/gtest.h"
+#include "gmock/gmock.h"
+
+#include "core/framework/allocator.h"
+#include "core/providers/cpu/llm/attention_softmax.h"
+
+namespace onnxruntime {
+namespace test {
+
+// Regression test for integer overflow in FP16 softmax allocation.
+// ComputeAttentionSoftmaxInplace<MLFloat16> previously used int for N and D, so N*D could overflow int32.
+// The fix changed parameters to size_t and uses SafeInt for the multiplication.
+//
+// This test calls ComputeAttentionSoftmaxInplace<MLFloat16> directly with overflow-triggering dimensions
+// (N=46341, D=46341, where N*D > INT_MAX).
+// A custom allocator intercepts the Alloc call to verify the requested size is computed correctly with size_t
+// arithmetic, without actually allocating the ~8GB buffer.
+//
+// On 32-bit builds, SafeInt<size_t> will signal an overflow for the requested size.
+TEST(AttentionSoftmaxTest, Fp16OverflowAllocation) {
+  // Custom exception thrown by the allocator to distinguish it from SafeInt overflow.
+  struct AllocationIntercepted : std::exception {
+    const char* what() const noexcept override { return "allocation intercepted"; }
+  };
+
+  // Custom allocator that records the requested allocation size and throws to avoid actually allocating the
+  // (very large) buffer.
+  class OverflowCheckAllocator : public IAllocator {
+   public:
+    OverflowCheckAllocator()
+        : IAllocator(OrtMemoryInfo(CPU, OrtDeviceAllocator)) {}
+    void* Alloc(size_t size) override {
+      last_alloc_size_ = size;
+      throw AllocationIntercepted();
+    }
+    void Free(void*) override {}
+    size_t LastAllocSize() const { return last_alloc_size_; }
+
+   private:
+    size_t last_alloc_size_ = 0;
+  };
+
+  constexpr size_t N = 46341;
+  constexpr size_t D = 46341;
+
+  // Verify at compile time that these dimensions would overflow int32.
+  static_assert(int64_t{N} * int64_t{D} > int64_t{std::numeric_limits<int>::max()},
+                "Test dimensions must cause int32 overflow in N*D");
+
+  auto alloc = std::make_shared<OverflowCheckAllocator>();
+  MLFloat16 dummy_score{0.0f};
+
+  // The allocation size must reflect correct size_t arithmetic: N * D * sizeof(float).
+  // With the old int parameters, N * D would overflow to a small/negative value, producing a wrong allocation size.
+  constexpr uintmax_t expected_allocation_size = uintmax_t{N} * D * sizeof(float);
+
+  if constexpr (expected_allocation_size <= uintmax_t{std::numeric_limits<size_t>::max()}) {
+    // Allocation size fits in size_t. The function reaches Alloc, which records the requested size and throws
+    // AllocationIntercepted.
+    EXPECT_THROW(ComputeAttentionSoftmaxInplace<MLFloat16>(&dummy_score, N, D, nullptr, alloc),
+                 AllocationIntercepted);
+
+    EXPECT_EQ(alloc->LastAllocSize(), static_cast<size_t>(expected_allocation_size));
+  } else {
+    // Allocation size overflows size_t (i.e., in a 32-bit build), so SafeInt<size_t> will throw an exception.
+    try {
+      ComputeAttentionSoftmaxInplace<MLFloat16>(&dummy_score, N, D, nullptr, alloc);
+      FAIL() << "Expected OnnxRuntimeException to be thrown";
+    } catch (const OnnxRuntimeException& e) {
+      EXPECT_THAT(e.what(), testing::HasSubstr("Integer overflow"));
+    }
+  }
+}
+
+}  // namespace test
+}  // namespace onnxruntime
+
+#endif  // !defined(ORT_NO_EXCEPTIONS)