Fix OneHot depth amplification DoS vulnerability

[GopalakrishnanN]

Description

Fix a Denial of Service vulnerability in the OneHot CPU/CUDA operator where an attacker-controlled depth parameter could cause unbounded memory allocation.

Root Cause

OneHotOp::Compute() reads depth from a scalar tensor input and uses it directly as an output tensor dimension. The only validation was depth_val <= 0 - there was no upper bound check. An attacker-supplied depth of 2^32 with a [1]-shape indices tensor creates a [1, 4294967296] float32 output requiring ~16 GB of memory, crashing servers.

Fix

• Overflow check in PrepareOutputShape(): Validates the total output tensor element count won't overflow int64 before any allocation. This blocks both CPU and CUDA paths since both share this function.
• Null check on Output(): Defense-in-depth check on the tensor allocation return value in both CPU and CUDA Compute functions.

Files Changed

• onnxruntime/core/providers/cpu/tensor/onehot.cc - overflow check + null guard
• onnxruntime/core/providers/cuda/tensor/onehot.cc - null guard (overflow check via shared PrepareOutputShape)
• onnxruntime/test/providers/cpu/tensor/onehot_op_test.cc - 2 new tests for overflow detection

Testing

All 30 OneHot tests pass (28 existing + 2 new)

Motivation and Context

MSRC security report: OneHot depth amplification - unbounded allocation DoS.

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