Add bounds checking for output tensor buffer in wasi-nn llama.cpp

[sumleo]

Summary

The get_output function in the wasi-nn llama.cpp backend copies data into output_tensor->buf without checking against output_tensor->size. This can lead to out-of-bounds writes when the model generates output longer than the caller-provided buffer.

Two vulnerable paths were identified:

1. Metadata path (index == 1): memcpy(output_tensor->buf, output_metadata, strlen(output_metadata)) copies up to 127 bytes of metadata JSON with no size check against the destination buffer.

2. Token output loop (index == 0): memcpy(output_tensor->buf + end_pos, buf, strlen(buf)) accumulates token pieces in a loop with no bounds checking, allowing unbounded writes past the buffer.

Fix

• For the metadata path: clamp the copy length to output_tensor->size before calling memcpy.
• For the token output loop: check whether appending the next token piece would exceed output_tensor->size, and break out of the loop if so.

Both fixes use the existing output_tensor->size field from the tensor_data struct defined in wasi_nn_types.h.

Test plan

• Verify that callers providing a sufficiently large buffer see no behavior change.
• Verify that callers providing a small buffer receive truncated output without memory corruption.