risc0 vulnerable to arbitrary code execution in guest via memory safety failure in `sys_read`

RISC Zero is a zero-knowledge verifiable general computing platform based on zk-STARKs and the RISC-V microarchitecture. In versions 2.0.2 and below of risc0-zkvm-platform, when the zkVM guest calls sys_read, the host is able to use a crafted response to write to an arbitrary memory location in the guest. This capability can be leveraged to execute arbitrary code within the guest. As sys_read is the mechanism by which input is requested by the guest, all guest programs built with the affected versions are vulnerable. This critically compromises the soundness guarantees of the guest program. Other affected packages include risc0-aggregation versions below 0.9, risc0-zkos-v1compat below 2.1.0, risc0-zkvm versions between 3.0.0-rc.1 and 3.0.1. This issue has been fixed in the following versions: risc0-zkvm-platform 2.1.0, risc0-zkos-v1compat 2.1.0, risc0-aggregation 0.9, and risc0-zkvm 2.3.2 and 3.0.3.