riscv: kvm: Fix out-of-bounds array access

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: riscv: kvm: Fix out-of-bounds array access In kvm_riscv_vcpu_sbi_init() the entry->ext_idx can contain an out-of-bound index. This is used as a special marker for the base extensions, that cannot be disabled. However, when traversing the extensions, that special marker is not checked prior indexing the array. Add an out-of-bounds check to the function.

References

• https://git.kernel.org/stable/c/3c49e1084a5df99807fc43dd318c491e6cbaa168
• https://git.kernel.org/stable/c/b1af648f0d610665c956ea4604d9f797e5c7e991
• https://git.kernel.org/stable/c/332fa4a802b16ccb727199da685294f85f9880cb