io_uring: prevent reg-wait speculations

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: io_uring: prevent reg-wait speculations With *ENTER_EXT_ARG_REG instead of passing a user pointer with arguments for the waiting loop the user can specify an offset into a pre-mapped region of memory, in which case the [offset, offset + sizeof(io_uring_reg_wait)) will be intepreted as the argument. As we address a kernel array using a user given index, it'd be a subject to speculation type of exploits. Use array_index_nospec() to prevent that. Make sure to pass not the full region size but truncate by the maximum offset allowed considering the structure size.

References

• https://git.kernel.org/stable/c/2a6de94df7bfa76d9850443547e7b3333f63a16a
• https://git.kernel.org/stable/c/29b95ac917927ce9f95bf38797e16333ecb489b1