io_uring: prevent reg-wait speculations

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.

Fixes: d617b3147d54c ("io_uring: restore back registered wait arguments")
Fixes: aa00f67adc2c0 ("io_uring: add support for fixed wait regions")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1e3d9da7c43d619de7bcf41d1cd277ab2688c443.1733694126.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>