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: d617b31 ("io_uring: restore back registered wait arguments")
Fixes: aa00f67 ("io_uring: add support for fixed wait regions")
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/r/1e3d9da7c43d619de7bcf41d1cd277ab2688c443.1733694126.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence

io_uring/io_uring.c

@@ -3211,6 +3211,7 @@ static struct io_uring_reg_wait *io_get_ext_arg_reg(struct io_ring_ctx *ctx,
 		     end > ctx->cq_wait_size))
 		return ERR_PTR(-EFAULT);
 
+	offset = array_index_nospec(offset, ctx->cq_wait_size - size);
 	return ctx->cq_wait_arg + offset;
 }