Re: [PATCH v2 4/8] cxl/mem: Add basic IOCTL interface

[Al Viro]

On Tue, Feb 09, 2021 at 04:02:55PM -0800, Ben Widawsky wrote:

> +static int handle_mailbox_cmd_from_user(struct cxl_memdev *cxlmd,
> +					const struct cxl_mem_command *cmd,
> +					u64 in_payload, u64 out_payload,
> +					struct cxl_send_command __user *s)
> +{
> +	struct cxl_mem *cxlm = cxlmd->cxlm;
> +	struct device *dev = &cxlmd->dev;
> +	struct mbox_cmd mbox_cmd = {
> +		.opcode = cmd->opcode,
> +		.size_in = cmd->info.size_in,
> +	};
> +	s32 user_size_out;
> +	int rc;
> +
> +	if (get_user(user_size_out, &s->out.size))
> +		return -EFAULT;

You have already copied it in.  Never reread stuff from userland - it *can*
change under you.