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

From: Ben Widawsky <ben.widawsky@intel.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-cxl@vger.kernel.org, linux-acpi@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-pci@vger.kernel.org, Bjorn Helgaas <helgaas@kernel.org>,
	Chris Browy <cbrowy@avery-design.com>,
	Christoph Hellwig <hch@infradead.org>,
	Dan Williams <dan.j.williams@intel.com>,
	David Hildenbrand <david@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Ira Weiny <ira.weiny@intel.com>, Jon Masters <jcm@jonmasters.org>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Rafael Wysocki <rafael.j.wysocki@intel.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Vishal Verma <vishal.l.verma@intel.com>,
	"John Groves (jgroves)" <jgroves@micron.com>,
	"Kelley, Sean V" <sean.v.kelley@intel.com>,
	kernel test robot <lkp@intel.com>,
	Dan Williams <dan.j.willams@intel.com>
Subject: Re: [PATCH v2 4/8] cxl/mem: Add basic IOCTL interface
Date: Sun, 14 Feb 2021 15:14:56 -0800	[thread overview]
Message-ID: <20210214231456.xnwitliczv6qwmjv@intel.com> (raw)
In-Reply-To: <YClQEefYBR+YKBUv@zeniv-ca.linux.org.uk>

On 21-02-14 16:30:09, Al Viro wrote:
> 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.

As it turns out, this is some leftover logic which doesn't need to exist at all,
and I'm happy to change it. Thanks for reviewing.

I wasn't familiar with this restriction though. For my edification could you
explain how that could happen? Also, is this something that should go in the
kdocs, because I don't see anything about this restriction there.

Thanks.
Ben