linux-kernel - Re: [PATCH v3 5/8] platform/x86: dell-wmi-smbios: introduce character device for userspace

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Date:   Tue, 3 Oct 2017 08:20:10 -0700
From:   Darren Hart <dvhart@...radead.org>
To:     Greg KH <greg@...ah.com>
Cc:     Mario Limonciello <mario.limonciello@...l.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>, quasisec@...gle.com,
        pali.rohar@...il.com
Subject: Re: [PATCH v3 5/8] platform/x86: dell-wmi-smbios: introduce
 character device for userspace

On Tue, Oct 03, 2017 at 11:26:11AM +0200, Greg KH wrote:
> On Wed, Sep 27, 2017 at 11:02:17PM -0500, Mario Limonciello wrote:
> > +static int dell_wmi_smbios_open(struct inode *inode, struct file *file)
> > +{
> > +	return nonseekable_open(inode, file);
> > +}
> > +
> > +static int dell_wmi_smbios_release(struct inode *inode, struct file *file)
> > +{
> > +	return 0;
> > +}
> 
> Why even declare an open/release if you don't do anything with them?
> Just leave them empty.
> 
> > +static long dell_wmi_smbios_ioctl(struct file *filp, unsigned int cmd,
> > +	unsigned long arg)
> > +{
> > +	void __user *p = (void __user *) arg;
> > +	size_t size;
> > +	int ret = 0;
> > +
> > +	if (_IOC_TYPE(cmd) != DELL_WMI_SMBIOS_IOC)
> > +		return -ENOTTY;
> > +
> > +	switch (cmd) {
> > +	case DELL_WMI_SMBIOS_CALL_CMD:
> > +		size = sizeof(struct wmi_calling_interface_buffer);
> > +		mutex_lock(&buffer_mutex);
> > +		if (copy_from_user(devfs_buffer, p, size)) {
> > +			ret = -EFAULT;
> > +			goto fail_smbios_cmd;
> > +		}
> > +		ret = run_wmi_smbios_call(devfs_buffer);
> 
> You _are_ checking that your structures are valid here, right?  I didn't
> see you really were, so I'll let you go audit your code paths for the
> next set of patches.
> 
> But really, ugh, this seems horrible.  It's a huge vague data blob going
> both ways, this feels ripe for abuse and other bad things.  Do you have
> a working userspace implementation for all of this to publish at the
> same time?
> 

This is the general challenge with WMI support, as it was designed to
provide userspace with a way to talk to the firmware.

For the more general case going forward the intent is to perform the MOF
parsing in the kernel, which will make it possible to do some automated
buffer validation.

This particular driver is an intermediate step improving on the
mechanism used for existing devices (libsmbios -> dcdbas -> SMM) to use
the safer WMI entry point (using an op region instead of passing a
physical memory pointer to SMM).

But, that said... Mario, in lieu of the MOF description of the buffer,
we should be able to do some driver specific validation of this buffer
here.

-- 
Darren Hart
VMware Open Source Technology Center