Re: [PATCH 3/4] drivers/misc: Add ASpeed LPC control driver

[Cyril Bur <cyrilbur-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Thu, 2017-01-12 at 08:47 +0100, Greg KH wrote:
> On Thu, Jan 12, 2017 at 11:29:09AM +1100, Cyril Bur wrote:
> > +static ssize_t lpc_ctrl_read(struct file *file, char __user *buf,
> > +				size_t count, loff_t *ppos)
> > +{
> > +	if (!access_ok(VERIFY_WRITE, buf, count))
> > +		return -EFAULT;
> > +
> > +	return -EPERM;
> > +}
> > +
> > +static ssize_t lpc_ctrl_write(struct file *file, const char __user *buf,
> > +				size_t count, loff_t *ppos)
> > +{
> > +	if (!access_ok(VERIFY_READ, buf, count))
> > +		return -EFAULT;
> > +
> > +	return -EPERM;
> > +}
> 

Hello Greg,

> Those functions don't actually do anything, so why even include them?
> 

Apologies, I should be more careful with what I send.

> And don't call access_ok(), it's racy and no driver should ever do that.
> 

Oh, duly noted. I'll be sure to check out how and why. Perhaps it
would be wise that no driver actually do that, I'm quite sure I used
other drivers as examples of best practice.

> > +static long lpc_ctrl_ioctl(struct file *file, unsigned int cmd,
> > +		unsigned long param)
> > +{
> > +	long rc;
> > +	struct lpc_mapping map;
> > +	struct lpc_ctrl *lpc_ctrl = file_lpc_ctrl(file);
> > +	void __user *p = (void __user *)param;
> > +
> > +	switch (cmd) {
> > +	case LPC_CTRL_IOCTL_SIZE:
> > +		return copy_to_user(p, &lpc_ctrl->size,
> > +			sizeof(lpc_ctrl->size)) ? -EFAULT : 0;
> > +	case LPC_CTRL_IOCTL_MAP:
> > +		if (copy_from_user(&map, p, sizeof(map)))
> > +			return -EFAULT;
> > +
> > +
> > +		/*
> > +		 * The top half of HICR7 is the MSB of the BMC address of the
> > +		 * mapping.
> > +		 * The bottom half of HICR7 is the MSB of the HOST LPC
> > +		 * firmware space address of the mapping.
> > +		 *
> > +		 * The 1 bits in the top of half of HICR8 represent the bits
> > +		 * (in the requested address) that should be ignored and
> > +		 * replaced with those from the top half of HICR7.
> > +		 * The 1 bits in the bottom half of HICR8 represent the bits
> > +		 * (in the requested address) that should be kept and pass
> > +		 * into the BMC address space.
> > +		 */
> > +
> > +		rc = regmap_write(lpc_ctrl->regmap, HICR7,
> > +				(lpc_ctrl->base | (map.hostaddr >> 16)));
> > +		if (rc)
> > +			return rc;
> > +
> > +		rc = regmap_write(lpc_ctrl->regmap, HICR8,
> > +			(~(map.size - 1)) | ((map.size >> 16) - 1));
> 
> Look Ma, a kernel exploit!
> 

So 'evil' input here could allow the host to control the bmc,
personally I file that under 'stupid' input. Also, stupid but not
accidental, I don't believe one could accidentally come up with such
input, although you never know what silly things human beings sometimes
do. For what its worth, I'm not even sure that can happen but I'll
grant you the benifit of the doubt.

> {sigh}
> 
> Your assignment is to go find a whiteboard/blackboard/whatever and write
> on it 100 times:
> 	All input is evil.
> 
> I want to see the picture of that before you send any more kernel patches.
> 
> > +static int lpc_ctrl_release(struct inode *inode, struct file *file)
> > +{
> > +	atomic_dec(&lpc_ctrl_open_count);
> 
> Totally unneeded and unnecessary, why do you care?
> 

My aim here was to only have one process playing with the LPC mapping
registers at a time.

> Again, use UIO, it will save you from yourself...
> 

Thank-you! This is the first I've heard of UIO and I'll investigate
furthur!



Sincerely,

Cyril

> thanks,
> 
> greg k-h
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