Re: [RFC PATCH 1/1] device-dax: check for vma range while dax_mmap.

[Dave Jiang]

On 08/02/2018 02:32 AM, Zhang,Yi wrote:
> 
> 
> On 2018å08æ02æ 03:40, Dave Jiang wrote:
>>
>> On 07/31/2018 04:46 AM, Zhang Yi wrote:
>>> It should be prevent user map an illegal vma range which larger than
>>> dax device phiscal resourse, as we don't have swap logic while page
>>> faulting in dax device.
>> This patch prevents a user mapping an illegal vma range that is larger
>> than a dax device physical resource.
>>
>>> Applications, especailly qemu, map the /dev/dax for virtual nvdimm's
>>> backend device, we defined the v-nvdimm label area at the end of mapped
>>> rang. By using an illegal size that exceeds the physical resource of
>>> /dev/dax, then it will triger qemu a signal fault while accessing these
>>> label area.
>> When qemu maps the dax device for virtual nvdimm's backend device, the
>> v-nvdimm label area is defined at the end of mapped range. By using an
>> illegal size that exceeds the range of the device dax, it will trigger a
>> fault with qemu.
> Thanks Dava, that's could be much better.
>>
>>> Signed-off-by: Zhang Yi <yi.z.zhang@xxxxxxxxxxxxxxx>
>>> ---
>>>  drivers/dax/device.c | 28 ++++++++++++++++++++++++++++
>>>  1 file changed, 28 insertions(+)
>>>
>>> diff --git a/drivers/dax/device.c b/drivers/dax/device.c
>>> index aff2c15..c9a50cd 100644
>>> --- a/drivers/dax/device.c
>>> +++ b/drivers/dax/device.c
>>> @@ -177,6 +177,32 @@ static const struct attribute_group *dax_attribute_groups[] = {
>>>  	NULL,
>>>  };
>>>  
>>> +static int check_vma_range(struct dev_dax *dev_dax, struct vm_area_struct *vma,
>>> +		const char *func)
>>> +{
>>> +	struct device *dev = &dev_dax->dev;
>>> +	struct resource *res;
>>> +	unsigned long size;
>>> +	int ret, i;
>>> +
>>> +	if (!dax_alive(dev_dax->dax_dev))
>>> +		return -ENXIO;
>>> +
>>> +	size = vma->vm_end - vma->vm_start + (vma->vm_pgoff << PAGE_SHIFT);
>>> +	ret = -EINVAL;
>>> +	for (i = 0; i < dev_dax->num_resources; i++) {
>>> +		res = &dev_dax->res[i];
>>> +		if (size > resource_size(res)) {
>>> +			dev_info(dev, "%s: %s: fail, vma range is overflow\n",
>>> +				current->comm, func);
>>> +			ret = -EINVAL;
>>> +			continue;
>>> +		} else
>>> +			return 0;
>>> +	}
>>> +	return ret;
>>> +}
>>> +
>>>  static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
>>>  		const char *func)
>>>  {
>>> @@ -465,6 +491,8 @@ static int dax_mmap(struct file *filp, struct vm_area_struct *vma)
>>>  	 */
>>>  	id = dax_read_lock();
>>>  	rc = check_vma(dev_dax, vma, __func__);
>>> +	if (!rc)
>>> +		rc |= check_vma_range(dev_dax, vma, __func__);

I don't see any reason to logical or the return code. It should be 0, so
you can just assign it to check_vma_range().

>> I think you want to augment check_vma() rather than adding another
>> function? If this is added inside check_vma() then you can also skip the
>> !dax_alive() check. Do you expect this function to be called anywhere else?
> since check_vma range also will be called while dax page faulting.  I
> don't wanna this check_vma_range introduce some additional workload
> while page fault. just let it checked in the mmap scenario

Good point. Ok.

>>
>>>  	dax_read_unlock(id);
>>>  	if (rc)
>>>  		return rc;
>>>
>