Re: [RFC v1 6/8] mshv: command line option to skip devices in PV-IOMMU

[Praveen Kumar <kumarpraveen@linux.microsoft.com>]

On 04-08-2021 03:26, Wei Liu wrote:
>>>  	struct iommu_domain domain;
>>> @@ -774,6 +784,41 @@ static struct iommu_device *hv_iommu_probe_device(struct device *dev)
>>>  	if (!dev_is_pci(dev))
>>>  		return ERR_PTR(-ENODEV);
>>>  
>>> +	/*
>>> +	 * Skip the PCI device specified in `pci_devs_to_skip`. This is a
>>> +	 * temporary solution until we figure out a way to extract information
>>> +	 * from the hypervisor what devices it is already using.
>>> +	 */
>>> +	if (pci_devs_to_skip && *pci_devs_to_skip) {
>>> +		int pos = 0;
>>> +		int parsed;
>>> +		int segment, bus, slot, func;
>>> +		struct pci_dev *pdev = to_pci_dev(dev);
>>> +
>>> +		do {
>>> +			parsed = 0;
>>> +
>>> +			sscanf(pci_devs_to_skip + pos,
>>> +				" (%x:%x:%x.%x) %n",
>>> +				&segment, &bus, &slot, &func, &parsed);
>>> +
>>> +			if (parsed <= 0)
>>> +				break;
>>> +
>>> +			if (pci_domain_nr(pdev->bus) == segment &&
>>> +				pdev->bus->number == bus &&
>>> +				PCI_SLOT(pdev->devfn) == slot &&
>>> +				PCI_FUNC(pdev->devfn) == func)
>>> +			{
>>> +				dev_info(dev, "skipped by MSHV IOMMU\n");
>>> +				return ERR_PTR(-ENODEV);
>>> +			}
>>> +
>>> +			pos += parsed;
>>> +
>>> +		} while (pci_devs_to_skip[pos]);
>>
>> Is there a possibility of pci_devs_to_skip + pos > sizeof(pci_devs_to_skip)
>> and also a valid memory ?
> 
> pos should point to the last parsed position. If parsing fails pos does
> not get updated and the code breaks out of the loop. If parsing is
> success pos should point to either the start of next element of '\0'
> (end of string). To me this is good enough.

The point is, hypothetically the address to pci_devs_to_skip + pos can be valid address (later to '\0'), and thus there is a possibility, that parsing may not fail.
Another, there is also a possibility of sscanf faulting accessing the illegal address, if pci_devs_to_skip[pos] turns out to be not NULL or valid address.

> 
>> I would recommend to have a check of size as well before accessing the
>> array content, just to be safer accessing any memory.
>>
> 
> What check do you have in mind?

Something like,
size_t len = strlen(pci_devs_to_skip);
do {

	len -= parsed;
} while (len);

OR

do {
...
	pos += parsed;
} while (pos < len);

Further, I'm also fine with the existing code, if you think this won't break and already been taken care. Thanks.

Regards,

~Praveen.