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

[Wei Liu]

On Wed, Aug 04, 2021 at 12:33:54PM +0530, Praveen Kumar wrote:
> 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.

Have you found an example how at any given point in time
pci_devs_to_skip + pos can point outside of user provided string?

> 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.

That depends on pci_devs_to_skip + pos can point to an invalid address
in the first place, so that goes back to the question above.

> 
> > 
> >> 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.

But in the loop somewhere you will still need to parse pci_devs_to_skip
+ some_offset. The new code structure does not remove that, right?

Given this is for debugging and is supposed to be temporary, I think the
code is good enough. But I want to make sure if there is anything I
missed.

Wei.

> 
> Regards,
> 
> ~Praveen.