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From: Wei Liu <wei.liu@kernel.org>
To: Praveen Kumar <kumarpraveen@linux.microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>,
	Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
	virtualization@lists.linux-foundation.org,
	Linux Kernel List <linux-kernel@vger.kernel.org>,
	Michael Kelley <mikelley@microsoft.com>,
	Vineeth Pillai <viremana@linux.microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Nuno Das Neves <nunodasneves@linux.microsoft.com>,
	pasha.tatashin@soleen.com, "K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Dexuan Cui <decui@microsoft.com>, Joerg Roedel <joro@8bytes.org>,
	Will Deacon <will@kernel.org>,
	"open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>
Subject: Re: [RFC v1 6/8] mshv: command line option to skip devices in PV-IOMMU
Date: Tue, 10 Aug 2021 10:04:19 +0000	[thread overview]
Message-ID: <20210810100419.g3rjj3xegydalyz3@liuwe-devbox-debian-v2> (raw)
In-Reply-To: <8d9b6b9a-62b1-95ea-1bb6-258e72c1800d@linux.microsoft.com>

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.

  reply	other threads:[~2021-08-10 10:04 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-09 11:43 [RFC v1 0/8] MSHV: add PV-IOMMU driver Wei Liu
2021-07-09 11:43 ` [RFC v1 1/8] x86/hyperv: export hv_build_pci_dev_id Wei Liu
2021-07-09 11:43 ` [RFC v1 2/8] asm-generic/hyperv: add device domain definitions Wei Liu
2021-07-09 11:43 ` [RFC v1 3/8] intel/vt-d: make DMAR table parsing code more flexible Wei Liu
2021-07-09 12:56   ` Robin Murphy
2021-07-09 13:42     ` Wei Liu
2021-07-09 11:43 ` [RFC v1 4/8] intel/vt-d: export intel_iommu_get_resv_regions Wei Liu
2021-07-09 14:17   ` Lu Baolu
2021-07-09 14:21     ` Wei Liu
2021-07-09 11:43 ` [RFC v1 5/8] mshv: add paravirtualized IOMMU support Wei Liu
2021-08-03 18:40   ` Praveen Kumar
2021-08-03 21:47     ` Wei Liu
2021-08-04  6:43       ` Praveen Kumar
2021-08-10 10:46         ` Wei Liu
2021-07-09 11:43 ` [RFC v1 6/8] mshv: command line option to skip devices in PV-IOMMU Wei Liu
2021-07-09 12:46   ` Robin Murphy
2021-07-09 13:34     ` Wei Liu
2021-08-03 18:50   ` Praveen Kumar
2021-08-03 21:56     ` Wei Liu
2021-08-04  7:03       ` Praveen Kumar
2021-08-10 10:04         ` Wei Liu [this message]
2021-07-09 11:43 ` [RFC v1 7/8] mshv: implement in-kernel device framework Wei Liu
2021-07-09 13:02   ` Matthew Wilcox
2021-07-09 13:50     ` Wei Liu
2021-07-09 15:32       ` Matthew Wilcox
2021-07-09 16:27         ` Wei Liu
2021-07-09 16:38           ` Matthew Wilcox
2021-07-09 19:14             ` Wei Liu
2021-07-09 19:48               ` Matthew Wilcox
2021-07-09 20:11                 ` Wei Liu
2021-08-03 19:12   ` Praveen Kumar
2021-08-03 22:04     ` Wei Liu
2021-07-09 11:43 ` [RFC v1 8/8] mshv: add vfio bridge device Wei Liu
2021-08-03 19:27   ` Praveen Kumar
2021-08-10 10:52     ` Wei Liu

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