linux-kernel - Re: [PATCH] PCI: avoid NULL deref in alloc_pcie_link

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Message-ID: <1372132661.30572.354.camel@ul30vt.home>
Date:	Mon, 24 Jun 2013 21:57:41 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Radim Krčmář <rkrcmar@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Myron Stowe <myron.stowe@...hat.com>,
	Joe Lawrence <Joe.Lawrence@...atus.com>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>
Subject: Re: [PATCH] PCI: avoid NULL deref in alloc_pcie_link_state

On Mon, 2013-06-24 at 21:35 -0600, Bjorn Helgaas wrote:
> On Mon, Jun 24, 2013 at 8:58 PM, Alex Williamson
> <alex.williamson@...hat.com> wrote:
> > On Mon, 2013-06-24 at 19:38 -0600, Bjorn Helgaas wrote:
> >> [+cc Michael, Alex, Isaku]
> >>
> >> On Wed, Jun 19, 2013 at 12:56 PM, Radim Krčmář <rkrcmar@...hat.com> wrote:
> >> > PCIe switch upstream port can be connected directly to the PCIe root bus
> >> > in QEMU; ASPM does not expect this topology and dereferences NULL pointer
> >> > when initializing.
> >> >
> >> > I have not confirmed this can happen on real hardware, but it is presented
> >> > as a feature in QEMU, so there is no reason to panic if we can recover.
> >>
> >> This doesn't seem like a valid hardware topology to me.  If this *can*
> >> occur on real hardware, we should fix it in Linux.  If not, maybe QEMU
> >> should be changed to disallow it.
> >
> > I think a quad-port 82576 plugged into an express slot is likely the
> > same topology.
> 
> I don't think that would be the same topology Radim described.  In
> Radim's case, we have this:
> 
>   00:03.0 upstream port
>   01:00.0 downstream port
> 
> and when we call alloc_pcie_link_state() for 01:00.0,
> 
>   pdev is 01:00.0
>   pdev->bus is bus 01
>   pdev->bus->parent is bus 00
>   pdev->bus->parent->self (the bridge device leading to bus 00) is NULL
> 
> But in the case of a quad 82576 plugged into a slot, there would be a
> root port or a downstream port leading to the slot's link, so my guess
> is we'd have something like this (based on lspci output I found at
> [1]):
> 
>   00:05.0 root port leading to slot (bridge to [bus 01-08])
>   01:00.0 upstream port of switch on card (bridge to [bus 02-08])
>   02:02.0 downstream port (bridge to [bus 03-05])
>   02:04.0 downstream port (bridge to [bus 06-08])
>   03:00.0 82576 port 0
>   03:00.1 82576 port 1
>   06:00.0 82576 port 2
>   06:00.1 82576 port 3
> 
> So when we call alloc_pcie_link_state() for 02:02.0,
> 
>   pdev is 02:02.0
>   pdev->bus is bus 02
>   pdev->bus->parent is bus 01
>   pdev->bus->parent->self (the bridge leading to bus 01) is 00:05.0


Oops, I misread his statement.  I don't think an upstream port connected
directly to a root complex is valid.  I thought he was having problems
with an upstream port connected to a root port, which is obviously
valid.  QEMU lets us attach nearly anything to the root complex, most of
them aren't valid.  Thanks,

Alex

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