Re: [PATCH v4 2/2] PCI: Add tango PCIe host bridge support

[Mason]

On 23/05/2017 20:35, Bjorn Helgaas wrote:
> On Tue, May 23, 2017 at 07:43:16PM +0200, Mason wrote:
>> On 23/05/2017 19:24, Bjorn Helgaas wrote:
>>> On Thu, Apr 20, 2017 at 04:31:25PM +0200, Marc Gonzalez wrote:
>>>> This driver is required to work around several hardware bugs in the
>>>> PCIe controller.
>>>>
>>>> NB: Revision 1 does not support legacy interrupts, or IO space.
>>>>
>>>> Signed-off-by: Marc Gonzalez <marc_gonzalez@xxxxxxxxxxxxxxxx>
>>>> ---
>>>>  Documentation/devicetree/bindings/pci/tango-pcie.txt |  32 ++++++++
>>>>  drivers/pci/host/Kconfig                             |   8 ++
>>>>  drivers/pci/host/Makefile                            |   1 +
>>>>  drivers/pci/host/pcie-tango.c                        | 161 +++++++++++++++++++++++++++++++++++++++
>>>>  include/linux/pci_ids.h                              |   2 +
>>>>  5 files changed, 204 insertions(+)
>>>> ...
>>>
>>>> +static int smp8759_config_read(struct pci_bus *bus,
>>>> +		unsigned int devfn, int where, int size, u32 *val)
>>>> +{
>>>> +	int ret;
>>>> +	struct pci_config_window *cfg = bus->sysdata;
>>>> +	struct tango_pcie *pcie = dev_get_drvdata(cfg->parent);
>>>> +
>>>> +	/*
>>>> +	 * QUIRK #1
>>>> +	 * Reads in configuration space outside devfn 0 return garbage.
>>>> +	 */
>>>> +	if (devfn != 0)
>>>> +		return PCIBIOS_FUNC_NOT_SUPPORTED;
>>>> +
>>>> +	/*
>>>> +	 * QUIRK #2
>>>> +	 * Unfortunately, config and mem spaces are muxed.
>>>> +	 * Linux does not support such a setting, since drivers are free
>>>> +	 * to access mem space directly, at any time.
>>>> +	 * Therefore, we can only PRAY that config and mem space accesses
>>>> +	 * NEVER occur concurrently.
>>>> +	 */
>>>> +	writel_relaxed(1, pcie->mux);
>>>> +	ret = pci_generic_config_read(bus, devfn, where, size, val);
>>>> +	writel_relaxed(0, pcie->mux);
>>>
>>> This is a major issue and possibly even a security problem.
>>> Unprivileged users can cause config accesses via lspci/setpci.
>>
>> Since the host bridge doesn't support hotplug in any way,
>> how about setting a flag once enumeration is complete,
>> to prevent all future config accesses?
> 
> I thought about that, too, but I'm not sure it will work becuase I
> think drivers will need to do at least a few config accesses, e.g.,
> pci_enable_device() may write PCI_COMMAND to set PCI_COMMAND_MEMORY,
> it may read PCI_INTERRUPT_PIN, etc.  And MSI setup requires config
> access, too.
> 
>>> I don't have a good suggestion for addressing it, but if you can't
>>> make this work reliably, you need at least a dev_err() in the probe
>>> function and probably a taint of the kernel (see add_taint()).
>>
>> There is a chance that the issue will get fixed in rev2
>> of the bridge. But obviously, that won't help for rev1
>> already in the wild.
> 
> Hopefully there will be a way to distinguish rev2 from rev1.

rev2 is embedded in a new SoC, so the DT node will specify a
different compatible string. A "preview" of the intent was
given in an older patch:
"[RFC PATCH v0.2] PCI: Add support for tango PCIe host bridge"

Regards.