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

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Message-ID: <7a46cfa6-3c8a-33a6-a821-da49535c5c8a@free.fr>
Date:   Tue, 23 May 2017 19:43:16 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Robin Murphy <robin.murphy@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        David Laight <david.laight@...lab.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Phuong Nguyen <phuong_nguyen@...madesigns.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] PCI: Add tango PCIe host bridge support

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@...madesigns.com>
>> ---
>>  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 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.

Regards.