linux-kernel - Re: [PATCH v3 3/4] lib: logic_pio: Reject accesses to unregistered CPU MMIO regions

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Date:   Mon, 8 Apr 2019 09:01:14 +0100
From:   John Garry <john.garry@...wei.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     Guenter Roeck <linux@...ck-us.net>, <wangkefeng.wang@...wei.com>,
        <lorenzo.pieralisi@....com>, <arnd@...db.de>, <rafael@...nel.org>,
        <linux-pci@...r.kernel.org>, <will.deacon@....com>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...wei.com>,
        <andy.shevchenko@...il.com>, <catalin.marinas@....com>,
        <bp@...e.de>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 3/4] lib: logic_pio: Reject accesses to unregistered
 CPU MMIO regions

On 05/04/2019 19:06, Bjorn Helgaas wrote:
> On Fri, Apr 05, 2019 at 09:10:27AM +0100, John Garry wrote:
>> On 04/04/2019 19:58, Bjorn Helgaas wrote:
>>> On Thu, Apr 04, 2019 at 10:43:36AM -0700, Guenter Roeck wrote:
>>>> On Thu, Apr 04, 2019 at 05:52:35PM +0100, John Garry wrote:
>>>>>>> Note that the f71805f driver does not call
>>>>>>> request_{muxed_}region(), as it should.
>>>>>
>>>>>> ... which is the real problem, one that is not solved by this
>>>>>> patch. This may result in parallel and descructive accesses if
>>>>>> there is another device on the LPC bus, and another driver
>>>>>> accessing that device. Personally I'd rather have
>>>>>> request_muxed_region() added to the f71805f driver.
>>>>>
>>>>> Right, we should and will still fix f71805f. If you recall, I did
>>>>> have the f71805f fix in the v1 series, but you committed that it
>>>>> was orthogonal, so I decided to take it out of this work for now.
>>>>>
>>>>> And even if we fix up f71805f and other known drivers which don't
>>>>> call request_muxed_region(), we still need to police against these
>>>>> rogue accesses, which is what this patch attempts to do.
>>>>>
>>>> Do we ? I am personally not convinced that LPC accesses _have_ to
>>>> occur through PCI on any given system.
>>>
>>> On current systems, I suspect ISA/LPC devices are typically connected
>>> via a PCI-to-ISA/LPC bridge.  But AFAIK there's no actual requirement
>>> for that bridge, and there certainly *were* systems with ISA devices
>>> but no PCI at all.
>>>
>>> IMO, if you want to build ISA drivers on your arch, you need to make
>>> sure the inb() probing done by those drivers works like it does on
>>> x86.  If there's no device there, the inb() should return 0xff with no
>>> fuss and no crash.
>>
>> Right, and this is what I am attempting to do here.
>>
>> So today a call to request_muxed_region() can still succeed even if no IO
>> space mapped.
>>
>> As such, even well-behaved drivers like f71882fg can still crash the system,
>> as noted in RFC patch 1/4 ("resource: Request IO port regions from children
>> of ioport_resource").
>

Correction:

"As such, *on arm* even well-behaved drivers like f71882fg can still 
crash the system, ...

So, yes, x86 - which has native IO ports - would not have this issue.

> Maybe I'm missing something, but on x86, drivers like f71882fg do not
> crash the system because inb() *never* causes a crash.
>
> If you want to build that driver for ARM, I think you need to make
> sure that inb() on ARM also *never* causes a crash.  I don't think
> changing f71882fg and all the similar drivers is the right answer.
>

Right, so this is the intention of this patch.

However it would be still good to find a way to fail a request to claim 
an IO port region if none is accessible or mapped.

Thanks,
John

> Bjorn
>
> .
>