Subject: [PATCH v2 4/5] PCI: generic: Correct, and avoid overflow, in bus_max calculation.

From: [email-masked] (David Daney)
Date: Wed, 23 Sep 2015 11:21:56 -0700

On 09/23/2015 11:01 AM, Will Deacon wrote:
> On Thu, Sep 17, 2015 at 11:02:11PM +0100, David Daney wrote:
[...]
>
>>   Properties of the /chosen node:
>> diff --git a/drivers/pci/host/pci-host-generic.c b/drivers/pci/host/pci-host-generic.c
>> index 77cf4bd..0a9c453 100644
>> --- a/drivers/pci/host/pci-host-generic.c
>> +++ b/drivers/pci/host/pci-host-generic.c
>> @@ -164,7 +164,7 @@ out_release_res:
>>   static int gen_pci_parse_map_cfg_windows(struct gen_pci *pci)
>>   {
>>   	int err;
>> -	u8 bus_max;
>> +	int bus_max;
>>   	resource_size_t busn;
>>   	struct resource *bus_range;
>>   	struct device *dev = pci->host.dev.parent;
>> @@ -177,8 +177,9 @@ static int gen_pci_parse_map_cfg_windows(struct gen_pci *pci)
>>   	}
>>
>>   	/* Limit the bus-range to fit within reg */
>> -	bus_max = pci->cfg.bus_range->start +
>> -		  (resource_size(&pci->cfg.res) >> pci->cfg.ops.bus_shift) - 1;
>> +	bus_max = (resource_size(&pci->cfg.res) >> pci->cfg.ops.bus_shift) - 1;
>> +	if (bus_max > 255)
>> +		bus_max = 255;
>
> I still don't understand the need for this part. If the cfg space is bigger
> than bus_max, isn't that simply an invalid resource? Given that the resource
> could be broken in other ways too, this check feels more like a specific
> workaround rather than generally useful code.

Imagine...

   bus-range [0x80 .. 0xff], this requires a cfg.res that will cover the 
entire range of 0..0xff.

   according to the calculations above, (resource_size(&pci->cfg.res) >> 
pci->cfg.ops.bus_shift) - 1 will have a value of 0xff, so...

   bus_max = 0x80 + 0xff -> OVERFLOW of u8!

That is not useful.  bus_max should represent the largest bus number 
that can be covered by cfg.res.  That is what my patch is attempting to 
accomplish.  Calculate the largest bus number that can be accommodated 
by cfg.res, and then clamp it to 0xff.

David Daney.