Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc/e500_pci: Fix an array overflow issue

[Alexander Graf]

On 27.09.2011, at 18:52, Scott Wood wrote:

> On 09/27/2011 07:45 AM, Alexander Graf wrote:
>> On 27.09.2011, at 10:17, Liu Yu wrote:
>>> ---
>>> hw/ppce500_pci.c |   26 ++++++++++++++++----------
>>> 1 files changed, 16 insertions(+), 10 deletions(-)
>>> 
>>> diff --git a/hw/ppce500_pci.c b/hw/ppce500_pci.c
>>> index 2db365d..3e24e85 100644
>>> --- a/hw/ppce500_pci.c
>>> +++ b/hw/ppce500_pci.c
>>> @@ -108,15 +108,18 @@ static uint32_t pci_reg_read4(void *opaque, 
>>> target_phys_addr_t addr)
>>> 
>>>    case PPCE500_PCI_IW3:
>>>    case PPCE500_PCI_IW2:
>>> -    case PPCE500_PCI_IW1:
>>> +    case PPCE500_PCI_IW1: {
>>> +        int idx = ((addr >> 5) & 0x3) - 1;
>> 
>> So this is the main change, right? Why the -1? A guest could potentially 
>> access pib[-1] using this, no?
> 
> Not with the values of addr that lead to this code.  The -1 is because
> IW1/2/3 are 0x1e0/0x1c0/0x1a0.  Previously IW1 would overflow the array.

We're matching on addr & 0xfe0 and do the switch based on that. Possible values 
are:

  0x1a0
  0x1c0
  0x1e0

Then we >> 5 them.

  0xd
  0xe
  0xf

... and & 0x3 them

  0x1
  0x2
  0x0

... and apply -1:

  0x0
  0x1
  -1

> 
>>>        switch (addr & 0xC) {
>>> -        case PCI_PITAR: value = pci->pib[(addr >> 5) & 0x3].pitar; break;
>>> -        case PCI_PIWBAR: value = pci->pib[(addr >> 5) & 0x3].piwbar; break;
>>> -        case PCI_PIWBEAR: value = pci->pib[(addr >> 5) & 0x3].piwbear; 
>>> break;
>>> -        case PCI_PIWAR: value = pci->pib[(addr >> 5) & 0x3].piwar; break;
>>> +        case PCI_PITAR: value = pci->pib[idx].pitar; break;
>>> +        case PCI_PIWBAR: value = pci->pib[idx].piwbar; break;
>>> +        case PCI_PIWBEAR: value = pci->pib[idx].piwbear; break;
>>> +        case PCI_PIWAR: value = pci->pib[idx].piwar; break;
>> 
>> I'm fairly sure this breaks checkpatch.pl.
> 
> So does the original code...
> 
> If this is to be fixed, the outbound window switch should be fixed too
> (and made to use idx, for consistency).

Yes, please. My preferred way to do this would be to send a cleanup patch that 
fixes the coding style issues first (can be in the same patch set) and then 
another patch for functional changes on top. Makes it easier to review.


Alex