Re: [Qemu-devel] [PATCH 3/3] scsi: megasas: check 'read_queue_head' index value

[Hannes Reinecke]

On 05/25/2016 01:20 PM, Alexander Graf wrote:
> On 05/25/2016 12:31 PM, P J P wrote:
>> From: Prasad J Pandit <address@hidden>
>>
>> While doing MegaRAID SAS controller command frame lookup, routine
>> 'megasas_lookup_frame' uses 'read_queue_head' value as an index
>> into 'frames[MEGASAS_MAX_FRAMES=2048]' array. Limit its value
>> within array bounds to avoid any OOB access.
>>
>> Reported-by: Li Qiang <address@hidden>
>> Signed-off-by: Prasad J Pandit <address@hidden>
>> ---
>>   hw/scsi/megasas.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/scsi/megasas.c b/hw/scsi/megasas.c
>> index 7c08932..76d57f9 100644
>> --- a/hw/scsi/megasas.c
>> +++ b/hw/scsi/megasas.c
>> @@ -649,8 +649,10 @@ static int megasas_init_firmware(MegasasState
>> *s, MegasasCmd *cmd)
>>       pa_lo = le32_to_cpu(initq->pi_addr_lo);
>>       pa_hi = le32_to_cpu(initq->pi_addr_hi);
>>       s->producer_pa = ((uint64_t) pa_hi << 32) | pa_lo;
>> -    s->reply_queue_head = ldl_le_pci_dma(pcid, s->producer_pa);
>> -    s->reply_queue_tail = ldl_le_pci_dma(pcid, s->consumer_pa);
>> +    s->reply_queue_head = \
>> +        ldl_le_pci_dma(pcid, s->producer_pa) % MEGASAS_MAX_FRAMES;
>> +    s->reply_queue_tail = \
>> +        ldl_le_pci_dma(pcid, s->consumer_pa) % MEGASAS_MAX_FRAMES;
> 
> You're using up 2 lines for each of those assignments now anyway, so
> why not just split it into
> 
>   s->reply_queue_head = ldl_le_pci_dma(pcid, s->producer_pa);
>   s->reply_queue_head %= MEGASAS_MAX_FRAMES;
> 
> and that way avoid a real multi-line statement (which is hard to read).
> 
> Whether the bitmask really is what hardware would do, I don't know.
> I'll let Hannes comment there. Maybe the hardware simply does
> 
>   s->reply_queue_head = MIN(ldl_le_pci_dma(pcid, s->producer_pa),
> MEGASAS_MAX_FRAMES);
> 
> But I agree that the mask is more likely.
> 
->reply_queue_head (and, consequently, ->producer_pa) is a ring
buffer, which are supposed to wrap at MEGASAS_MAX_FRAMES.
So 'MIN' is wrong; we need the mask as we _want_ the overflow to happen.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Teamlead Storage & Networking
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