Re: [Qemu-devel] [PATCH] macio: fix overflow in lba to offset conversion for ATAPI devices

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 04/01/16 21:03, John Snow wrote:

> On 01/04/2016 03:54 PM, Mark Cave-Ayland wrote:
>> On 04/01/16 20:36, John Snow wrote:
>>
>>> On 01/04/2016 02:15 PM, Mark Cave-Ayland wrote:
>>>> On 04/01/16 19:04, P J P wrote:
>>>>
>>>>> +-- On Mon, 4 Jan 2016, Mark Cave-Ayland wrote --+
>>>>> |      /* Calculate current offset */
>>>>> | -    offset = (int64_t)(s->lba << 11) + s->io_buffer_index;
>>>>> | +    offset = ((int64_t)(s->lba) << 11) + s->io_buffer_index;
>>>>>
>>>>> Maybe ((int64_t)s->lba << 11) ? No parenthesis around s->lba.
>>>>
>>>> Yes that works here too (perhaps I was just being over-cautious).
>>>> Alex/John, please let me know if you want me to resubmit.
>>>>
>>>
>>> PJP's version should work just fine. I won't ask you to resubmit, though...
>>
>> Great, thanks :)
>>
>>> ...But, well, while we're here, I have a question for you:
>>>
>>> So s->lba is an int that we left shift by 11 for a max of (2^43 - 2^11)
>>> then we add it against s->io_buffer_index, a uint64_t, so this statement
>>> could still in theory overflow.
>>>
>>> Except not really, since io_buffer_index is bounded (in general) by
>>> io_buffer_total_len, which is usually (IDE_DMA_BUF_SECTORS*512 + 4) ->
>>> ~132K.
>>>
>>> I don't think there's any rigorous bounds-checking of io_buffer_index,
>>> just ad-hoc checking when we're good enough to remember to do it. And we
>>> don't seem to do it anywhere in macio. Is it worth peppering in an
>>> assert somewhere that io_buffer_index is reasonably small?
>>
>> The DBDMA engine is limited to 16-bit transfers so the maximum transfer
>> size is 64K, and s->io_buffer_index is used to hold the current position
>> within this transfer so unless we get some very large disks I think we
>> should be okay here?
>>
> 
> For all non-malicious uses of the code, yes.
> 
> If I want to apply some rigorous checking to this bound I should just
> add a function to manipulate it centrally in core.c, I think.

That sounds good - any solution that avoids having to maintain changes
across IDE core and macio separately is always good!

> I'll pull this and edit it to PJP's suggestion.

Brilliant - thanks a lot!


ATB,

Mark.