Re: [Qemu-block] [PATCH] block/parallels.c: avoid integer overflow in allocate_clusters()

[Denis V. Lunev]

On 03/31/2017 04:47 PM, Max Reitz wrote:
> On 31.03.2017 15:13, Peter Maydell wrote:
>> Coverity (CID 1307776) points out that in the multiply:
>>   space = to_allocate * s->tracks;
>> we are trying to calculate a 64 bit result but the types
>> of to_allocate and s->tracks mean that we actually calculate
>> a 32 bit result. Add an explicit cast to force a 64 bit
>> multiply.
>>
>> Signed-off-by: Peter Maydell <address@hidden>
>> ---
>> NB: compile-and-make-check tested only...
>> ---
>>  block/parallels.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/parallels.c b/block/parallels.c
>> index 4173b3f..3886c30 100644
>> --- a/block/parallels.c
>> +++ b/block/parallels.c
>> @@ -206,7 +206,7 @@ static int64_t allocate_clusters(BlockDriverState *bs, 
>> int64_t sector_num,
>>      }
>>  
>>      to_allocate = DIV_ROUND_UP(sector_num + *pnum, s->tracks) - idx;
>> -    space = to_allocate * s->tracks;
>> +    space = (int64_t)to_allocate * s->tracks;
>>      if (s->data_end + space > bdrv_getlength(bs->file->bs) >> 
>> BDRV_SECTOR_BITS) {
>>          int ret;
>>          space += s->prealloc_size;
> I think the division is technically fine because to_allocate will
> roughly be *pnum / s->tracks (and since *pnum is an int, the
> multiplication cannot overflow).
>
> However, it's still good to fix this, but I would do it differently:
> Make idx, to_allocate, and i all uint64_t or int64_t instead of
> uint32_t. This would also prevent accidental overflow when storing the
> result of the division in:
>
> idx = sector_num / s->tracks;
> if (idx >= s->bat_size) {
>     [...]
>
> The much greater problem to me appears to be that we don't check that
> idx + to_allocate <= s->bat_size. I'm not sure whether there can be a
> buffer overflow in the for loop below, but I'm not sure I really want to
> know either... I think the block_status() call limits *pnum so that
> there will not be an overflow, but then we should at least assert this.
>
> Max
>
technically we are protected by the check in

static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
    BdrvTrackedRequest *req, int64_t offset, unsigned int bytes,
    int64_t align, QEMUIOVector *qiov, int flags)
...
    /* Forward the request to the BlockDriver, possibly fragmenting it */
    total_bytes = bdrv_getlength(bs);
    if (total_bytes < 0) {
        ret = total_bytes;
        goto out;
    }

    max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
    if (bytes <= max_bytes && bytes <= max_transfer) {
        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, 0);
        goto out;
    }

which guarantees that the request is always inside the length of the
device. Thus we should be on the safe side with the mentioned
access as bat_size is calculated from the size of the entire virtual
disk.

Den