Re: [PATCH v3 05/10] migration/dirtyrate: Record hash results for each sampled page

[Zheng Chuan]

On 2020/8/21 20:39, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (berrange@redhat.com) wrote:
>> On Fri, Aug 21, 2020 at 08:22:06PM +0800, Zheng Chuan wrote:
>>>
>>>
>>> On 2020/8/21 1:55, Dr. David Alan Gilbert wrote:
>>>> * Daniel P. Berrangé (berrange@redhat.com) wrote:
>>>>> On Thu, Aug 20, 2020 at 06:30:09PM +0100, Dr. David Alan Gilbert wrote:
>>>>>> * Chuan Zheng (zhengchuan@huawei.com) wrote:
>>>>>>> Record hash results for each sampled page.
>>>>>>>
>>>>>>> Signed-off-by: Chuan Zheng <zhengchuan@huawei.com>
>>>>>>> Signed-off-by: YanYing Zhuang <ann.zhuangyanying@huawei.com>
>>>>>>> ---
>>>>>>>  migration/dirtyrate.c | 144 
>>>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>  migration/dirtyrate.h |   7 +++
>>>>>>>  2 files changed, 151 insertions(+)
>>>>>>>
>>>>>>> diff --git a/migration/dirtyrate.c b/migration/dirtyrate.c
>>>>>>> index c4304ef..62b6f69 100644
>>>>>>> --- a/migration/dirtyrate.c
>>>>>>> +++ b/migration/dirtyrate.c
>>>>>>> @@ -25,6 +25,7 @@
>>>>>>>  #include "dirtyrate.h"
>>>>>>>  
>>>>>>>  CalculatingDirtyRateState CalculatingState = CAL_DIRTY_RATE_INIT;
>>>>>>> +static unsigned long int qcrypto_hash_len = QCRYPTO_HASH_LEN;
>>>>>>
>>>>>> Why do we need this static rather than just using the QCRYPTO_HASH_LEN ?
>>>>>> It's never going to change is it?
>>>>>> (and anyway it's just a MD5 len?)
>>>>>
>>>>> I wouldn't want to bet on that given that this is use of MD5. We might
>>>>> claim this isn't security critical, but surprises happen, and we will
>>>>> certainly be dinged on security audits for introducing new use of MD5
>>>>> no matter what.
>>>>>
>>>>> If a cryptographic hash is required, then sha256 should be the choice
>>>>> for any new code that doesn't have back compat requirements.
>>>>>
>>>>> If a cryptographic hash is not required then how about crc32 
>>>>
>>>> It doesn't need to be cryptographic; is crc32 the fastest reasonable hash 
>>>> for use
>>>> in large areas?
>>>>
>>>> Dave
>>>>
>>>>> IOW, it doesn't make a whole lot of sense to say we need a cryptographic
>>>>> hash, but then pick the most insecure one.
>>>>>
>>>>> sha256 is slower than md5, but it is conceivable that in future we might
>>>>> gain support for something like Blake2b which is similar security level
>>>>> to SHA3, while being faster than MD5.
>>>>>
>>>>> Overall I'm pretty unethusiastic about use of MD5 being introduced and
>>>>> worse, being hardcoded as the only option.
>>>>>
>>>>> Regards,
>>>>> Daniel
>>>>> -- 
>>>>> |: https://berrange.com      -o-    
>>>>> https://www.flickr.com/photos/dberrange :|
>>>>> |: https://libvirt.org         -o-            
>>>>> https://fstop138.berrange.com :|
>>>>> |: https://entangle-photo.org    -o-    
>>>>> https://www.instagram.com/dberrange :|
>>>
>>> Hi, Daniel, Dave.
>>>
>>> I do compare MD5 and SHA256 with vm memory of 128G under mempress of 100G.
>>>
>>> 1. Calculation speed
>>> 1） MD5 takes about 500ms to sample and hash all pages by 
>>> record_ramblock_hash_info().
>>> 2)  SHA256 takes about 750ms to sample all pages by 
>>> record_ramblock_hash_info().
>>>
>>> 2. CPU Consumption
>>> 1)  MD5 may have instant rise up to 48% for dirtyrate thread
>>> 2)  SHA256 may have instant rise up to 75% for dirtyrate thread
>>>
>>> 3. Memory Consumption
>>> SHA256 may need twice memory than MD5 due to its HASH_LEN.
>>>
>>> I am trying to consider if crc32 is more faster and takes less memory and 
>>> is more safer than MD5?
>>
>> No, crc32 is absolutely *weaker* than MD5. It is NOT a cryptographic
>> hash so does not try to guarantee collision resistance. It only has
>> 2^32 possible outputs.
>>
>> MD5 does try to guarantee collision resistance, but MD5 is considered
>> broken these days, so a malicious attacker can cause collisions if they
>> are motivated enough.
>>
>> IOW if you need collision resistance that SHA256 should be used.
> 
> There's no need to guard against malicious behaviour here - this is just
> a stat to guide migration.
> If CRC32 is likely to be faster than md5 I suspect it's enough.
> 
> Dave
> 
OK, i'll take a test by crc32.

>>
>> Regards,
>> Daniel
>> -- 
>> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange 
>> :|
>> |: https://libvirt.org         -o-            https://fstop138.berrange.com 
>> :|
>> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange 
>> :|