* Daniel P. Berrangé (berra...@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 (berra...@redhat.com) wrote: > > >> On Thu, Aug 20, 2020 at 06:30:09PM +0100, Dr. David Alan Gilbert wrote: > > >>> * Chuan Zheng (zhengch...@huawei.com) wrote: > > >>>> Record hash results for each sampled page. > > >>>> > > >>>> Signed-off-by: Chuan Zheng <zhengch...@huawei.com> > > >>>> Signed-off-by: YanYing Zhuang <ann.zhuangyany...@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 > > 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 :| -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
