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. 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 :|
