On Thu, 2021-04-01 at 18:02 +0200, Rafael J. Wysocki wrote: > On Thu, Apr 1, 2021 at 3:54 PM Ard Biesheuvel <a...@kernel.org> wrote: > > On Thu, 1 Apr 2021 at 15:38, Rafael J. Wysocki <raf...@kernel.org> wrote: > > > On Thu, Apr 1, 2021 at 10:47 AM Ard Biesheuvel <a...@kernel.org> wrote: > > > > On Tue, 30 Mar 2021 at 21:56, Simo Sorce <s...@redhat.com> wrote: > > > > > On Tue, 2021-03-30 at 21:45 +0200, Ard Biesheuvel wrote: > > > > > > On Tue, 30 Mar 2021 at 20:05, Simo Sorce <s...@redhat.com> wrote: > > > > > > > On Tue, 2021-03-30 at 16:46 +0200, Rafael J. Wysocki wrote: > > > > > > > > On Tue, Mar 30, 2021 at 12:14 AM Dexuan Cui > > > > > > > > <de...@microsoft.com> wrote: > > > > > > > > > Hi, > > > > > > > > > MD5 was marked incompliant with FIPS in 2009: > > > > > > > > > a3bef3a31a19 ("crypto: testmgr - Skip algs not flagged > > > > > > > > > fips_allowed in fips mode") > > > > > > > > > a1915d51e8e7 ("crypto: testmgr - Mark algs allowed in fips > > > > > > > > > mode") > > > > > > > > > > > > > > > > > > But hibernation_e820_save() is still using MD5, and fails in > > > > > > > > > FIPS mode > > > > > > > > > due to the 2018 patch: > > > > > > > > > 749fa17093ff ("PM / hibernate: Check the success of > > > > > > > > > generating md5 digest before hibernation") > > > > > > > > > > > > > > > > > > As a result, hibernation doesn't work when FIPS is on. > > > > > > > > > > > > > > > > > > Do you think if hibernation_e820_save() should be changed to > > > > > > > > > use a > > > > > > > > > FIPS-compliant algorithm like SHA-1? > > > > > > > > > > > > > > > > I would say yes, it should. > > > > > > > > > > > > > > > > > PS, currently it looks like FIPS mode is broken in the > > > > > > > > > mainline: > > > > > > > > > https://www.mail-archive.com/linux-crypto@vger.kernel.org/msg49414.html > > > > > > > > > > > > > > FYI, SHA-1 is not a good choice, it is only permitted in HMAC > > > > > > > constructions and only for specified uses. If you need to change > > > > > > > algorithm you should go straight to SHA-2 or SHA-3 based hashes. > > > > > > > > > > > > > > > > > > > What is the reason for using a [broken] cryptographic hash here? if > > > > > > this is just an integrity check, better use CRC32 > > > > > > Not really. > > > > > > CRC32 is not really sufficient for integrity checking here AFAICS. It > > > might be made a fallback option if MD5 is not available, but making it > > > the default would be somewhat over the top IMO. > > > > > > > > If the integrity check is used exclusively to verify there were no > > > > > accidental changes and is not used as a security measure, by all means > > > > > I agree that using crc32 is a better idea. > > > > > > > > > > > > > Looking at 62a03defeabd58f74e07ca030d6c21e069d4d88e which introduced > > > > this, it is only a best effort check which is simply omitted if md5 > > > > happens to be unavailable, so there is definitely no need for crypto > > > > here. > > > > > > Yes, it is about integrity checking only. No, CRC32 is not equivalent > > > to MD5 in that respect AFAICS. > > > > > > > There are two possibilities: > > - we care about an adversary attempting to forge a collision, in which > > case you need a cryptographic hash which is not broken; > > - we only care about integrity, in which case crypto is overkill, and > > CRC32 is sufficient. (Note that the likelihood of an honest, > > inadvertent modification not being caught by CRC32 is 1 in 4 billion) > > That depends on how you count. > > Surely, there are modifications caught by MD5 that will not be caught by > CRC32.
This is a technically correct statement, but does it matter in this context? (Hint, probably not) > > MD5 does not meet either requirement, given that it is known to be > > broken, and overkill for simple integrity checks. MD5 should be phased > > out and removed, and moving this code onto the correct abstraction > > would be a reasonable step towards that goal. > > This clearly is a matter of opinion. Sorry, but this is not a matter of opinion. The only reason to use a cryptographic hash is that you want to protect from active tampering, rather than from accidental changes. And if you need to protect from active tampering then you cannot use a known broken hash, there is no point. OTOH if you do not care for active tampering but only to catch transmission/storage errors then all you care for is error checking. In that case a cryptographic hash is overkill because it entails a lot more computation than is needed. > I'm not religious about it though. If there is a general consensus > that CRC32 is sufficient for error detection in hibernation files, > then it can be used. So is there such a consensus and if so, can you > give me a pointer to some research that it is based on? CRC32 is an industry standard to check for accidental modifications of a bit stream. The chances of missing an accidental change are 1 in 4 billion. Does your application require a higher threshold? If so you should justify why you think your case is not fulfilled by an industry standard. HTH, Simo. -- Simo Sorce RHEL Crypto Team Red Hat, Inc