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Subject: Re: Fix hibernation in FIPS mode?
From:   Simo Sorce <simo@redhat.com>
To:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>
Cc:     Dexuan Cui <decui@microsoft.com>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "crecklin@redhat.com" <crecklin@redhat.com>,
        "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Date:   Thu, 01 Apr 2021 12:22:18 -0400
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On Thu, 2021-04-01 at 18:02 +0200, Rafael J. Wysocki wrote:
> On Thu, Apr 1, 2021 at 3:54 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > On Thu, 1 Apr 2021 at 15:38, Rafael J. Wysocki <rafael@kernel.org> wrote:
> > > On Thu, Apr 1, 2021 at 10:47 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > On Tue, 30 Mar 2021 at 21:56, Simo Sorce <simo@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 <simo@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 <decui@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