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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 1 Apr 2021 18:02:12 +0200
Message-ID: <CAJZ5v0iPneWK69GTzWigdXjjb6VN6Hyd5=hLCdGasfnTxXCYNg@mail.gmail.com>
Subject: Re: Fix hibernation in FIPS mode?
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Simo Sorce <simo@redhat.com>, 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>
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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.

> 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.

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?