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Subject: Re: [PATCH v5 1/1] use crc32 instead of md5 for hibernation e820
 integrity check
From:   Simo Sorce <simo@redhat.com>
To:     Eric Biggers <ebiggers@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Chris von Recklinghausen <crecklin@redhat.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Dexuan Cui <decui@microsoft.com>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Thu, 08 Apr 2021 11:52:31 -0400
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References: <20210408131506.17941-1-crecklin@redhat.com>
         <CAJZ5v0ib+jmbsD9taGW0RujY5c9BCK8yLHv065u44mb0AwO9vQ@mail.gmail.com>
         <YG8gqZoZGutPmROz@sol.localdomain>
Organization: Red Hat, Inc.
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On Thu, 2021-04-08 at 08:26 -0700, Eric Biggers wrote:
> On Thu, Apr 08, 2021 at 03:32:38PM +0200, Rafael J. Wysocki wrote:
> > On Thu, Apr 8, 2021 at 3:15 PM Chris von Recklinghausen
> > <crecklin@redhat.com> wrote:
> > > Suspend fails on a system in fips mode because md5 is used for the e820
> > > integrity check and is not available. Use crc32 instead.
> > > 
> > > This patch changes the integrity check algorithm from md5 to
> > > crc32. This integrity check is used only to verify accidental
> > > corruption of the hybernation data
> > 
> > It isn't used for that.
> > 
> > In fact, it is used to detect differences between the memory map used
> > before hibernation and the one made available by the BIOS during the
> > subsequent resume.  And the check is there, because it is generally
> > unsafe to load the hibernation image into memory if the current memory
> > map doesn't match the one used when the image was created.
> 
> So what types of "differences" are you trying to detect?  If you need to detect
> differences caused by someone who maliciously made changes ("malicious" implies
> they may try to avoid detection), then you need to use a cryptographic hash
> function (or a cryptographic MAC if the hash value isn't stored separately).  If
> you only need to detect non-malicious changes (normally these would be called
> "accidental" changes, but sure, it could be changes that are "intentionally"
> made provided that the other side can be trusted to not try to avoid
> detection...), then a non-cryptographic checksum would be sufficient.

Wouldn't you also need a signature with a TPM key in that case?
An attacker that can change memory maps can also change the hash on
disk ? Unless the hash is in an encrypted partition I guess...

Simo.

-- 
Simo Sorce
RHEL Crypto Team
Red Hat, Inc