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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 8 Apr 2021 18:05:33 +0200
Message-ID: <CAJZ5v0g65irXKmy7pdgD8-5KWrxdtwiWbJsBD2A=PKf1D3RVZg@mail.gmail.com>
Subject: Re: [PATCH v5 1/1] use crc32 instead of md5 for hibernation e820
 integrity check
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Chris von Recklinghausen <crecklin@redhat.com>,
        Ard Biesheuvel <ardb@kernel.org>, Simo Sorce <simo@redhat.com>,
        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>
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On Thu, Apr 8, 2021 at 5:26 PM Eric Biggers <ebiggers@kernel.org> 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...)

That's the case here.

> then a non-cryptographic checksum would be sufficient.