Re: [PATCH v5 1/1] use crc32 instead of md5 for hibernation e820 integrity check

[Simo Sorce]

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@xxxxxxxxxx> 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