linux-kernel - Re: [PATCH v6 1/1] use crc32 instead of md5 for hibernation e820 integrity check

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Date:   Mon, 12 Apr 2021 12:20:33 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Chris von Recklinghausen <crecklin@...hat.com>
Cc:     ardb@...nel.org, simo@...hat.com, rafael@...nel.org,
        decui@...rosoft.com, linux-pm@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/1] use crc32 instead of md5 for hibernation e820
 integrity check

On Mon, Apr 12, 2021 at 03:04:58PM -0400, Chris von Recklinghausen wrote:
> On 4/12/21 1:45 PM, Eric Biggers wrote:
> > On Mon, Apr 12, 2021 at 10:09:32AM -0400, Chris von Recklinghausen 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.
> > > 
> > > The purpose of the integrity check is to detect possible differences
> > > between the memory map used at the time when the hibernation image is
> > > about to be loaded into memory and the memory map used at the image
> > > creation time, because it is generally unsafe to load the image if the
> > > current memory map doesn't match the one used when it was created. so
> > > it is not intended as a cryptographic integrity check.
> > This still doesn't actually explain why a non-cryptographic checksum is
> > sufficient.  "Detection of possible differences" could very well require
> > cryptographic authentication; it depends on whether malicious changes need to be
> > detected or not.
> 
> Hi Eric,
> 
> The cases that the commit comments for 62a03defeabd mention are the same as
> for this patch, e.g.
> 
>     1. Without this patch applied, it is possible that BIOS has
>        provided an inconsistent memory map, but the resume kernel is still
>        able to restore the image anyway(e.g, E820_RAM region is the superset
>        of the previous one), although the system might be unstable. So this
>        patch tries to treat any inconsistent e820 as illegal.
> 
>     2. Another case is, this patch replies on comparing the e820_saved, but
>        currently the e820_save might not be strictly the same across
>        hibernation, even if BIOS has provided consistent e820 map - In
>        theory mptable might modify the BIOS-provided e820_saved dynamically
>        in early_reserve_e820_mpc_new, which would allocate a buffer from
>        E820_RAM, and marks it from E820_RAM to E820_RESERVED).
>        This is a potential and rare case we need to deal with in OS in
>        the future.
> 
> Maybe they should be added to the comments with this patch as well? In any
> case, the above comments only mention detecting consequences of BIOS
> issues/actions on the e820 map and not intrusions from attackers requiring
> cryptographic protection. Does that seem to be a reasonable explanation to
> you? If so I can add these to the commit comments.
> 
> I'll make the other changes you suggest below.
> 
> Thanks,
> 

Those details are still missing the high-level point.  Is this just meant to
detect non-malicious changes (presumably caused by BIOS bugs), or is it meant to
detect malicious changes?  That's all that really needs to be mentioned.

- Eric