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

[Chris von Recklinghausen]

On 4/12/21 3:20 PM, Eric Biggers wrote:
> 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.


Ok, I'll say the intent is to detect non-malicious changes presumably 
from BIOS issues.

Thanks,

Chris

> - Eric