Re: [RFC] Persist ima logs to disk

[Mimi Zohar <zohar@linux.ibm.com>]

On Thu, 2021-01-07 at 08:42 -0800, James Bottomley wrote:
> On Thu, 2021-01-07 at 10:06 -0500, Mimi Zohar wrote:
> > [Cc: Amir Goldstein]
> > 
> > On Tue, 2021-01-05 at 11:57 -0800, Raphael Gianotti wrote:
> > > IMA measures files and buffer data and some systems may end up
> > > generating lots of entries in the IMA measurement list. This list
> > > is kept in kernel memoryc and as it grows in size it could end up
> > > taking too many resources, causing the system to run out of
> > > available memory. During kexec, the IMA measurement list can be
> > > carried over in memory, but it's possible for the list to become
> > > too large for that to happen.
> > > 
> > > The Kconfig introduced in this series enables admins to configure a
> > > maximum number of entries and a file to export the IMA measurement
> > > list to whenever the set limit is reached.
> > > 
> > > The list is written out in append mode, so the system will keep
> > > writing new entries as long as it stays running or runs out of
> > > space. Whenever the export file is set, it's truncated. If writing
> > > to the export list fails, a flag is set to prevent further exports,
> > > as the file is likely in a bad state. Setting a new export file
> > > resets this flag, allowing exports to resume and giving admins a
> > > way to recover from this state if necessary.
> > > 
> > > In the case of kexec, if the list is too large too be carried over
> > > in memory and an export file is configured, the list will be
> > > exported, preventing the measurements from being lost during kexec.
> > > 
> > > This code is based off of a previous RFC sent by Janne Karhunen[1],
> > > and is intended to pick up where that was left off.
> > > 
> > > In a thread with Janne Karhunen[2], it was mentioned that another
> > > approach, using mm had been considered. Upon some investigation the
> > > approach used in this RFC still seemed adequate for solving this
> > > problem.
> > > 
> > > [1] 
> > > https://patchwork.kernel.org/project/linux-integrity/patch/201912
> > > 20074929.8191-1-janne.karhunen@gmail.com/
> > > [2] 
> > > https://lore.kernel.org/linux-integrity/CAE=NcrbdS-3gVvnnEwdNSOLO
> > > vTenLjyppDz2aJACGRgBYSh=Gw@mail.gmail.com/
> > > 
> > > Signed-off-by: Raphael Gianotti <raphgi@linux.microsoft.com>
> > 
> > My original concerns of truncating the IMA measurement list have not
> > been addressed.  Once the IMA measurement list has been truncated,
> > quoting and then verifying any of the PCRs contained in the
> > measurement list will fail, unless the measurements have been
> > preserved and are readily accessible.
> > 
> > Amir's suggestion addresses kernel memory constraints without
> > truncating the IMA measurement list.
> 
> What about having a log entry that's the current PCR value?  Then
> stretches of the log starting with these entries would be independently
> verifiable provided you had a way of trusting the PCR value.  It might
> be possible to get the TPM to add a signed quote as an optional part of
> the log entry (of course this brings other problems like which key do
> you use for the signing and how does it get verified) which would
> provide the trust and would definitively allow you to archive log
> segments and still make the rest of the log useful.

The current PCR values are aggregated and stored in the boot_aggregate
record.  As part of the new boot_aggregate record format, the
individual PCR values could be included.

But this doesn't address where the offloaded measurement list will be
stored, how long the list will be retained, nor who guarantees the
integrity of the offloaded list.  In addition, different form factors
will have different requirements.

Mimi