Re: [PATCH v2] ima: export the measurement list when needed

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

On Tue, 2020-02-11 at 11:10 -0500, david.safford@gmail.com wrote:
> On Mon, 2020-02-10 at 15:24 -0500, Mimi Zohar wrote:
> > On Mon, 2020-02-10 at 13:18 -0500, david.safford@gmail.com wrote:
> > > On Thu, 2020-02-06 at 09:13 -0500, Mimi Zohar wrote:
> > > > Hi Janne,
> > > > 
> > > > On Fri, 2020-01-10 at 10:48 +0200, Janne Karhunen wrote:
> > > > > On Wed, Jan 8, 2020 at 1:18 PM Janne Karhunen <janne.karhunen@gmail.com> wrote:
> > > > > > Some systems can end up carrying lots of entries in the ima
> > > > > > measurement list. Since every entry is using a bit of kernel
> > > > > > memory, allow the sysadmin to export the measurement list to
> > > > > > the filesystem to free up some memory.
> > > > > 
> > > > > Hopefully this addressed comments from everyone. The flush event can
> > > > > now be triggered by the admin anytime and unique file names can be
> > > > > used for each flush (log.1, log.2, ...) etc, so getting to the correct
> > > > > item should be easy.
> > > > > 
> > > > > While it can now be argued that since this is an admin-driven event,
> > > > > kernel does not need to write the file. However, the intention is to
> > > > > bring out a second patch a bit later that adds a variable to define
> > > > > the max number of entries to be kept in the kernel memory and
> > > > > workqueue based automatic flushing. In those cases the kernel has to
> > > > > be able to write the file without any help from the admin..
> > > > 
> > > > The implications of exporting and removing records from the IMA-
> > > > measurement list needs to be considered carefully.  Verifying a TPM
> > > > quote will become dependent on knowing where the measurements are
> > > > stored.  The existing measurement list is stored in kernel memory and,
> > > > barring a kernel memory attack, is protected from modification.
> > > >  Before upstreaming this or a similar patch, there needs to be a
> > > > discussion as to how the measurement list will be protected once is it
> > > > exported to userspace.
> > > 
> > > "Protected" here can mean two different aspects: cryptographically
> > > protected from tampering, which is covered with the TPM_QUOTE, and
> > > availability protected from even accidental deletion, which is what
> > > I suspect you are concerned about. Certainly my original TLV patches
> > > were too flippant about this, as userspace had to be trusted not to
> > > drop any records. In this patch, the kernel writes the data in an
> > > atomic fashion. Either all records are successfully written, or none
> > > are, and an error is returned.
> > 
> > A third aspect, which I'm concerned about, is removing records from
> > the measurement list.  This changes the existing userspace
> > expectations of returning the entire measurement list.  Now userspace
> > will need some out of band method of knowing where to look for the
> > measurements.
> 
> This is a feature, not a bug. :-)
> There is no reason to resend the same data for every attestation,
> nor is there any reason to store already attested measurements anywhere
> on the client. By versioning the log file names, userspace gets a
> simple way to know what has and has not been attested, and for small
> embedded devices we don't need to waste memory or filesystem space
> on the data already attested.

This new feature will require setting up some infrastructure for
storing the partial measurement list(s) in order to validate a TPM
quote.  Userspace already can save partial measurement list(s) without
any kernel changes.  The entire measurement list does not need to be
read each time.  lseek can read past the last record previously read.
 The only new aspect is truncating the in kernel measurement list in
order to free kernel memory.

< snip> 

> > > > Instead of exporting the measurement records, one option as suggested
> > > > by Amir Goldstein, would be to use a vfs_tmpfile() to get an anonymous
> > > > file for backing store.  The existing securityfs measurement lists
> > > > would then read from this private copy of the anonymous file.
> > > 
> > > This doesn't help in use cases where we really do want to
> > > export to a persistent file, without userspace help.
> > 
> > Is to prevent needing to carry the measurement list across kexec the
> > only reason for the kernel needing to write to a persistent file?
> 
> Well, that and the other reasons mentioned, such as completely freeing
> the data from the client after attestation, and simplicity of the
> mechanism.

Until there is proof that the measurement list can be exported to a
file before kexec, instead of carrying the measurement list across
kexec, and a TPM quote can be validated after the kexec, there isn't a
compelling reason for the kernel needing to truncate the measurement
list.

Mimi