Re: [RFC] IMA Log Snapshotting Design Proposal

Paul Moore
2023-08-29T19:34:34Z

On Mon, Aug 21, 2023 at 7:08 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> On Mon, 2023-08-21 at 15:05 -0700, Sush Shringarputale wrote:
> > On 8/14/2023 3:02 PM, Mimi Zohar wrote:
> > > On Mon, 2023-08-14 at 14:42 -0700, Sush Shringarputale wrote:
> > >>> This design seems overly complex and requires synchronization between
> > >>> the "snapshot" record and exporting the records from the measurement
> > >>> list. None of this would be necessary if the measurements were copied
> > >>> from kernel memory to a backing file (e.g. tmpfs), as described in [1].
> > Even if the Kernel maintains the link between a tmpfs exported and an
> > in-memory IMA log - it still has to copy the tmpfs portion to the
> > Kernel memory during kexec soft boot. tmpfs is cleared during kexec,
> > so this copying of tmpfs back to kernel memory is necessary to preserve
> > the integrity of the log during kexec. But the copying would add back
> > the memory pressure on the node during kexec (which may result in
> > out-of-memory), defeating the purpose of the overall effort/feature.
> > Copying to a regular *persistent* protected file seems a cleaner
> > approach, compared to tmpfs.
>
> From a kernel perspective, it doesn't make a difference if userspace
> provides a tmpfs or persistent file. As per the discussion
> https://lore.kernel.org/linux-integrity/CAOQ4uxj4Pv2Wr1wgvBCDR-tnA5dsZT3rvdDzKgAH1aEV_-r9Qg@mail.gmail.com/#t
> , userspace provides the kernel with the file descriptor of the opened
> file.
>
> > We prototyped this solution, however it
> > does not seem to be a common pattern within the Kernel to write state
> > directly to files on disk file systems. We considered two potential
> > options:
>
> If no file descriptor is provided, then the measurements aren't copied
> and removed from the securityfs file. If there are write errors, the
> measurements aren't removed from the securityfs file until the write
> errors are resolved.

It sounds like this approach would require the file/filesystem to be
continuously available for the life of the system once the log was
snapshotted/overflowed to persistent storage, yes? Assuming that is
the case, what happens if the file/filesystem becomes inaccessible at
some point and an attestation client attempts to read the entire log?

--
paul-moore.com