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. 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:
Option (A): (RECOMMENDED)
Let Kernel write the file using KM file APIs
--------------------------------------------
Use Kernel mode file APIs such as _file_open_root_, _vfs_llseek_,
and _vfs_write_ to use a persistent file on disk. There is not
sufficient precedent for this pattern in the Kernel currently, so
we need guidance from area experts on the best mechanism to
implement this.
As for the location of the file, we suggest setting this in
KConfig. The file will be created by the Kernel, so it should be
protected from UM access. Additionally, on a full boot, the
file should be cleared by the Kernel.
POTENTIAL ISSUES AND MITIGATIONS
- handling IO errors from KM
A potential mitigation for this is to retry the failed write.
This assumes that the vfs_write Kernel method can handle any
failures gracefully without causing crashes.
- using file system paths to resolve the file from KM
Using the file_open_root seems to provide sufficient resiliency
against this. The file can be located at a well known location
to minimize potential concerns. However, any guidance in
minimizing squatting risks would be greatly appreciated.
- the file could be tampered by UM
There needs to be a lock/kernel-only-permission on the file
so that a UM process cannot tamper with the file. A description
on how this file would be protected was provided at [2].
Option (B): (NOT RECOMMENDED)
Hand over the file writing to UM
--------------------------------------------
Alternately, UM could write the file contents and decide the
location of the file on disk. If we want to preserve the Kernel
behavior of rendering the IMA log as a single monolithic log on
client (as ascii/binary_runtime_measurements file), the
Kernel will have to read from a file written by UM and combine it
with the remaining in-kernel-memory IMA log events.
This approach of a Kernel reading from a UM written file as an
IMA log violates the call stack, and thus it is not recommended.
If UM is to handle writing the IMA log to disk, the Kernel cannot read
those snapshotted events again. Integrity of the IMA log can still be
maintained - a solution implementing this was in the original proposal
we submitted [1].
We appreciate the community's feedback on helping mold this feature to a
suitable implementation.
Thanks,
Sush and Tushar.
References:
[1]
https://lore.kernel.org/linux-integrity/c5737141-7827-1c83-ab38-0119dcfea485@xxxxxxxxxxxxxxxxxxx/
[2]
https://lore.kernel.org/linux-integrity/CAOQ4uxiBAGKco1BKgyLOMY54r_Ck2jnvz8RCFODD-V87CGqLEw@xxxxxxxxxxxxxx/
What is the real problem - kernel memory pressure, memory pressure in
general, or disk space? Is the intention to remove or offload the
exported measurements?
The main concern is the memory pressure on both the kernel and the
attestation client
when it sends the request. The concern you bring up is valid and we are
working on
creating a prototype. There is no intention to remove the exported
measurements.
Glad to hear that you're not intending to remove the exported
measurements.
Defining and including a new record in the measurement list measurement
is fine, if it helps with attestation and doesn't require pausing the
measurements.
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