Re: [PATCH v2 00/15] ima: digest list feature
From: Roberto Sassu
Date: Tue Nov 07 2017 - 11:45:53 EST
On 11/7/2017 2:37 PM, Mimi Zohar wrote:
Hi Roberto,
On Tue, 2017-11-07 at 11:36 +0100, Roberto Sassu wrote:
IMA is a security module with the objective of reporting or enforcing the
integrity of a system, by measuring files accessed with the execve(),
mmap() and open() system calls. For reporting, it takes advantage of the
TPM and extends a PCR with the digest of an evaluated event. For enforcing,
it returns a value which is zero if the operation should be allowed,
negative if it should be denied.
Measuring files of an operating system introduces three main issues. First,
since the overhead introduced by the TPM is noticeable, the performance of
the system decreases linearly with the number of measurements taken. This
can be seen especially at boot time.
I've said this previously. ÂThe solution IS FIRST to improve the
performance of the TPM device driver, before finding solutions around
it.
TPM performance patches:
a233a0289cf9 "tpm: msleep() delays - replace with usleep_range() in i2c nuvoton driver"
0afb7118ae02 "tpm: add sleep only for retry in i2c_nuvoton_write_status()"
9f3fc7bcddcb "tpm: replace msleep() with usleep_range() in TPM 1.2/2.0 generic drivers"
Nayna Jain submitted additional performance improvements, that were posted
https://www.spinics.net/lists/linux-integrity/msg00238.htmlÂand are
currently being tested.
Even after these TPM performance improvements, there are still more
TPM performance improvements.
Hi Mimi
I applied the patches you mentioned, and indeed the performance
improvement is great, from 1 minute and 30 seconds to 24 seconds
compared to the normal boot time of 8.5 seconds. However, especially for
embedded devices performances requirements might be more strict, and
much more files might be measured. For desktops, also it would be
desirable to have low latency.
Second, managing large measurement
lists requires computation power and network bandwidth.
"Large" for whom? ÂLarge for the attestation server? ÂLarge for the
client? ÂSmaller devices would have fewer measurements than larger
devices. ÂWe're not discussing gigabytes/terabytes of data here.
ÂAttestation servers should be able to handle the bandwidth. ÂIf it
becomes a problem, then the attestation server/client communication
could be optimized to send just the recent measurements, not the
entire measurement list.
I'm not considering an individual system, but a datacenter with several
nodes. An attestation server processing for example 200 measurement
lists with 5000 entries must be much more powerful than one that
processes the same number of lists with 10 entries.
Third, it is
necessary to obtain reference measurements (i.e. digests of software known
to be good) to evaluate/enforce the integrity of the system. If file
signatures are used to enforce access, Linux distribution vendors have to
modify their building systems in order to include signatures in their
packages.
Although IMA-appraisal verifies file integrity based on either a file
hash or signature, they are not equivalent. ÂFile signatures provide
file provenance. ÂNot only does the file hash have to match, but a
certificate used to sign the file data must be loaded onto the IMA
keyring. ÂFile hashes should be limited to mutable files.
Digest lists work the same. If appraisal is in enforcing mode, digest
lists must have a valid digital signature.
Instead of working around the problem of a lack of file signatures in
software packages, help promote including them so that there are
measurement and signature chains of trust anchored in hardware.
One of the key point of digest lists feature is that it reuses
information that is already available, while providing the same security
properties. I find it difficult to promote a solution that would
introduce redundant information and complicate the management of Linux
distributions.
Digest lists aim at mitigating these issues. A digest list is a list of
digests that are taken by IMA as reference measurements and loaded before
files are accessed. Then, IMA compares calculated digests of accessed files
with digests from loaded digest lists. If the digest is found, measurement,
appraisal and audit are not performed.
Although the previous patch set did not break userspace per-se, it
changed the existing meaning of the IMA measurement list. ÂWithout
taking into account my previous comments, this patch set makes similar
changes to IMA-appraisal and IMA-measurement.
For appraisal, I think only the verification method changes: instead of
verifying file signatures individually, IMA verifies one signature per
many file digests.
Instead of including the individual file measurements, the previous
version of this patch set, I assume it hasn't changed, includes a hash
of a file containing a list of all potential file measurements, not
the actual file measurements.
Digest lists do not introduce false positives due to not including a
measurement. Files whose digest is included in a digest list must be
considered as accessed. This can be improved later, for example by
introducing a new digest list type contaning digests of files that must
be accessed. IMA would create a new measurement entry when the last file
is accessed.
Instead of changing the meaning of the IMA measurement list, I
previously suggested defining a new securityfs file for this purpose.
The meaning of the measurement list is determined by the policy, which
is measured. As the same, the meaning of the measurement list can be
determined from the digest of digest list metadata. Either a verifier
detects that digest lists have been loaded (if he supports them), or
must consider the system as compromised, as the digest of digest list
metadata is unknown (he does not support digest lists).
Multiple digest lists can be loaded at the same time, by providing to IMA
metadata for each list: digest, signature and path. The digest is specified
so that loaded digest lists can be identified only with the measurement of
metadata. The signature is used for appraisal. If the verification
succeeds, IMA loads the digest list even if security.ima is missing.
Previously IMA-appraisal verified the file signature of the file
containing the file hashes. ÂIt now sounds like even this guarantee is
gone.
IMA obtains reference values from digest lists instead of security.ima.
This is comparable to file signatures, because digest lists must be
signed for appraisal.
Normally, the protection of kernel memory is out of scope for IMA.
This patch set introduces an in kernel white list, which would be a
prime target for attackers looking for ways of by-passing IMA-
measurement, IMA-appraisal and IMA-audit. ÂOthers might disagree, but
from my perspective, this risk is too high.
It would be much easier for an attacker to just set ima_policy_flag to
zero.
Roberto
Digest lists address the first issue because the TPM is used only if the
digest of a measured file is unknown. On a minimal system, 10 of 1400
measurements are unknown because of mutable files (e.g. log files).
Digest lists mitigate the second issue because, since digest lists do not
change, they don't have to be sent at every remote attestation. Sending
unknown measurements and a reference to digest lists would be sufficient.
Finally, digest lists address also the third issue because Linux
distribution vendors already provide the digests of files included in each
RPM package. The digest list is stored in the RPM header, signed by the
vendor.
When using digest lists, a limitation must be considered. Since a
measurement is not reported if the digest of an accessed file is found in a
digest list, the measurement list does not show which files have been
actually accessed, and in which sequence.
A possible solution would be to load a list with digest of files which are
usually accessed. Also, it is possible to selectively enable digest list
lookup only for a subset of IMA policy rules. For example, a policy could
enable digest lookup only for file accesses from the TCB and disable it
for execve() and mmap() from regular users.
Changelog
v1:
- added new policy option digest_list to selectively enable digest lookup
- added support for appraisal
- added support for immutable/mutable files
Roberto Sassu (15):
ima: generalize ima_read_policy()
ima: generalize ima_write_policy()
ima: generalize policy file operations
ima: use ima_show_htable_value to show hash table data
ima: add functions to manage digest lists
ima: add parser of digest lists metadata
ima: add parser of compact digest list
ima: add parser of RPM package headers
ima: introduce securityfs interfaces for digest lists
ima: disable digest lookup if digest lists are not checked
ima: add policy action digest_list
ima: do not update security.ima if appraisal status is not
INTEGRITY_PASS
evm: add kernel command line option to select protected xattrs
ima: add support for appraisal with digest lists
ima: add Documentation/security/IMA-digest-lists.txt
Documentation/admin-guide/kernel-parameters.txt | 4 +
Documentation/security/IMA-digest-lists.txt | 161 ++++++++++++
include/linux/evm.h | 6 +
include/linux/fs.h | 2 +
security/integrity/evm/evm_main.c | 36 +++
security/integrity/iint.c | 1 +
security/integrity/ima/Kconfig | 19 ++
security/integrity/ima/Makefile | 1 +
security/integrity/ima/ima.h | 33 ++-
security/integrity/ima/ima_api.c | 7 +-
security/integrity/ima/ima_appraise.c | 52 +++-
security/integrity/ima/ima_digest_list.c | 326 ++++++++++++++++++++++++
security/integrity/ima/ima_fs.c | 181 ++++++++-----
security/integrity/ima/ima_main.c | 47 +++-
security/integrity/ima/ima_policy.c | 33 ++-
security/integrity/ima/ima_queue.c | 42 +++
security/integrity/integrity.h | 11 +
17 files changed, 877 insertions(+), 85 deletions(-)
create mode 100644 Documentation/security/IMA-digest-lists.txt
create mode 100644 security/integrity/ima/ima_digest_list.c
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