[RFC PATCH v7 00/16] Integrity Policy Enforcement (IPE)

deven.desai
2021-10-13T19:06:46Z

From: Deven Bowers <deven.desai@linux.microsoft.com>

Overview:
---------

IPE is a Linux Security Module which takes a complimentary approach to
access control. Whereas existing systems approach use labels or paths
which control access to a resource, IPE controls access to a resource
based on the system's trust of said resource.

Trust requirements are established via IPE's policy, sourcing multiple
different implementations within the kernel to build a cohesive trust
model, based on how the system was built.

Trust, with respect to computing, is a concept that designates a set
of entities who will endorse a set of resources as non-malicious.
Traditionally, this is done via signatures, which is the act of endorsing
a resource.

Integrity, on the other hand, is the concept of ensuring that a resource
has not been modified since a point of time. This is typically done through
cryptographic hashes or signatures.

Trust and integrity are very closely tied together concepts, as integrity
is the way you can prove trust for a resource; otherwise it could have
been modified by an entity who is untrusted.

IPE provides a way for a user to express trust requirements of resources,
by using pre-existing systems which provide the integrity half of the
equation.

IPE is compiled under CONFIG_SECURITY_IPE.

Use Cases
---------

IPE works best in fixed-function devices: Devices in which their purpose
is clearly defined and not supposed to be changed (e.g. network firewall
device in a data center, an IoT device, etcetera), where all software and
configuration is built and provisioned by the system owner.

IPE is a long-way off for use in general-purpose computing:
the Linux community as a whole tends to follow a decentralized trust
model, known as the Web of Trust, which IPE has no support for as of yet.
Instead, IPE supports the PKI Trust Model, which generally designates a
set of entities that provide a measure absolute trust.

Additionally, while most packages are signed today, the files inside
the packages (for instance, the executables), tend to be unsigned. This
makes it difficult to utilize IPE in systems where a package manager is
expected to be functional, without major changes to the package manager
and ecosystem behind it.

Policy:
-------

IPE policy is a plain-text [#]_ policy composed of multiple statements
over several lines. There is one required line, at the top of the
policy, indicating the policy name, and the policy version, for
instance:

policy_name="Ex Policy" policy_version=0.0.0

The policy version indicates the current version of the policy (NOT the
policy syntax version). This is used to prevent roll-back of policy to
potentially insecure previous versions of the policy.

The next portion of IPE policy, are rules. Rules are formed by key=value
pairs, known as properties. IPE rules require two properties: "action",
which determines what IPE does when it encounters a match against the
policy, and "op", which determines when that rule should be evaluated.
Thus, a minimal rule is:

op=EXECUTE action=ALLOW

This example will allow any execution. Additional properties are used to
restrict attributes about the files being evaluated. These properties are
intended to be deterministic attributes that are resident in the kernel.
Available properties for IPE described in the documentation patch of this
series.

A rule is required to have the "op" property as the first token of a rule,
and the "action" as the last token of the rule. Rules are evaluated
top-to-bottom. As a result, any revocation rules, or denies should be
placed early in the file to ensure that these rules are evaluated before
a rule with "action=ALLOW" is hit.

Any unknown syntax in IPE policy will result in a fatal error to parse
the policy. User mode can interrogate the kernel to understand what
properties and the associated versions through the securityfs node,
$securityfs/ipe/config, which will return a string of form:

key1=version1
key2=version2
.
.
.
keyN=versionN

User-mode should correlate these versions with the supported values
identified in the documentation to determine whether a policy should
be accepted by the system without actually trying to deploy the policy.

Additionally, a DEFAULT operation must be set for all understood
operations within IPE. For policies to remain completely forwards
compatible, it is recommended that users add a "DEFAULT action=ALLOW"
and override the defaults on a per-operation basis.

For more information about the policy syntax, the kernel documentation
page.

Early Usermode Protection:
--------------------------

IPE can be provided with a policy at startup to load and enforce.
This is intended to be a minimal policy to get the system to a state
where userland is setup and ready to receive commands, at which
point a policy can be deployed via securityfs. This "boot policy" can be
specified via the config, SECURITY_IPE_BOOT_POLICY, which accepts a path
to a plain-text version of the IPE policy to apply. This policy will be
compiled into the kernel. If not specified, IPE will be disabled until a
policy is deployed and activated through the method above.

Policy Examples:
----------------

Allow all:

policy_name="Allow All" policy_version=0.0.0
DEFAULT action=ALLOW

Allow only initial superblock:

policy_name="Allow All Initial SB" policy_version=0.0.0
DEFAULT action=DENY

op=EXECUTE boot_verified=TRUE action=ALLOW

Allow any signed dm-verity volume and the initial superblock:

policy_name="AllowSignedAndInitial" policy_version=0.0.0
DEFAULT action=DENY

op=EXECUTE boot_verified=TRUE action=ALLOW
op=EXECUTE dmverity_signature=TRUE action=ALLOW

Prohibit execution from a specific dm-verity volume:

policy_name="AllowSignedAndInitial" policy_version=0.0.0
DEFAULT action=DENY

op=EXECUTE dmverity_roothash=401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=DENY
op=EXECUTE boot_verified=TRUE action=ALLOW
op=EXECUTE dmverity_signature=TRUE action=ALLOW

Allow only a specific dm-verity volume:

policy_name="AllowSignedAndInitial" policy_version=0.0.0
DEFAULT action=DENY

op=EXECUTE dmverity_roothash=401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW

Deploying Policies:
-------------------

First sign a plain text policy, with a certificate that is present in
the SYSTEM_TRUSTED_KEYRING of your test machine. Through openssl, the
signing can be done via:

openssl smime -sign -in "$MY_POLICY" -signer "$MY_CERTIFICATE" \
-inkey "$MY_PRIVATE_KEY" -binary -outform der -noattr -nodetach \
-out "$MY_POLICY.p7s"

Then, simply cat the file into the IPE's "new_policy" securityfs node:

cat "$MY_POLICY.p7s" > /sys/kernel/security/ipe/new_policy

The policy should now be present under the policies/ subdirectory, under
its "policy_name" attribute.

The policy is now present in the kernel and can be marked as active,
via the securityfs node:

echo "1" > "/sys/kernel/security/ipe/$MY_POLICY_NAME/active"

This will now mark the policy as active and the system will be enforcing
$MY_POLICY_NAME.

There is one requirement when marking a policy as active, the policy_version
attribute must either increase, or remain the same as the currently running
policy.

Policies can be updated via:

cat "$MY_UPDATED_POLICY.p7s" > \
"/sys/kernel/security/ipe/policies/$MY_POLICY_NAME/update"

Additionally, policies can be deleted via the "delete" securityfs
node. Simply write "1" to the corresponding node in the policy folder:

echo "1" > "/sys/kernel/security/ipe/policies/$MY_POLICY_NAME/delete"

There is only one requirement to delete policies, the policy being
deleted must not be the active policy.

NOTE: The securityfs commands will require CAP_MAC_ADMIN.

Integrations:
-------------

This patch series adds support for fsverity via digest and signature
(fsverity_signature and fsverity_digest), dm-verity by digest and
signature (dmverity_signature and dmverity_roothash), and trust for
the initramfs (boot_verified).

Please see the documentation patch for more information about the
integrations available.

Testing:
--------

KUnit Tests are available. Recommended kunitconfig:

CONFIG_KUNIT=y
CONFIG_SECURITY=y
CONFIG_SECURITYFS=y
CONFIG_PKCS7_MESSAGE_PARSER=y
CONFIG_SYSTEM_DATA_VERIFICATION=y
CONFIG_FS_VERITY=y
CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y
CONFIG_BLOCK=y
CONFIG_MD=y
CONFIG_BLK_DEV_DM=y
CONFIG_DM_VERITY=y
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y

CONFIG_SECURITY_IPE=y
CONFIG_SECURITY_IPE_KUNIT_TEST=y
CONFIG_IPE_PROP_BOOT_VERIFIED=y
CONFIG_IPE_PROP_DM_VERITY_SIGNATURE=y
CONFIG_IPE_PROP_DM_VERITY_ROOTHASH=y
CONFIG_IPE_PROP_FS_VERITY_SIGNATURE=y
CONFIG_IPE_PROP_FS_VERITY_DIGEST=y

Simply run:

make ARCH=um mrproper
./tools/testing/kunit/kunit.py run --kunitconfig <path/to/config>

And the tests will execute and report the result. For more indepth testing,
it will require you to create and mount a dm-verity volume or fs-verity
enabled file.

Documentation:
--------------

There is both documentation available on github at
https://microsoft.github.io/ipe, and Documentation in this patch series,
to be added in-tree. This includes architectural block diagrams.

Known Gaps:
-----------

IPE has two known gaps:

1. IPE cannot verify the integrity of anonymous executable memory, such as
the trampolines created by gcc closures and libffi (<3.4.2), or JIT'd code.
Unfortunately, as this is dynamically generated code, there is no way
for IPE to ensure the integrity of this code to form a trust basis. In all
cases, the return result for these operations will be whatever the admin
configures the DEFAULT action for "EXECUTE".

2. IPE cannot verify the integrity of interpreted languages' programs when
these scripts invoked via ``<interpreter> <file>``. This is because the
way interpreters execute these files, the scripts themselves are not
evaluated as executable code through one of IPE's hooks. Interpreters
can be enlightened to the usage of IPE by trying to mmap a file into
executable memory (+X), after opening the file and responding to the
error code appropriately. This also applies to included files, or high
value files, such as configuration files of critical system components.
However, there is a patchset that is looking to address this gap [1].

Appendix:
---------

A. IPE Github Repository: https://github.com/microsoft/ipe
B. IPE Users' Guide: Documentation/admin-guide/LSM/ipe.rst

References:
-----------

[1] https://lore.kernel.org/all/20211012192410.2356090-1-mic@digikod.net/

FAQ:
----

Q: What's the difference between other LSMs which provide trust-based
access control, for instance, IMA?

A: IMA is a fantastic option when needing measurement in addition to the
trust-based access model. All of IMA is centered around their measurement
hashes, so you save time when doing both actions. IPE, on the other hand,
is a highly performant system that does not rely (and explicitly prohibits),
generating its own integrity mechanisms - separating measurement and access
control. Simply put, IPE provides only the enforcement of trust, while other
subsystems provide the integrity guarantee that IPE needs to determine the
trust of a resource. IMA provides both the integrity guarantee, the
enforcement of trust, and a whole host of other features that may not be
needed.

Changelog:
----------

Changes since v1:
Split the second patch of the previous series into two.
Minor corrections in the cover-letter and documentation
comments regarding CAP_MAC_ADMIN checks in IPE.

Changes since v2:
Address various comments by Jann Horn. Highlights:
Switch various audit allocators to GFP_KERNEL.
Utilize rcu_access_pointer() in various locations.
Strip out the caching system for properties
Strip comments from headers
Move functions around in patches
Remove kernel command line parameters
Reconcile the race condition on the delete node for policy by
expanding the policy critical section.

Address a few comments by Jonathan Corbet around the documentation
pages for IPE.

Fix an issue with the initialization of IPE policy with a "-0"
version, caused by not initializing the hlist entries before
freeing.

Changes since v3:
Address a concern around IPE's behavior with unknown syntax.
Specifically, make any unknown syntax a fatal error instead of a
warning, as suggested by Mickaël Salaün.
Introduce a new securityfs node, $securityfs/ipe/property_config,
which provides a listing of what properties are enabled by the
kernel and their versions. This allows usermode to predict what
policies should be allowed.
Strip some comments from c files that I missed.
Clarify some documentation comments around 'boot_verified'.
While this currently does not functionally change the property
itself, the distinction is important when IPE can enforce verified
reads. Additionally, 'KERNEL_READ' was omitted from the documentation.
This has been corrected.
Change SecurityFS and SHA1 to a reverse dependency.
Update the cover-letter with the updated behavior of unknown syntax.
Remove all sysctls, making an equivalent function in securityfs.
Rework the active/delete mechanism to be a node under the policy in
$securityfs/ipe/policies.
The kernel command line parameters ipe.enforce and ipe.success_audit
have returned as this functionality is no longer exposed through
sysfs.

Changes since v4:
Correct some grammatical errors reported by Randy Dunlap.
Fix some warnings reported by kernel test bot.
Change convention around security_bdev_setsecurity. -ENOSYS
is now expected if an LSM does not implement a particular @name,
as suggested by Casey Schaufler.
Minor string corrections related to the move from sysfs to securityfs
Correct a spelling of an #ifdef for the permissive argument.
Add the kernel parameters re-added to the documentation.
Fix a minor bug where the mode being audited on permissive switch
was the original mode, not the mode being swapped to.
Cleanup doc comments, fix some whitespace alignment issues.

Changes since v5:
Change if statement condition in security_bdev_setsecurity to be
more concise, as suggested by Casey Schaufler and Al Viro
Drop the 6th patch in the series, "dm-verity move signature check..."
due to numerous issues, and it ultimately providing no real value.
Fix the patch tree - the previous iteration appears to have been in a
torn state (patches 8+9 were merged). This has since been corrected.

Changes since v6:
* Reword cover letter to more accurate convey IPE's purpose
and latest updates.
* Refactor series to:
1. Support a context structure, enabling:
1. Easier Testing via KUNIT
2. A better architecture for future designs
2. Make parser code cleaner
* Move patch 01/12 to [14/16] of the series
* Split up patch 02/12 into four parts:
1. context creation [01/16]
2. audit [07/16]
3. evaluation loop [03/16]
4. access control hooks [05/16]
5. permissive mode [08/16]
* Split up patch 03/12 into two parts:
1. parser [02/16]
2. userspace interface [04/16]
* Reword and refactor patch 04/12 to [09/16]
* Squash patch 05/12, 07/12, 09/12 to [10/16]
* Squash patch 08/12, 10/12 to [11/16]
* Change audit records to MAC region (14XX) from Integrity region (18XX)
* Add FSVerity Support
* Interface changes:
1. "raw" was renamed to "pkcs7" and made read only
2. "raw"'s write functionality (update a policy) moved to "update"
3. introduced "version", "policy_name" nodes.
4. "content" renamed to "policy"
5. The boot policy can now be updated like any other policy.
* Add additional developer-level documentation
* Update admin-guide docs to reflect changes.
* Kunit tests
* Dropped CONFIG_SECURITY_IPE_PERMISSIVE_SWITCH - functionality can
easily come later with a small patch.
* Use partition0 for block_device for dm-verity patch

Deven Bowers (14):
security: add ipe lsm & initial context creation
ipe: add policy parser
ipe: add evaluation loop
ipe: add userspace interface
ipe: add LSM hooks on execution and kernel read
uapi|audit: add trust audit message definitions
ipe: add auditing support
ipe: add permissive toggle
ipe: introduce 'boot_verified' as a trust provider
fs|dm-verity: add block_dev LSM blob and submit dm-verity data
ipe: add support for dm-verity as a trust provider
scripts: add boot policy generation program
ipe: kunit tests
documentation: add ipe documentation

Fan Wu (2):
fsverity|security: add security hooks to fsverity digest and signature
ipe: enable support for fs-verity as a trust provider

Documentation/admin-guide/LSM/index.rst | 1 +
Documentation/admin-guide/LSM/ipe.rst | 587 ++++++++++
.../admin-guide/kernel-parameters.txt | 12 +
Documentation/security/index.rst | 1 +
Documentation/security/ipe.rst | 339 ++++++
MAINTAINERS | 9 +
block/bdev.c | 7 +
drivers/md/dm-verity-target.c | 20 +-
drivers/md/dm-verity-verify-sig.c | 16 +-
drivers/md/dm-verity-verify-sig.h | 10 +-
fs/verity/open.c | 12 +
fs/verity/signature.c | 5 +-
include/asm-generic/vmlinux.lds.h | 16 +
include/linux/blk_types.h | 1 +
include/linux/device-mapper.h | 3 +
include/linux/fsverity.h | 3 +
include/linux/lsm_hook_defs.h | 5 +
include/linux/lsm_hooks.h | 12 +
include/linux/security.h | 22 +
include/uapi/linux/audit.h | 4 +
scripts/Makefile | 1 +
scripts/ipe/Makefile | 2 +
scripts/ipe/polgen/.gitignore | 1 +
scripts/ipe/polgen/Makefile | 6 +
scripts/ipe/polgen/polgen.c | 145 +++
security/Kconfig | 11 +-
security/Makefile | 1 +
security/ipe/.gitignore | 1 +
security/ipe/Kconfig | 100 ++
security/ipe/Makefile | 39 +
security/ipe/audit.c | 304 +++++
security/ipe/audit.h | 41 +
security/ipe/ctx.c | 381 ++++++
security/ipe/ctx.h | 43 +
security/ipe/ctx_test.c | 732 ++++++++++++
security/ipe/eval.c | 237 ++++
security/ipe/eval.h | 57 +
security/ipe/fs.c | 327 ++++++
security/ipe/fs.h | 13 +
security/ipe/hooks.c | 328 ++++++
security/ipe/hooks.h | 56 +
security/ipe/ipe.c | 143 +++
security/ipe/ipe.h | 27 +
security/ipe/ipe_parser.h | 59 +
security/ipe/modules.c | 134 +++
security/ipe/modules.h | 17 +
security/ipe/modules/Kconfig | 66 ++
security/ipe/modules/Makefile | 12 +
security/ipe/modules/boot_verified.c | 24 +
security/ipe/modules/dmverity_roothash.c | 80 ++
security/ipe/modules/dmverity_signature.c | 25 +
security/ipe/modules/fsverity_digest.c | 80 ++
security/ipe/modules/fsverity_signature.c | 33 +
security/ipe/modules/ipe_module.h | 40 +
security/ipe/parsers.c | 139 +++
security/ipe/parsers/Makefile | 12 +
security/ipe/parsers/default.c | 106 ++
security/ipe/parsers/policy_header.c | 126 ++
security/ipe/policy.c | 1037 +++++++++++++++++
security/ipe/policy.h | 113 ++
security/ipe/policy_parser_tests.c | 299 +++++
security/ipe/policyfs.c | 528 +++++++++
security/security.c | 76 +-
63 files changed, 7069 insertions(+), 18 deletions(-)
create mode 100644 Documentation/admin-guide/LSM/ipe.rst
create mode 100644 Documentation/security/ipe.rst
create mode 100644 scripts/ipe/Makefile
create mode 100644 scripts/ipe/polgen/.gitignore
create mode 100644 scripts/ipe/polgen/Makefile
create mode 100644 scripts/ipe/polgen/polgen.c
create mode 100644 security/ipe/.gitignore
create mode 100644 security/ipe/Kconfig
create mode 100644 security/ipe/Makefile
create mode 100644 security/ipe/audit.c
create mode 100644 security/ipe/audit.h
create mode 100644 security/ipe/ctx.c
create mode 100644 security/ipe/ctx.h
create mode 100644 security/ipe/ctx_test.c
create mode 100644 security/ipe/eval.c
create mode 100644 security/ipe/eval.h
create mode 100644 security/ipe/fs.c
create mode 100644 security/ipe/fs.h
create mode 100644 security/ipe/hooks.c
create mode 100644 security/ipe/hooks.h
create mode 100644 security/ipe/ipe.c
create mode 100644 security/ipe/ipe.h
create mode 100644 security/ipe/ipe_parser.h
create mode 100644 security/ipe/modules.c
create mode 100644 security/ipe/modules.h
create mode 100644 security/ipe/modules/Kconfig
create mode 100644 security/ipe/modules/Makefile
create mode 100644 security/ipe/modules/boot_verified.c
create mode 100644 security/ipe/modules/dmverity_roothash.c
create mode 100644 security/ipe/modules/dmverity_signature.c
create mode 100644 security/ipe/modules/fsverity_digest.c
create mode 100644 security/ipe/modules/fsverity_signature.c
create mode 100644 security/ipe/modules/ipe_module.h
create mode 100644 security/ipe/parsers.c
create mode 100644 security/ipe/parsers/Makefile
create mode 100644 security/ipe/parsers/default.c
create mode 100644 security/ipe/parsers/policy_header.c
create mode 100644 security/ipe/policy.c
create mode 100644 security/ipe/policy.h
create mode 100644 security/ipe/policy_parser_tests.c
create mode 100644 security/ipe/policyfs.c

--
2.33.0