Re: [RFC PATCH v9 00/16] Integrity Policy Enforcement LSM (IPE)

[Roberto Sassu]

On Mon, 2023-01-30 at 14:57 -0800, Fan Wu wrote:
> Overview:
> ---------
> 
> IPE is a Linux Security Module which takes a complimentary approach to
> access control. Whereas existing mandatory access control mechanisms
> base their decisions on labels and paths, IPE instead determines
> whether or not an operation should be allowed based on immutable
> security properties of the system component the operation is being
> performed on.
> 
> IPE itself does not mandate how the security property should be
> evaluated, but relies on an extensible set of external property providers
> to evaluate the component. IPE makes its decision based on reference
> values for the selected properties, specified in the IPE policy.
> 
> The reference values represent the value that the policy writer and the
> local system administrator (based on the policy signature) trust for the
> system to accomplish the desired tasks.
> 
> One such provider is for example dm-verity, which is able to represent
> the integrity property of a partition (its immutable state) with a digest.
> 
> 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.
> There are exceptions, such as the case where a Linux distribution
> vendor trusts only their own keys, where IPE can successfully be used
> to enforce the trust requirement.
> 
> 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.
> 
> DIGLIM[1] is a system that when combined with IPE, could be used to
> enable general purpose computing scenarios.
> 
> 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. 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 keys: "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 rule will allow any execution. 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. 
> 
> 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. 
> 
> For example:
> 
>   op=EXECUTE dmverity_signature=FALSE action=DENY
> 
> This rule with property dmverity_signature will deny any file not from
> a signed dmverity volume to be executed. 
> 
> All available properties for IPE described in the documentation patch of
> this series.
> 
> 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. 
> 
> 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, see 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 userspace 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, while allowing
> all signed volumes and the initial superblock:
> 
>   policy_name=ProhibitSingleVolume policy_version=0.0.0
>   DEFAULT action=DENY
> 
>   op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 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=AllowSpecific policy_version=0.0.0
>   DEFAULT action=DENY
> 
>   op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW
> 
> Allow any signed fs-verity file
> 
>   policy_name="AllowSignedFSVerity" policy_version=0.0.0
>   DEFAULT action=DENY
> 
>   op=EXECUTE fsverity_signature=TRUE action=ALLOW
> 
> Deny a specific fs-verity file:
> 
>   policy_name="ProhibitSpecificFSVF" policy_version=0.0.0
>   DEFAULT action=DENY
> 
>   op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=DENY
>   op=EXECUTE boot_verified=TRUE action=ALLOW
>   op=EXECUTE dmverity_signature=TRUE 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 "$MY_PRIVATE_KEY" -outform der -noattr -nodetach \
>     -out "$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: Any securityfs write to IPE's nodes 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_NET=y
>     CONFIG_AUDIT=y
>     CONFIG_AUDITSYSCALL=y
> 
>     CONFIG_SECURITY_IPE=y
>     CONFIG_IPE_PROP_DM_VERITY=y
>     CONFIG_IPE_PROP_FS_VERITY=y
>     CONFIG_SECURITY_IPE_KUNIT_TEST=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.
> 
> 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".

I think it would be useful to handle special cases, for example you
could allow a process that created a file with memfd to use it, at the
condition that nobody else writes it.

This would be required during the boot, otherwise services could fail
to start (depending on the policy).

> 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.

Ok, it is a well known issue. Hopefully, it will be fixed soon.

Roberto

> 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/bpf/4d6932e96d774227b42721d9f645ba51@xxxxxxxxxx/T/
> 
> FAQ:
> ----
> 
> Q: What is the difference between IMA and IPE?
> 
> A: See the documentation patch for more on this topic.
> 
> Previous Postings
> -----------------
> 
> v1: https://lore.kernel.org/all/20200406181045.1024164-1-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v2: https://lore.kernel.org/all/20200406221439.1469862-1-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v3: https://lore.kernel.org/all/20200415162550.2324-1-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v4: https://lore.kernel.org/all/20200717230941.1190744-1-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v5: https://lore.kernel.org/all/20200728213614.586312-1-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v6: https://lore.kernel.org/all/20200730003113.2561644-1-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v7: https://lore.kernel.org/all/1634151995-16266-1-git-send-email-deven.desai@xxxxxxxxxxxxxxxxxxx/
> v8: https://lore.kernel.org/all/1654714889-26728-1-git-send-email-deven.desai@xxxxxxxxxxxxxxxxxxx/
> 
> Changelog:
> ----------
> 
> v2:
>   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.
> 
> v3:
>   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.
> 
> v4:
>   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.
> 
> v5:
>   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.
> 
> v6:
>   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.
> 
> v7:
>   * 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
> 
> v8:
>   * Add changelog information to individual commits
>   * A large number of changes to the audit patch.
>   * split fs/ & security/ changes to two separate patches.
>   * split block/, security/ & drivers/md/ changes to separate patches.
>   * Add some historical context to what lead to the creation of IPE
>     in the documentation patch.
>   * Cover-letter changes suggested by Roberto Sassu.
> 
> v9:
>   * Rewrite IPE parser to use kernel match_table parser.
>   * Adapt existing IPE properties to the new parser.
>   * Remove ipe_context, quote policy syntax, kernel_read for simplicity.
>   * Add new function in the security file system to delete IPE policy.
>   * Make IPE audit builtin and change several audit formats.
>   * Make boot_verified property builtin
> 
> Deven Bowers (13):
>   security: add ipe lsm
>   ipe: add policy parser
>   ipe: add evaluation loop and introduce 'boot_verified' as a trust
>     provider
>   ipe: add userspace interface
>   ipe: add LSM hooks on execution and kernel read
>   uapi|audit|ipe: add ipe auditing support
>   ipe: add permissive toggle
>   block|security: add LSM blob to block_device
>   dm-verity: consume root hash digest and signature data via LSM hook
>   ipe: add support for dm-verity as a trust provider
>   scripts: add boot policy generation program
>   ipe: kunit test for parser
>   documentation: add ipe documentation
> 
> Fan Wu (3):
>   security: add new securityfs delete function
>   fsverity: consume builtin signature via LSM hook
>   ipe: enable support for fs-verity as a trust provider
> 
>  Documentation/admin-guide/LSM/index.rst       |   1 +
>  Documentation/admin-guide/LSM/ipe.rst         | 729 ++++++++++++++++++
>  .../admin-guide/kernel-parameters.txt         |  12 +
>  Documentation/security/index.rst              |   1 +
>  Documentation/security/ipe.rst                | 436 +++++++++++
>  MAINTAINERS                                   |   8 +
>  block/bdev.c                                  |   7 +
>  drivers/md/dm-verity-target.c                 |  25 +-
>  drivers/md/dm-verity-verify-sig.c             |  16 +-
>  drivers/md/dm-verity-verify-sig.h             |  10 +-
>  fs/verity/fsverity_private.h                  |   2 +-
>  fs/verity/open.c                              |  13 +-
>  fs/verity/signature.c                         |   1 +
>  include/linux/blk_types.h                     |   3 +
>  include/linux/dm-verity.h                     |  19 +
>  include/linux/fsverity.h                      |   2 +
>  include/linux/lsm_hook_defs.h                 |   5 +
>  include/linux/lsm_hooks.h                     |  12 +
>  include/linux/security.h                      |  23 +
>  include/uapi/linux/audit.h                    |   1 +
>  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/inode.c                              |  25 +
>  security/ipe/.gitignore                       |   1 +
>  security/ipe/Kconfig                          |  75 ++
>  security/ipe/Makefile                         |  32 +
>  security/ipe/audit.c                          | 279 +++++++
>  security/ipe/audit.h                          |  19 +
>  security/ipe/digest.c                         | 144 ++++
>  security/ipe/digest.h                         |  26 +
>  security/ipe/eval.c                           | 424 ++++++++++
>  security/ipe/eval.h                           |  60 ++
>  security/ipe/fs.c                             | 242 ++++++
>  security/ipe/fs.h                             |  17 +
>  security/ipe/hooks.c                          | 275 +++++++
>  security/ipe/hooks.h                          |  42 +
>  security/ipe/ipe.c                            |  97 +++
>  security/ipe/ipe.h                            |  22 +
>  security/ipe/policy.c                         | 259 +++++++
>  security/ipe/policy.h                         |  93 +++
>  security/ipe/policy_fs.c                      | 459 +++++++++++
>  security/ipe/policy_parser.c                  | 545 +++++++++++++
>  security/ipe/policy_parser.h                  |  11 +
>  security/ipe/policy_tests.c                   | 294 +++++++
>  security/security.c                           |  70 ++
>  50 files changed, 4988 insertions(+), 16 deletions(-)
>  create mode 100644 Documentation/admin-guide/LSM/ipe.rst
>  create mode 100644 Documentation/security/ipe.rst
>  create mode 100644 include/linux/dm-verity.h
>  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/digest.c
>  create mode 100644 security/ipe/digest.h
>  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/policy.c
>  create mode 100644 security/ipe/policy.h
>  create mode 100644 security/ipe/policy_fs.c
>  create mode 100644 security/ipe/policy_parser.c
>  create mode 100644 security/ipe/policy_parser.h
>  create mode 100644 security/ipe/policy_tests.c
>