Integrity Policy Enforcement (IPE) — The Linux Kernel 6.12.93-gnu-rt18-parabola1-1-rt-lts documentation

# Integrity Policy Enforcement (IPE)

Note

This is the documentation for admins, system builders, or individuals attempting to use IPE. If you’re looking for more developer-focused documentation about IPE please see the design docs.

## Overview

Integrity Policy Enforcement (IPE) is a Linux Security Module that takes a complementary approach to access control. Unlike traditional access control mechanisms that rely on labels and paths for decision-making, IPE focuses on the immutable security properties inherent to system components. These properties are fundamental attributes or features of a system component that cannot be altered, ensuring a consistent and reliable basis for security decisions.

To elaborate, in the context of IPE, system components primarily refer to files or the devices these files reside on. However, this is just a starting point. The concept of system components is flexible and can be extended to include new elements as the system evolves. The immutable properties include the origin of a file, which remains constant and unchangeable over time. For example, IPE policies can be crafted to trust files originating from the initramfs. Since initramfs is typically verified by the bootloader, its files are deemed trustworthy; “file is from initramfs” becomes an immutable property under IPE’s consideration.

The immutable property concept extends to the security features enabled on a file’s origin, such as dm-verity or fs-verity, which provide a layer of integrity and trust. For example, IPE allows the definition of policies that trust files from a dm-verity protected device. dm-verity ensures the integrity of an entire device by providing a verifiable and immutable state of its contents. Similarly, fs-verity offers filesystem-level integrity checks, allowing IPE to enforce policies that trust files protected by fs-verity. These two features cannot be turned off once established, so they are considered immutable properties. These examples demonstrate how IPE leverages immutable properties, such as a file’s origin and its integrity protection mechanisms, to make access control decisions.

For the IPE policy, specifically, it grants the ability to enforce stringent access controls by assessing security properties against reference values defined within the policy. This assessment can be based on the existence of a security property (e.g., verifying if a file originates from initramfs) or evaluating the internal state of an immutable security property. The latter includes checking the roothash of a dm-verity protected device, determining whether dm-verity possesses a valid signature, assessing the digest of a fs-verity protected file, or determining whether fs-verity possesses a valid built-in signature. This nuanced approach to policy enforcement enables a highly secure and customizable system defense mechanism, tailored to specific security requirements and trust models.

To enable IPE, ensure that `CONFIG_SECURITY_IPE` (under Security -> Integrity Policy Enforcement (IPE)) config option is enabled.

## 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 it yet. Instead, IPE supports PKI (public key infrastructure), which generally designates a set of trusted entities that provide a measure of 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.

The digest_cache LSM [1] is a system that when combined with IPE, could be used to enable and support general-purpose computing use cases.

## Known Limitations

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.

IPE cannot verify the integrity of programs written in interpreted languages when these scripts are invoked by passing these program files to the interpreter. This is because the way interpreters execute these files; the scripts themselves are not evaluated as executable code through one of IPE’s hooks, but they are merely text files that are read (as opposed to compiled executables) [2].

## Threat Model

IPE specifically targets the risk of tampering with user-space executable code after the kernel has initially booted, including the kernel modules loaded from userspace via `modprobe` or `insmod`.

To illustrate, consider a scenario where an untrusted binary, possibly malicious, is downloaded along with all necessary dependencies, including a loader and libc. The primary function of IPE in this context is to prevent the execution of such binaries and their dependencies.

IPE achieves this by verifying the integrity and authenticity of all executable code before allowing them to run. It conducts a thorough check to ensure that the code’s integrity is intact and that they match an authorized reference value (digest, signature, etc) as per the defined policy. If a binary does not pass this verification process, either because its integrity has been compromised or it does not meet the authorization criteria, IPE will deny its execution. Additionally, IPE generates audit logs which may be utilized to detect and analyze failures resulting from policy violation.

Tampering threat scenarios include modification or replacement of executable code by a range of actors including:

- Actors with physical access to the hardware

- Actors with local network access to the system

- Actors with access to the deployment system

- Compromised internal systems under external control

- Malicious end users of the system

- Compromised end users of the system

- Remote (external) compromise of the system

IPE does not mitigate threats arising from malicious but authorized developers (with access to a signing certificate), or compromised developer tools used by them (i.e. return-oriented programming attacks). Additionally, IPE draws hard security boundary between userspace and kernelspace. As a result, kernel-level exploits are considered outside the scope of IPE and mitigation is left to other mechanisms.

## Policy

IPE policy is a plain-text [3] 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 name is a unique key identifying this policy in a human readable name. This is used to create nodes under securityfs as well as uniquely identify policies to deploy new policies vs update existing policies.

The policy version indicates the current version of the policy (NOT the policy syntax version). This is used to prevent rollback 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 rule, and `op`, which determines when the rule should be evaluated. The ordering is significant, a rule must start with `op`, and end with `action`. Thus, a minimal rule is:

    op=EXECUTE action=ALLOW

This example will allow any execution. Additional properties are used to assess immutable security properties about the files being evaluated. These properties are intended to be descriptions of systems within the kernel that can provide a measure of integrity verification, such that IPE can determine the trust of the resource based on the value of the property.

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`.

IPE policy supports comments. The character ‘#’ will function as a comment, ignoring all characters to the right of ‘#’ until the newline.

The default behavior of IPE evaluations can also be expressed in policy, through the `DEFAULT` statement. This can be done at a global level, or a per-operation level:

    # Global
    DEFAULT action=ALLOW

    # Operation Specific
    DEFAULT op=EXECUTE action=ALLOW

A default must be set for all known operations in IPE. If you want to preserve older policies being compatible with newer kernels that can introduce new operations, set a global default of `ALLOW`, then override the defaults on a per-operation basis (as above).

With configurable policy-based LSMs, there’s several issues with enforcing the configurable policies at startup, around reading and parsing the policy:

1. The kernel should not read files from userspace, so directly reading the policy file is prohibited.

2. The kernel command line has a character limit, and one kernel module should not reserve the entire character limit for its own configuration.

3. There are various boot loaders in the kernel ecosystem, so handing off a memory block would be costly to maintain.

As a result, IPE has addressed this problem through a concept of a “boot policy”. A boot policy is a minimal policy which is compiled into the kernel. This policy is intended to get the system to a state where userspace is set up and ready to receive commands, at which point a more complex policy can be deployed via securityfs. The boot policy can be specified via `SECURITY_IPE_BOOT_POLICY` config option, 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 securityfs.

### Deploying Policies

Policies can be deployed from userspace through securityfs. These policies are signed through the PKCS#7 message format to enforce some level of authorization of the policies (prohibiting an attacker from gaining unconstrained root, and deploying an “allow all” policy). These policies must be signed by a certificate that chains to the `SYSTEM_TRUSTED_KEYRING`, or to the secondary and/or platform keyrings if `CONFIG_IPE_POLICY_SIG_SECONDARY_KEYRING` and/or `CONFIG_IPE_POLICY_SIG_PLATFORM_KEYRING` are enabled, respectively. With openssl, the policy can be signed by:

    openssl smime -sign \
       -in "$MY_POLICY" \
       -signer "$MY_CERTIFICATE" \
       -inkey "$MY_PRIVATE_KEY" \
       -noattr \
       -nodetach \
       -nosmimecap \
       -outform der \
       -out "$MY_POLICY.p7b"

Deploying the policies is done through securityfs, through the `new_policy` node. To deploy a policy, simply cat the file into the securityfs node:

    cat "$MY_POLICY.p7b" > /sys/kernel/security/ipe/new_policy

Upon success, this will create one subdirectory under `/sys/kernel/security/ipe/policies/`. The subdirectory will be the `policy_name` field of the policy deployed, so for the example above, the directory will be `/sys/kernel/security/ipe/policies/Ex_Policy`. Within this directory, there will be seven files: `pkcs7`, `policy`, `name`, `version`, `active`, `update`, and `delete`.

The `pkcs7` file is read-only. Reading it returns the raw PKCS#7 data that was provided to the kernel, representing the policy. If the policy being read is the boot policy, this will return `ENOENT`, as it is not signed.

The `policy` file is read only. Reading it returns the PKCS#7 inner content of the policy, which will be the plain text policy.

The `active` file is used to set a policy as the currently active policy. This file is rw, and accepts a value of `"1"` to set the policy as active. Since only a single policy can be active at one time, all other policies will be marked inactive. The policy being marked active must have a policy version greater or equal to the currently-running version.

The `update` file is used to update a policy that is already present in the kernel. This file is write-only and accepts a PKCS#7 signed policy. Two checks will always be performed on this policy: First, the `policy_names` must match with the updated version and the existing version. Second the updated policy must have a policy version greater than the currently-running version. This is to prevent rollback attacks.

The `delete` file is used to remove a policy that is no longer needed. This file is write-only and accepts a value of `1` to delete the policy. On deletion, the securityfs node representing the policy will be removed. However, delete the current active policy is not allowed and will return an operation not permitted error.

Similarly, writing to both `update` and `new_policy` could result in bad message(policy syntax error) or file exists error. The latter error happens when trying to deploy a policy with a `policy_name` while the kernel already has a deployed policy with the same `policy_name`.

Deploying a policy will not cause IPE to start enforcing the policy. IPE will only enforce the policy marked active. Note that only one policy can be active at a time.

Once deployment is successful, the policy can be activated, by writing file `/sys/kernel/security/ipe/policies/$policy_name/active`. For example, the `Ex_Policy` can be activated by:

    echo 1 > "/sys/kernel/security/ipe/policies/Ex_Policy/active"

From above point on, `Ex_Policy` is now the enforced policy on the system.

IPE also provides a way to delete policies. This can be done via the `delete` securityfs node, `/sys/kernel/security/ipe/policies/$policy_name/delete`. Writing `1` to that file deletes the policy:

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

There is only one requirement to delete a policy: the policy being deleted must be inactive.

Note

If a traditional MAC system is enabled (SELinux, apparmor, smack), all writes to ipe’s securityfs nodes require `CAP_MAC_ADMIN`.

### Modes

IPE supports two modes of operation: permissive (similar to SELinux’s permissive mode) and enforced. In permissive mode, all events are checked and policy violations are logged, but the policy is not really enforced. This allows users to test policies before enforcing them.

The default mode is enforce, and can be changed via the kernel command line parameter `ipe.enforce=(0|1)`, or the securityfs node `/sys/kernel/security/ipe/enforce`.

Note

If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), all writes to ipe’s securityfs nodes require `CAP_MAC_ADMIN`.

### Audit Events

#### 1420 AUDIT_IPE_ACCESS

Event Examples:

    type=1420 audit(1653364370.067:61): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2241 comm="ld-linux.so" path="/deny/lib/libc.so.6" dev="sda2" ino=14549020 rule="DEFAULT action=DENY"
    type=1300 audit(1653364370.067:61): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=7f1105a28000 a1=195000 a2=5 a3=812 items=0 ppid=2219 pid=2241 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="ld-linux.so" exe="/tmp/ipe-test/lib/ld-linux.so" subj=unconfined key=(null)
    type=1327 audit(1653364370.067:61): 707974686F6E3300746573742F6D61696E2E7079002D6E00

    type=1420 audit(1653364735.161:64): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2472 comm="mmap_test" path=? dev=? ino=? rule="DEFAULT action=DENY"
    type=1300 audit(1653364735.161:64): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=0 a1=1000 a2=4 a3=21 items=0 ppid=2219 pid=2472 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="mmap_test" exe="/root/overlake_test/upstream_test/vol_fsverity/bin/mmap_test" subj=unconfined key=(null)
    type=1327 audit(1653364735.161:64): 707974686F6E3300746573742F6D61696E2E7079002D6E00

This event indicates that IPE made an access control decision; the IPE specific record (1420) is always emitted in conjunction with a `AUDITSYSCALL` record.

Determining whether IPE is in permissive or enforced mode can be derived from `success` property and exit code of the `AUDITSYSCALL` record.

Field descriptions:

Field | Value Type | Optional? | Description of Value
--- | --- | --- | ---
ipe_op | string | No | The IPE operation name associated with the log
ipe_hook | string | No | The name of the LSM hook that triggered the IPE event
enforcing | integer | No | The current IPE enforcing state 1 is in enforcing mode, 0 is in permissive mode
pid | integer | No | The pid of the process that triggered the IPE event.
comm | string | No | The command line program name of the process that triggered the IPE event
path | string | Yes | The absolute path to the evaluated file
ino | integer | Yes | The inode number of the evaluated file
dev | string | Yes | The device name of the evaluated file, e.g. vda
rule | string | No | The matched policy rule

#### 1421 AUDIT_IPE_CONFIG_CHANGE

Event Example:

    type=1421 audit(1653425583.136:54): old_active_pol_name="Allow_All" old_active_pol_version=0.0.0 old_policy_digest=sha256:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855 new_active_pol_name="boot_verified" new_active_pol_version=0.0.0 new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1
    type=1300 audit(1653425583.136:54): SYSCALL arch=c000003e syscall=1 success=yes exit=2 a0=3 a1=5596fcae1fb0 a2=2 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null)
    type=1327 audit(1653425583.136:54): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2

This event indicates that IPE switched the active poliy from one to another along with the version and the hash digest of the two policies. Note IPE can only have one policy active at a time, all access decision evaluation is based on the current active policy. The normal procedure to deploy a new policy is loading the policy to deploy into the kernel first, then switch the active policy to it.

This record will always be emitted in conjunction with a `AUDITSYSCALL` record for the `write` syscall.

Field descriptions:

Field | Value Type | Optional? | Description of Value
--- | --- | --- | ---
old_active_pol_name | string | Yes | The name of previous active policy
old_active_pol_version | string | Yes | The version of previous active policy
old_policy_digest | string | Yes | The hash of previous active policy
new_active_pol_name | string | No | The name of current active policy
new_active_pol_version | string | No | The version of current active policy
new_policy_digest | string | No | The hash of current active policy
auid | integer | No | The login user ID
ses | integer | No | The login session ID
lsm | string | No | The lsm name associated with the event
res | integer | No | The result of the audited operation(success/fail)

#### 1422 AUDIT_IPE_POLICY_LOAD

Event Example:

    type=1422 audit(1653425529.927:53): policy_name="boot_verified" policy_version=0.0.0 policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1
    type=1300 audit(1653425529.927:53): arch=c000003e syscall=1 success=yes exit=2567 a0=3 a1=5596fcae1fb0 a2=a07 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null)
    type=1327 audit(1653425529.927:53): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2E

This record indicates a new policy has been loaded into the kernel with the policy name, policy version and policy hash.

This record will always be emitted in conjunction with a `AUDITSYSCALL` record for the `write` syscall.

Field descriptions:

Field | Value Type | Optional? | Description of Value
--- | --- | --- | ---
policy_name | string | No | The policy_name
policy_version | string | No | The policy_version
policy_digest | string | No | The policy hash
auid | integer | No | The login user ID
ses | integer | No | The login session ID
lsm | string | No | The lsm name associated with the event
res | integer | No | The result of the audited operation(success/fail)

#### 1404 AUDIT_MAC_STATUS

Event Examples:

    type=1404 audit(1653425689.008:55): enforcing=0 old_enforcing=1 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1
    type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=)
    type=1327 audit(1653425689.008:55): proctitle="-bash"

    type=1404 audit(1653425689.008:55): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1
    type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=)
    type=1327 audit(1653425689.008:55): proctitle="-bash"

This record will always be emitted in conjunction with a `AUDITSYSCALL` record for the `write` syscall.

Field descriptions:

Field | Value Type | Optional? | Description of Value
--- | --- | --- | ---
enforcing | integer | No | The enforcing state IPE is being switched to, 1 is in enforcing mode, 0 is in permissive mode
old_enforcing | integer | No | The enforcing state IPE is being switched from, 1 is in enforcing mode, 0 is in permissive mode
auid | integer | No | The login user ID
ses | integer | No | The login session ID
enabled | integer | No | The new TTY audit enabled setting
old-enabled | integer | No | The old TTY audit enabled setting
lsm | string | No | The lsm name associated with the event
res | integer | No | The result of the audited operation(success/fail)

#### Success Auditing

IPE supports success auditing. When enabled, all events that pass IPE policy and are not blocked will emit an audit event. This is disabled by default, and can be enabled via the kernel command line `ipe.success_audit=(0|1)` or `/sys/kernel/security/ipe/success_audit` securityfs file.

This is very noisy, as IPE will check every userspace binary on the system, but is useful for debugging policies.

Note

If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), all writes to ipe’s securityfs nodes require `CAP_MAC_ADMIN`.

## Properties

As explained above, IPE properties are `key=value` pairs expressed in IPE policy. Two properties are built-into the policy parser: ‘op’ and ‘action’. The other properties are used to restrict immutable security properties about the files being evaluated. Currently those properties are: ‘`boot_verified`’, ‘`dmverity_signature`’, ‘`dmverity_roothash`’, ‘`fsverity_signature`’, ‘`fsverity_digest`’. A description of all properties supported by IPE are listed below:

### op

Indicates the operation for a rule to apply to. Must be in every rule, as the first token. IPE supports the following operations:

> `EXECUTE`

> > Pertains to any file attempting to be executed, or loaded as an executable.

> `FIRMWARE`:

> > Pertains to firmware being loaded via the firmware_class interface. This covers both the preallocated buffer and the firmware file itself.

> `KMODULE`:

> > Pertains to loading kernel modules via `modprobe` or `insmod`.

> `KEXEC_IMAGE`:

> > Pertains to kernel images loading via `kexec`.

> `KEXEC_INITRAMFS`

> > Pertains to initrd images loading via `kexec --initrd`.

> `POLICY`:

> > Controls loading policies via reading a kernel-space initiated read.

> > An example of such is loading IMA policies by writing the path to the policy file to `$securityfs/ima/policy`

> `X509_CERT`:

> > Controls loading IMA certificates through the Kconfigs, `CONFIG_IMA_X509_PATH` and `CONFIG_EVM_X509_PATH`.

### action

> Determines what IPE should do when a rule matches. Must be in every rule, as the final clause. Can be one of:

> `ALLOW`:

> > If the rule matches, explicitly allow access to the resource to proceed without executing any more rules.

> `DENY`:

> > If the rule matches, explicitly prohibit access to the resource to proceed without executing any more rules.

### boot_verified

> This property can be utilized for authorization of files from initramfs. The format of this property is:

>     boot_verified=(TRUE|FALSE)

> Warning

> This property will trust files from initramfs(rootfs). It should only be used during early booting stage. Before mounting the real rootfs on top of the initramfs, initramfs script will recursively remove all files and directories on the initramfs. This is typically implemented by using switch_root(8) [4]. Therefore the initramfs will be empty and not accessible after the real rootfs takes over. It is advised to switch to a different policy that doesn’t rely on the property after this point. This ensures that the trust policies remain relevant and effective throughout the system’s operation.

### dmverity_roothash

> This property can be utilized for authorization or revocation of specific dm-verity volumes, identified via their root hashes. It has a dependency on the DM_VERITY module. This property is controlled by the `IPE_PROP_DM_VERITY` config option, it will be automatically selected when `SECURITY_IPE` and `DM_VERITY` are all enabled. The format of this property is:

>     dmverity_roothash=DigestName:HexadecimalString

> The supported DigestNames for dmverity_roothash are [5]

> > - blake2b-512

> > - blake2s-256

> > - sha256

> > - sha384

> > - sha512

> > - sha3-224

> > - sha3-256

> > - sha3-384

> > - sha3-512

> > - sm3

> > - rmd160

### dmverity_signature

> This property can be utilized for authorization of all dm-verity volumes that have a signed roothash that validated by a keyring specified by dm-verity’s configuration, either the system trusted keyring, or the secondary keyring. It depends on `DM_VERITY_VERIFY_ROOTHASH_SIG` config option and is controlled by the `IPE_PROP_DM_VERITY_SIGNATURE` config option, it will be automatically selected when `SECURITY_IPE`, `DM_VERITY` and `DM_VERITY_VERIFY_ROOTHASH_SIG` are all enabled. The format of this property is:

>     dmverity_signature=(TRUE|FALSE)

### fsverity_digest

> This property can be utilized for authorization of specific fsverity enabled files, identified via their fsverity digests. It depends on `FS_VERITY` config option and is controlled by the `IPE_PROP_FS_VERITY` config option, it will be automatically selected when `SECURITY_IPE` and `FS_VERITY` are all enabled. The format of this property is:

>     fsverity_digest=DigestName:HexadecimalString

> The supported DigestNames for fsverity_digest are [6]

> > - sha256

> > - sha512

### fsverity_signature

> This property is used to authorize all fs-verity enabled files that have been verified by fs-verity’s built-in signature mechanism. The signature verification relies on a key stored within the “.fs-verity” keyring. It depends on `FS_VERITY_BUILTIN_SIGNATURES` config option and it is controlled by the `IPE_PROP_FS_VERITY` config option, it will be automatically selected when `SECURITY_IPE`, `FS_VERITY` and `FS_VERITY_BUILTIN_SIGNATURES` are all enabled. The format of this property is:

>     fsverity_signature=(TRUE|FALSE)

## Policy Examples

### Allow all

    policy_name=Allow_All policy_version=0.0.0
    DEFAULT action=ALLOW

### Allow only initramfs

    policy_name=Allow_Initramfs policy_version=0.0.0
    DEFAULT action=DENY

    op=EXECUTE boot_verified=TRUE action=ALLOW

### Allow any signed and validated dm-verity volume and the initramfs

    policy_name=Allow_Signed_DMV_And_Initramfs 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=Deny_DMV_By_Roothash policy_version=0.0.0
    DEFAULT action=DENY

    op=EXECUTE dmverity_roothash=sha256:cd2c5bae7c6c579edaae4353049d58eb5f2e8be0244bf05345bc8e5ed257baff 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=Allow_DMV_By_Roothash policy_version=0.0.0
    DEFAULT action=DENY

    op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW

### Allow any fs-verity file with a valid built-in signature

    policy_name=Allow_Signed_And_Validated_FSVerity policy_version=0.0.0
    DEFAULT action=DENY

    op=EXECUTE fsverity_signature=TRUE action=ALLOW

### Allow execution of a specific fs-verity file

    policy_name=ALLOW_FSV_By_Digest policy_version=0.0.0
    DEFAULT action=DENY

    op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=ALLOW

## Additional Information

- Github Repository

- Developer and design docs for IPE

## FAQ

Q:

  What’s the difference between other LSMs which provide a measure of trust-based access control?

A:

> In general, there’s two other LSMs that can provide similar functionality: IMA, and Loadpin.

> IMA and IPE are functionally very similar. The significant difference between the two is the policy. [3]

> Loadpin and IPE differ fairly dramatically, as Loadpin only covers the IPE’s kernel read operations, whereas IPE is capable of controlling execution on top of kernel read. The trust model is also different; Loadpin roots its trust in the initial super-block, whereas trust in IPE is stemmed from kernel itself (via `SYSTEM_TRUSTED_KEYS`).

[1]

https://lore.kernel.org/lkml/20240415142436.2545003-1-roberto.sassu@huaweicloud.com/

[2]

There is some interest in solving this issue.

[3] (1,2)

Please see the design docs for more on this topic.

[4]

https://man7.org/linux/man-pages/man8/switch_root.8.html

[5]

These hash algorithms are based on values accepted by the Linux crypto API; IPE does not impose any restrictions on the digest algorithm itself; thus, this list may be out of date.

[6]

These hash algorithms are based on values accepted by the kernel’s fsverity support; IPE does not impose any restrictions on the digest algorithm itself; thus, this list may be out of date.