The chip guard feature helps detect if attackers have replaced a Cisco router’s Application Specific Integrated Circuit (ASIC) chip or CPU chip with a counterfeit one when the device is in the manufacturing supply chain. The ASIC performs critical functions, such as scanning an egress queue for error causes and a CPU runs the operating system. If these chips are counterfeited, the performance, reliability, and security of the router is compromised. During the hardware integrity check, at the time of device boot, if the chip guard feature identifies a counterfeit ASIC or CPU, it halts the secure boot process and displays a warning to inform that the supply chain integrity has been compromised.

Each Linux user is mapped to an SELinux user through an SELinux policy. This allows Linux users to inherit the restrictions placed on SELinux users.

If an unconfined Linux user executes an application, which an SELinux policy defines as an application that can transition from the unconfined_t domain to its own confined domain, the unconfined Linux user is subject to the restrictions of that confined domain. The security benefit is that, even though a Linux user is running in unconfined mode, the application remains confined. Therefore, the exploitation of a flaw in the application is limited by the policy.

A confined Linux user is restricted by a confined user domain against the unconfined_t domain. The SELinux policy can also define a transition from a confined user domain to its own target confined domain. In such a case, confined Linux users are subject to the restrictions of that target confined domain.

There are three SELinux modes:

• Enforcing: When SELinux is running in enforcing mode, it enforces the SELinux policy and denies access based on SELinux policy rules.

• Permissive: In permissive mode, the SELinux does not enforce policy, but logs any denials. Permissive mode is used for debugging and policy development. This is the default mode.

• Disabled: In disabled mode, no SELinux policy is loaded. The mode may be changed in the boot loader, SELinux config, or at runtime with setenforce .

SELinux plays an important role during system startup. Because all processes must be labeled with their proper domain, the init process performs essential actions early in the boot process that synchronize labeling and policy enforcement.

RPMs are signed using Cisco keys during the build process.

The install component of the Cisco IOS XR automatically performs various actions on the RPMs, such as verification, activation, deactivation, and removal. Many of these actions invoke the underlying DNF installer. During each of these actions, the DNF installer verifies the signature of the RPM to ensure that it operates on a legitimate package.

Cisco RPMs are signed using GPG keys. The RPM format has an area dedicated to hold the signature of the header and payload and these are verified and validated via DNF package managers.

The XR Install enforces signature validation using the ‘gpgcheck’ option of DNF. Thus, any Third-Party RPM packages installation fails if done through the XR Install (which uses the DNF).

DM-Crypt is a Linux kernel module that provides disk encryption. The module takes advantage of the Linux kernel’s device-mapper (DM) infrastructure. The DM provides a way to create virtual layers of block devices.

DM-crypt is a device-mapper target and provides transparent encryption of block devices using the kernel crypto API. Data written to the block device is encrypted; whereas, data to be read is decrypted. See the following figure.

Intel's Advanced Encryption Standard New Instructions (AES-NI) is a hardware-assisted engine that enables high-speed hardware encryption and decryption. This process leaves the CPU free to do other tasks.

When the input-output operations are started, the read-write requests that are directed at the encrypted block device are passed to the DM-Crypt. DM-Crypt then sends multiple cryptographic requests to the Cryptographic Framework. The crypto framework is designed to take advantage of off-chip hardware accelerators and provides software implementations when accelerators are not available. See the following image.

DM-Crypt relies on user space tools, such as cryptsetup to set up cryptographic volumes. Cryptsetup is a command-line-interface (CLI) tool that interacts with DM-Crypt for creating, accessing, and managing encrypted devices.

An encrypted logical volume (LV) can be created during software installation on the following Cisco NCS 540 router variants:

• N540-28Z4C-SYS-A/D

• N540X-16Z4G8Q2C-A/D

• N540-12Z20G-SYS-A/D

• N540X-12Z16G-SYS-A/D

• N540X-6Z18G-SYS-A/D

You can activate or deactivate the encrypted disk partition on demand. In addition to being activated, all sensitive files are also migrated from the unencrypted disk partition to the encrypted disk partition. The encrypted files can be migrated back during deactivation.

You can activate the data encryption by using the disk-encryption activate location command. A sample output is as follows:

Router#disk-encryption activate location 0/RP0/CPU0 
Tue Apr 16 14:35:00.939 UTC

Preparing system for backup. This may take a few minutes especially for large configurations.
	Status report: node0_RP0_CPU0: START TO BACKUP 
Router#	Status report: node0_RP0_CPU0: BACKUP HAS COMPLETED SUCCESSFULLY 
[Done]

The encrypted logical volume capacity is 150MB of disk space and is available as /var/xr/enc for applications to access.

Note	Although applications can choose to use this space for storage, that data is not be part of the data migration if the software image is downgraded to a version that does not support encryption.

When encryption is activated on a system, each card generates a random encryption key and stores it in its own secure storage—the Trust Anchor module (TAm). During successive reboots, the encryption key is read from the TAm and applied to unlock the encrypted device. Since each card stores its encryption key locally on the TAm, an SSD that is removed from one card and inserted into another cannot be unlocked by the key stored on that card, thereby making the SSD unusable.

If encryption is activated, the encrypted LV can only be unlocked by using the key stored in the TAm. So, if an encrypted SSD is removed and moved to another line card, the SSD cannot be unlocked. In other words, when you activate encryption, the SSD is bound to the card it is inserted in.

Zeroization refers to the process of deleting sensitive data from a cryptographic module.

Note	In case of a Return Material Authorization (RMA), you must factory reset the data.

You can perform zeroization by using the factory reset location command from the XR prompt.

Caution

Running this command while encryption is activated, deletes the master encryption key from the TAm and renders the motherboard unusable after the subsequent reload.

IMA appraisal provides an added runtime security level that can detect if a file has been modified – either accidentally or maliciously and either remotely or locally.

The kernel achieves this by validating the hash measurement of the file against a known good value (KGV). The encrypted KGV in the form of a signature is stored in the file’s extended attribute and enforces local file integrity. The enforced mode strictly enforces the file integrity check whenever a file is opened for either reading or executing.

There are three categories of system files that require protection – Linux, XR, and third-party applications.

1. Linux System Files: System files are comprised of Executable and Linkable Format (ELF) binary executables, shared libraries, scripts (such as, Bash, Python, PERL, and Tcl), configuration files, and password files that are part of the Linux distribution packages. Integrity protection of the said files ensures that remote or local modification of the data does not remain undetected and access to such tampered data is either forbidden or logged or both. To guarantee the integrity of these files, they must have a valid IMA signature for the lifetime of the files. Executables and scripts must be appraised and measured. All other immutable files must be measured. Files that don’t require appraisal and measurement are runtime files, logs, memory-mapped files like devices, and shared memory objects.

2. XR System Files: XR system files are comprised of XR applications, shared libraries, kernel modules, scripts, data files, configuration files and secret files like keys and user credentials. Integrity of these files must be maintained in order for XR to operate properly. To keep the integrity of these files protected all system files must have a valid IMA signature for the lifetime of the files. Executables and scripts must be appraised and measured. All other immutable files must be measured. Files that don’t require appraisal and measurement are runtime files, logs, memory-mapped files like devices, and shared memory objects.

3. Third-party Applications (TPAs): All TPAs are not appraised. There are two types of TPAs:

   • native running applications: For native running applications the system files are installed on the disk from an rpm package or directly copied to the disk. All immutable files are only measured. Executables and scripts must be appraised and measured. All other immutable files must be measured. Files that don’t require appraisal and measurement are runtime files, logs, and memory mapped-files like shared memory objects.

   • containerized applications: For containerized applications the system files are packaged in the container image such as docker as part of the filesystem layers. When the container is launched, the system files are only accessible from within the container unless it is bind mounted on the host. In this case, only container image files are measured.

There are other frequently updated files that are created by the IOS XR (Linux, XR) at runtime, such as runtime files, logs, memory mapped files like devices and shared memory objects. These files contain runtime data and logs that are constantly updated by the applications. By default, they do not require an IMA signature and are excluded from appraisal to avoid possible access failure.

In this release, the following files are not signed with an IMA key, so they do not have an IMA signature. However, the system still allows their execution:

• Zero Touch Provisioning (ZTP) bash scripts with execute permission

• ZTP bash scripts without execute permission

• Third-party bash scripts without execute permission

• Bash scripts downloaded through file transfer operation like secure copy (SCP) or Secure File Transfer Protocol (SFTP)

• Open Programmability System (OPS) 1.0 scripts, whether downloaded or created on the router

Note	In Cisco IOS XR Release 7.4.1, enforced appraisal is enabled only on the XR system files.

IMA audit generates an event log every time it finds a file opened for reading or executing that has a mismatch between the measured file hash and the one stored in the extended attribute.

This data integrity verification event is recorded in the audit log.

There are three reasons an integrity log is recorded in the audit log – invalid signature, invalid hash and missing hash. The audit log has the following key information:

• type - INTEGRITY_DATA - Triggered to record a data integrity verification event run by the kernel.

• pid - Process ID of the calling process that opened the file with integrity verification failure.

• subject - SELinux file context label. SELinux runs in Permissive mode. Any access control violation is only logged in the audit log and the application is still allowed to run.

• op - Operation (appraise_data).

• cause - Reason for integrity verification failure (invalid-signature, invalid-hash, missing-hash).

• comm - Calling process.

• name – Name of the file with full path that was appraised.

The following output shows an instance where the IMA appraisal causes the execution of a tampered binary executable to fail. The integrity violation logged is Invalid Signature, the integrity violation log type is Integrity Data, and the appraised file is /usr/bin/zip.

RP/0/RP0/CPU0:NCS-540-C-LNT#run
Mon Apr 29 08:39:26.793 UTC
[node0_RP0_CPU0:~]$cat /var/log/audit/audit.log | grep -i integ | grep zip | fold -w 100
type=INTEGRITY_DATA msg=audit(1714378019.193:866): pid=2236 uid=0 auid=4294967295 ses=4294967295 sub
j=iosxradmin_u:iosxradmin_r:iosxradmin_t:s0 op="appraise_data" cause="invalid-signature" comm="sh" n
ame="/usr/bin/zip" dev="dm-14" ino=147881 res=0
type=INTEGRITY_DATA msg=audit(1714378019.197:867): pid=2236 uid=0 auid=4294967295 ses=4294967295 sub
j=iosxradmin_u:iosxradmin_r:iosxradmin_t:s0 op="appraise_data" cause="invalid-signature" comm="sh" n
ame="/usr/bin/zip" dev="dm-14" ino=147881 res=0

The following output shows an instance when an audit log was recorded because the file was missing an IMA signature and was opened for either reading or execution. This resulted in a “missing-hash” event log.

[node0_RP0_CPU0:/ima-appraisal]$zip --version | head -2
sh: /usr/bin/zip: Permission denied
[node0_RP0_CPU0:/ima-appraisal]$
[node0_RP0_CPU0:/ima-appraisal]$cat /var/log/audit/audit.log | grep -i integ | fold -w 100
type=INTEGRITY_DATA msg=audit(1714500558.187:556): pid=52560 uid=0 auid=4294967295 ses=4294967295 su
bj=iosxradmin_u:iosxradmin_r:iosxradmin_t:s0 op=appraise_data cause=missing-hash comm="sh" name="/us
r/bin/zip" dev="dm-11" ino=1507384 res=0

The IMA policy is not user-defined and is created by default. It contains a policy rule set that defines exactly which files on the file system should be measured or appraised.

Each policy rule must start with one of the following directives:

• measure: Perform IMA measurement

• dont_measure: Exclude from IMA measurement

• appraise: Perform IMA appraisal

• dont_appraise: Exclude from IMA appraisal

Note	IMA policy is protected at runtime – it cannot be read or modified.

To display the content of the IMA appraisal mode, query the kernel command line and look for “ima_appraise=enforce”.

$ cat /proc/cmdline

To query the content of the IMA measurement logs:

$ cat /sys/kernel/security/ima/ascii_runtime_measurements

To display the total number of files measured:

$ cat /sys/kernel/security/ima/runtime_measurements_count

To display the total number of integrity violations:

$ cat /sys/kernel/security/ima/violations

To access other user space interfaces in sysfs that are specific to the cisco_ima measurement:

$ ls /sys/kernel/security/cisco_ima

The IMA appraisal provides local integrity, validation, and enforcement of the measurement against a known good value stored as an extended attribute—security.ima. The method for validating file data integrity is based on a digital signature, which in addition to providing file data integrity also provides authenticity. Each file (RPM) shipped in the image is signed by Cisco during the build and packaging process and validated at runtime using the IMA public certificate stored in the TAm.

All RPMs contain Cisco IMA signatures of the files packaged in the RPM, which are embedded in the RPM header. The IMA signature of the individual file is stored in its extended attribute during RPM installation. This protects against modification of the Cisco RPMs.

The IMA signature format used for IMA can have multiple lines and every line has comma-separated fields. Each line entry will have the filename, hash, and signature as illustrated below.

• File – Filename with the full path of the file hashed and signed

• Hash – SHA256 hash of the file

• Signature – RSA2048 key-based signature