IMA/EVM: Linux Integrity Measurement and Appraisal

Introduction

IMA (Integrity Measurement Architecture) and EVM (Extended Verification Module) are Linux kernel security subsystems that provide file integrity measurement and appraisal. Together, they form the Linux integrity subsystem, ensuring that files haven’t been tampered with and that only trusted code executes.

IMA measures file contents by computing cryptographic hashes when files are executed, opened, or read. EVM extends this by protecting file metadata (permissions, ownership, extended attributes) from tampering. Both rely on the kernel keyring and TPM (Trusted Platform Module) hardware for cryptographic operations and trust anchoring.

Key capabilities:

• Measurement — log hashes of executed/loaded files (IMA)
• Appraisal — enforce integrity checks before file access (IMA-appraisal)
• Metadata protection — prevent tampering with file attributes (EVM)
• Digital signatures — verify file authenticity using PKI (IMA-sig)
• Remote attestation — prove system integrity to remote parties

Architecture Overview

graph TD
    subgraph "User Space"
        EXEC["execve()"]
        OPEN["open()"]
        READ["read()"]
    end
    subgraph "LSM Framework"
        BPRM_CHECK["bprm_check_security"]
        FILE_OPEN["file_open"]
        FILE_FREE["file_free_security"]
    end
    subgraph "IMA Subsystem"
        MEASURE["IMA Measure<br>→ PCR extend + log"]
        APPRAISE["IMA Appraise<br>→ verify hash/signature"]
        AUDIT["IMA Audit<br>→ security audit log"]
    end
    subgraph "EVM Subsystem"
        EVM_VERIFY["EVM Verify<br>→ check metadata HMAC"]
        EVM_PROTECT["EVM Protect<br>→ HMAC over xattrs"]
    end
    subgraph "Trust Anchors"
        TPM["TPM Chip<br>PCR registers"]
        KEYRING[".ima keyring<br>trusted keys"]
        XATTR["security.ima<br>file xattr"]
    end

    EXEC --> BPRM_CHECK
    OPEN --> FILE_OPEN
    BPRM_CHECK --> MEASURE
    BPRM_CHECK --> APPRAISE
    FILE_OPEN --> MEASURE
    FILE_OPEN --> APPRAISE
    MEASURE --> TPM
    APPRAISE --> KEYRING
    APPRAISE --> XATTR
    EVM_VERIFY --> XATTR
    EVM_PROTECT --> XATTR

    style MEASURE fill:#3182ce,color:#fff
    style APPRAISE fill:#e53e3e,color:#fff
    style EVM_VERIFY fill:#d69e2e,color:#000

IMA: Integrity Measurement Architecture

What IMA Does

IMA maintains a measurement list — a running log of every file that has been executed or opened for reading. Each entry contains:

• File path
• SHA256/SHA1 hash of file contents
• Who triggered the measurement

The measurement list is analogous to a bank’s transaction log — it records what happened, not whether it was allowed.

IMA Policy

IMA uses a policy to determine which files are measured and/or appraised. The policy is loaded via /sys/kernel/security/ima/policy.

# Basic IMA policy
# Measure all executed files
measure func=BPRM_CHECK

# Measure all files opened for reading by UID 0
measure func=FILE_CHECK mask=MAY_READ uid=0

# Appraise all executed files (enforce integrity)
appraise func=BPRM_CHECK

# Appraise all files opened for writing
appraise fowner=0 func=FILE_CHECK mask=MAY_WRITE

IMA Policy Syntax

[action] [condition...]

Actions:

Conditions:

Loading IMA Policy

# Load policy at boot (kernel command line)
# ima_policy=tcb  (default: measure executed files)

# Load custom policy
echo 'measure func=BPRM_CHECK' > /sys/kernel/security/ima/policy

# Read current policy
cat /sys/kernel/security/ima/policy

# View measurement log
cat /sys/kernel/security/ima/ascii_runtime_measurements

# View binary measurement log (for TPM attestation)
cat /sys/kernel/security/ima/binary_runtime_measurements

IMA Measurement Log Format

10 <PCR> <hash> <template> <path>

Example:

10 0 92b3ed25c1781d0f31f23b4c1e4e6a8c1b3d7f8a ima-ng sha256:abc123... /usr/bin/cat
10 0 a7c4e91f2b3d5e6f8a9b0c1d2e3f4a5b6c7d8e9f ima-sig sha256:def456... /usr/bin/ls

IMA Appraisal

IMA appraisal enforces integrity by verifying file hashes or digital signatures before allowing access.

Appraisal Modes

graph TD
    subgraph "Appraisal Modes"
        LOG["log<br>Default: measure + log failures"]
        ENFORCE["enforce<br>Block access if integrity check fails"]
        FIX["fix<br>Measure + write security.ima xattr"]
    end
    LOG -->|"ima_appraise=log"| LOGDESC["Logs tampering,<br>doesn't block"]
    ENFORCE -->|"ima_appraise=enforce"| ENFDESC["Blocks tampered files,<br>requires signed files"]
    FIX -->|"ima_appraise=fix"| FIXDESC["Auto-generates<br>integrity labels"]

    style ENFORCE fill:#e53e3e,color:#fff
    style LOG fill:#d69e2e,color:#000
    style FIX fill:#3182ce,color:#fff

Setting Up IMA Appraisal

# Enable IMA appraisal at boot (kernel command line)
# ima_appraise=enforce ima_policy=tcb

# Generate IMA signatures for files
evmctl ima_sign --hashalgo sha256 --key /path/to/signing_key.pem /usr/bin/cat

# Verify a signature
evmctl ima_verify --key /path_to/x509_cert.der /usr/bin/cat

# Set simple hash (for fix mode)
evmctl ima_hash /usr/bin/cat

File Signature Workflow

sequenceDiagram
    participant Admin as System Admin
    participant evmctl as evmctl
    participant File as /usr/bin/cat
    participant Xattr as security.ima xattr

    Admin->>evmctl: ima_sign --key signing_key.pem /usr/bin/cat
    evmctl->>File: Read file contents
    evmctl->>evmctl: Compute SHA256 hash
    evmctl->>evmctl: Sign hash with private key
    evmctl->>Xattr: Set security.ima = signature

    Note over File,Xattr: At execution time:
    File->>File: execve() called
    File->>Xattr: Read security.ima
    File->>File: Verify signature with .ima keyring
    File->>File: Allow or deny execution

EVM: Extended Verification Module

What EVM Protects

While IMA protects file contents, EVM protects file metadata:

• File permissions (st_mode)
• File ownership (st_uid, st_gid)
• File modification time (st_mtime)
• Extended attributes (security.*, system.*, user.*)

EVM computes an HMAC over these metadata fields and stores it in the security.evm extended attribute.

EVM Architecture

graph TD
    subgraph "Protected Metadata"
        MODE["st_mode<br>permissions"]
        UID["st_uid / st_gid<br>ownership"]
        MTIME["st_mtime<br>modification time"]
        XATTRS["security.ima<br>IMA hash/signature"]
        CAPS["security.capability<br>file capabilities"]
    end
    subgraph "EVM HMAC"
        HMAC["HMAC(key, metadata + IMA hash)"]
    end
    subgraph "Storage"
        EVMA["security.evm<br>HMAC stored in xattr"]
    end

    MODE --> HMAC
    UID --> HMAC
    MTIME --> HMAC
    XATTRS --> HMAC
    CAPS --> HMAC
    HMAC --> EVMA

    style HMAC fill:#e53e3e,color:#fff
    style EVMA fill:#3182ce,color:#fff

EVM Keys

EVM uses a symmetric HMAC key:

# Generate EVM HMAC key
openssl rand -out /etc/keys/evm-key 32

# Load key into kernel keyring (requires TPM-sealed key in production)
keyctl add user evm-key "$(cat /etc/keys/evm-key)" @u

# Enable EVM
echo 1 > /sys/kernel/security/evm

EVM Modes

# EVM mode (set at boot via kernel command line)
# evm=fix    — compute and store HMAC (development)
# evm= HMAC  — verify HMAC on every access (enforcement)
# (default)   — no EVM unless enabled at runtime

# Check current EVM status
cat /sys/kernel/security/evm
# 0: disabled
# 1: HMAC verification enabled
# 2: digital signature verification enabled

# Enable EVM at runtime
echo 1 > /sys/kernel/security/evm

EVM Key Types

Digital Signatures in IMA/EVM

IMA Signature Format

IMA supports X.509-based digital signatures stored in the security.ima extended attribute:

security.ima = 0x04 | signature

The first byte indicates the type:

• 0x04 — IMA signature (v2 format)
• 0x03 — IMA hash (legacy)
• 0x05 — IMA signature with modsig (module signature)

Signing Files

# Install evmctl
sudo apt install evm-utils    # Debian/Ubuntu
sudo dnf install evm-utils    # Fedora

# Generate signing key pair
openssl req -new -x509 -newkey rsa:2048 -keyout signing_key.pem \
    -outform DER -out signing_cert.der -days 365 \
    -subj "/CN=IMA Signing Key" -nodes

# Sign a file
evmctl ima_sign --hashalgo sha256 --key signing_key.pem /usr/bin/cat

# Verify signature
evmctl ima_verify --key signing_cert.der /usr/bin/cat

# Load public key into kernel .ima keyring
evmctl import signing_cert.der

Keyring Hierarchy

graph TD
    ROOT[".ima keyring<br>(IMA trusted keys)"]
    SECONDARY[".evm keyring<br>(EVM trusted keys)"]
    PLATFORM[".platform keyring<br>(firmware keys)"]
    TRUST["Trusted Keys<br>(TPM-sealed)"]

    ROOT --> SECONDARY
    PLATFORM --> ROOT
    TRUST --> ROOT
    TRUST --> SECONDARY

    style ROOT fill:#e53e3e,color:#fff
    style SECONDARY fill:#3182ce,color:#fff

Configuring IMA/EVM on a Real System

Kernel Configuration

CONFIG_IMA=y                    # Enable IMA
CONFIG_IMA_APPRAISE=y           # Enable IMA appraisal
CONFIG_IMA_APPRAISE_SIGNED_POLICY=y  # Signed policy support
CONFIG_IMA_TRUSTED_KEYRING=y    # Use trusted keyring
CONFIG_EVM=y                    # Enable EVM
CONFIG_EVM_ATTR_FSUUID=y        # Include filesystem UUID in HMAC
CONFIG_INTEGRITY=y              # Integrity subsystem base
CONFIG_TCG_TPM=y                # TPM driver
CONFIG_TCG_TIS=y                # TPM Interface Specification

Boot Configuration (GRUB)

# /etc/default/grub
GRUB_CMDLINE_LINUX="... ima_policy=tcb ima_appraise=enforce evm=fix"

Complete Setup Script

#!/bin/bash
# setup-ima-evm.sh — Configure IMA/EVM on a running system

set -e

echo "=== IMA/EVM Setup ==="

# Step 1: Load EVM HMAC key
if [ -f /etc/keys/evm-key ]; then
    keyctl add user evm-key "$(cat /etc/keys/evm-key)" @u
    echo "[+] EVM key loaded"
fi

# Step 2: Enable EVM (fix mode first to generate HMACs)
echo 1 > /sys/kernel/security/evm
echo "[+] EVM enabled"

# Step 3: Load IMA policy
cat > /tmp/ima-policy << 'EOF'
# Measure all executables
measure func=BPRM_CHECK

# Measure all libraries
measure func=FILE_CHECK mask=MAY_READ fowner=0

# Appraise all executables
appraise func=BPRM_CHECK

# Appraise all kernel modules
appraise func=MODULE_CHECK

# Don't appraise pseudo-filesystems
dont_appraise fsmagic=0x9fa0
dont_appraise fsmagic=0x62656572
dont_appraise fsmagic=0x64626720
dont_appraise fsmagic=0x01021994
dont_appraise fsmagic=0x858458f6
dont_appraise fsmagic=0x73636673
EOF

echo 'measure func=BPRM_CHECK' > /sys/kernel/security/ima/policy
echo "[+] IMA policy loaded"

# Step 4: Generate EVM HMACs for system files
find /usr -type f -executable | while read f; do
    evmctl ima_hash "$f" 2>/dev/null
done
echo "[+] EVM HMACs generated"

echo "=== Setup complete ==="

Remote Attestation

IMA measurement lists can be sent to a remote verifier to prove system integrity:

sequenceDiagram
    participant Host as Linux Host
    participant TPM as TPM Chip
    participant Verifier as Remote Verifier

    Host->>Host: Execute files (IMA measures them)
    Host->>TPM: Extend PCR with file hashes
    Host->>Host: Build measurement list
    Verifier->>Host: Request attestation
    Host->>TPM: Quote PCR values
    TPM-->>Host: Signed PCR quote
    Host->>Verifier: Send measurement list + TPM quote
    Verifier->>Verifier: Verify TPM signature
    Verifier->>Verifier: Check measurement list against known-good
    Verifier-->>Host: Attestation result (pass/fail)

Using keyctl for TPM Operations

# List keys in .ima keyring
keyctl list %:.ima

# Show key details
keyctl show

# Read IMA measurement log
head -20 /sys/kernel/security/ima/ascii_runtime_measurements

# Count measurement entries
wc -l /sys/kernel/security/ima/ascii_runtime_measurements

Troubleshooting

Common Issues

Debugging

# Check IMA/EVM kernel support
grep -E 'IMA|EVM|INTEGRITY' /boot/config-$(uname -r)

# View IMA audit messages
ausearch -m INTEGRITY_DATA -i

# Check IMA policy
cat /sys/kernel/security/ima/policy

# Check EVM status
cat /sys/kernel/security/evm

# View kernel integrity messages
dmesg | grep -i 'ima\|evm\|integrity'

# List xattrs on a file
getfattr -n security.ima -n security.evm /usr/bin/cat

# Verify IMA signature manually
evmctl ima_verify --key /path/to/cert.der /usr/bin/ls

Audit Events

# IMA generates audit records for:
# - INTEGRITY_DATA: hash mismatches
# - INTEGRITY_STATUS: appraisal results
# - INTEGRITY_HASH: computed hashes
# - INTEGRITY_PCR: TPM PCR events

# Search for integrity violations
ausearch -m INTEGRITY_DATA --interpret

# Watch integrity events in real-time
tail -f /var/log/audit/audit.log | grep INTEGRITY

Use Cases

Secure Boot Chain

graph TD
    FW["Firmware (UEFI Secure Boot)"] --> GRUB["GRUB (verified)"]
    GRUB --> KERNEL["Kernel (signed)"]
    KERNEL --> INITRD["initrd (signed)"]
    INITRD --> SYSTEM["System binaries<br>(IMA appraised)"]
    SYSTEM --> USER["User applications<br>(IMA appraised)"]

    style FW fill:#e53e3e,color:#fff
    style KERNEL fill:#3182ce,color:#fff
    style SYSTEM fill:#38a169,color:#fff

Container Image Verification

# IMA can verify container image contents
# At image build time: sign all files
find /rootfs -type f | xargs -I{} evmctl ima_sign --key key.pem {}

# At container runtime: IMA appraises files
# Container runtime sets IMA policy for container mounts

Further Reading

• Linux IMA documentation
• IMA/EVM Wiki
• LWN: IMA and EVM
• evmctl man page
• LWN: Digital signatures for IMA
• TPM 2.0 and Linux

See Also

• Secure Boot — UEFI secure boot chain
• SELinux — mandatory access control
• Keyring — kernel key management
• Audit — security audit framework
• Cryptography — kernel crypto API