evmctl man page

evmctl — IMA/EVM signing utility

Synopsis

evmctl [options] <command> [Options]

Description

The evmctl utility can be used for producing and verifying digital signatures, which are used by Linux kernel integrity subsystem (IMA/EVM). It can be also used to import keys into the kernel keyring.

Commands

--version
help <command>
import [--rsa] pubkey keyring
sign [-r] [--imahash | --imasig ] [--key key] [--pass password] file
verify file
ima_sign [--sigfile] [--key key] [--pass password] file
ima_verify file
ima_hash file
ima_measurement file
ima_fix [-t fdsxm] path
sign_hash [--key key] [--pass password]
hmac [--imahash | --imasig ] file

Options

-a, --hashalgo     sha1 (default), sha224, sha256, sha384, sha512
-s, --imasig       make IMA signature
-d, --imahash      make IMA hash
-f, --sigfile      store IMA signature in .sig file instead of xattr
    --rsa          use RSA key type and signing scheme v1
-k, --key          path to signing key (default: /etc/keys/{privkey,pubkey}_evm.pem)
-p, --pass         password for encrypted signing key
-r, --recursive    recurse into directories (sign)
-t, --type         file types to fix 'fdsxm' (f: file, d: directory, s: block/char/symlink)
                   x - skip fixing if both ima and evm xattrs exist (use with caution)
                   m - stay on the same filesystem (like 'find -xdev')
-n                 print result to stdout instead of setting xattr
-u, --uuid         use custom FS UUID for EVM (unspecified: from FS, empty: do not use)
    --smack        use extra SMACK xattrs for EVM
    --m32          force EVM hmac/signature for 32 bit target system
    --m64          force EVM hmac/signature for 64 bit target system
-v                 increase verbosity level
-h, --help         display this help and exit

Introduction

Linux kernel integrity subsystem is comprised of a number of different components including the Integrity Measurement Architecture (IMA), Extended Verification Module (EVM), IMA-appraisal extension, digital signature verification extension and audit measurement log support.

The evmctl utility is used for producing and verifying digital signatures, which are used by the Linux kernel integrity subsystem. It is also used for importing keys into the kernel keyring.

Linux integrity subsystem allows to use IMA and EVM signatures. EVM signature protects file metadata, such as file attributes and extended attributes. IMA signature protects file content.

For more detailed information about integrity subsystem it is recommended to follow resources in Resources section.

Evm Hmac and Signature Metadata

EVM protects file metadata by including following attributes into HMAC and signature calculation: inode number, inode generation, UID, GID, file mode, security.selinux, security.SMACK64, security.ima, security.capability.

EVM HMAC and signature in may also include additional file and file system attributes. Currently supported additional attributes are filesystem UUID and extra SMACK extended attributes.

Kernel configuration option CONFIG_EVM_ATTR_FSUUID controls whether to include filesystem UUID into HMAC and enabled by default. Therefore evmctl also includes fsuuid by default. Providing --uuid option without parameter allows to disable usage of fs uuid. Providing --uuid=UUID option with parameter allows to use custom UUID.

Kernel configuration option CONFIG_EVM_EXTRA_SMACK_XATTRS controls whether to include additional SMACK extended attributes into HMAC. They are following: security.SMACK64EXEC, security.SMACK64TRANSMUTE and security.SMACK64MMAP. evmctl --smack options enables that.

Key and Signature Formats

Linux integrity subsystem supports two type of signature and respectively two key formats.

First key format (v1) is pure RSA key encoded in PEM a format and uses own signature format. It is now non-default format and requires to provide evmctl --rsa option for signing and importing the key.

Second key format uses X509 DER encoded public key certificates and uses asymmetric key support in the kernel (since kernel 3.9). CONFIG_INTEGRITY_ASYMMETRIC_KEYS must be enabled (default).

Integrity Keyrings

Integrity subsystem uses dedicated IMA/EVM keyrings to search for signature verification keys - _ima and _evm respectively.

Since 3.13 IMA allows to declare IMA keyring as trusted. It allows only to load keys, signed by a key from the system keyring (.system). It means self-signed keys are not allowed. This is a default behavior unless CONFIG_IMA_TRUSTED_KEYRING is undefined. IMA trusted keyring is has different name .ima. Trusted keyring requires X509 public key certificates. Old version RSA public keys are not compatible with trusted keyring.

Generate Evm Encrypted Keys

EVM encrypted key is used for EVM HMAC calculation:

# create and save the key kernel master key (user type)
# LMK is used to encrypt encrypted keys
keyctl add user kmk "`dd if=/dev/urandom bs=1 count=32 2>/dev/null`" @u
keyctl pipe `keyctl search @u user kmk` > /etc/keys/kmk
# create the EVM encrypted key
keyctl add encrypted evm-key "new user:kmk 32" @u
keyctl pipe `keyctl search @u encrypted evm-key` >/etc/keys/evm-key

Generate Evm Trusted Keys (Tpm Based)

Trusted EVM keys are keys which a generate with the help of TPM. They are not related to integrity trusted keys.

# create and save the key kernel master key (user type)
keyctl add trusted kmk "new 32" @u
keyctl pipe `keyctl search @u trusted kmk` >kmk
# create the EVM trusted key
keyctl add encrypted evm-key "new trusted:kmk 32" @u
keyctl pipe `keyctl search @u encrypted evm-key` >evm-key

Generate Signing and Verification Keys

Generate private key in plain text format:

openssl genrsa -out privkey_evm.pem 1024

Generate encrypted private key:

openssl genrsa -des3 -out privkey_evm.pem 1024

Make encrypted private key from unencrypted:

openssl rsa -in /etc/keys/privkey_evm.pem -out privkey_evm_enc.pem -des3

Generate self-signed X509 public key certificate and private key for using kernel asymmetric keys support:

openssl req -new -nodes -utf8 -sha1 -days 36500 -batch \
            -x509 -config x509_evm.genkey \
            -outform DER -out x509_evm.der -keyout privkey_evm.pem

Configuration file x509_evm.genkey:

# Begining of the file
[ req ]
default_bits = 1024
distinguished_name = req_distinguished_name
prompt = no
string_mask = utf8only
x509_extensions = myexts
[ req_distinguished_name ]
O = Magrathea
CN = Glacier signing key
emailAddress = slartibartfast@magrathea.h2g2
[ myexts ]
basicConstraints=critical,CA:FALSE
keyUsage=digitalSignature
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid
# EOF

Generate public key for using RSA key format:

openssl rsa -pubout -in privkey_evm.pem -out pubkey_evm.pem

Copy keys to /etc/keys:

   cp pubkey_evm.pem /etc/keys
   scp pubkey_evm.pem target:/etc/keys
or
   cp x509_evm.pem /etc/keys
   scp x509_evm.pem target:/etc/keys

Generate Trusted Keys

Generation of trusted keys is a bit more complicated process and involves following steps:

Configuration file ima-local-ca.genkey:

# Begining of the file
[ req ]
default_bits = 2048
distinguished_name = req_distinguished_name
prompt = no
string_mask = utf8only
x509_extensions = v3_ca
[ req_distinguished_name ]
O = IMA-CA
CN = IMA/EVM certificate signing key
emailAddress = ca@ima-ca
[ v3_ca ]
basicConstraints=CA:TRUE
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid:always,issuer
# keyUsage = cRLSign, keyCertSign
# EOF

Generate private key and X509 public key certificate:

openssl req -new -x509 -utf8 -sha1 -days 3650 -batch -config $GENKEY \
            -outform DER -out ima-local-ca.x509 -keyout ima-local-ca.priv

Produce X509 in DER format for using while building the kernel:

openssl x509 -inform DER -in ima-local-ca.x509 -out ima-local-ca.pem

Configuration file ima.genkey:

# Begining of the file
[ req ]
default_bits = 1024
distinguished_name = req_distinguished_name
prompt = no
string_mask = utf8only
x509_extensions = v3_usr
[ req_distinguished_name ]
O = `hostname`
CN = `whoami` signing key
emailAddress = `whoami`@`hostname`
[ v3_usr ]
basicConstraints=critical,CA:FALSE
#basicConstraints=CA:FALSE
keyUsage=digitalSignature
#keyUsage = nonRepudiation, digitalSignature, keyEncipherment
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid
#authorityKeyIdentifier=keyid,issuer
# EOF

Generate private key and X509 public key certificate signing request:

openssl req -new -nodes -utf8 -sha1 -days 365 -batch -config $GENKEY \
            -out csr_ima.pem -keyout privkey_ima.pem

Sign X509 public key certificate signing request with local IMA CA private key:

openssl x509 -req -in csr_ima.pem -days 365 -extfile $GENKEY -extensions v3_usr \
             -CA ima-local-ca.pem -CAkey ima-local-ca.priv -CAcreateserial \
             -outform DER -out x509_ima.der

Sign File Data and Metadata

Default key locations:

Private RSA key: /etc/keys/privkey_evm.pem
Public RSA key: /etc/keys/pubkey_evm.pem
X509 certificate: /etc/keys/x509_evm.der

Options to remember: -k, -r, --rsa, --uuid, --smack.

Sign file with EVM signature and calculate hash value for IMA:

evmctl sign --imahash test.txt

Sign file with both IMA and EVM signatures:

evmctl sign --imasig test.txt:

Sign file with IMA signature:

evmctl ima_sign test.txt

Sign recursively whole filesystem:

evmctl -r sign --imahash /

Fix recursively whole filesystem:

evmctl -r ima_fix /

Sign filesystem selectively using find command:

find / \( -fstype rootfs -o -fstype ext4 \) -exec evmctl sign --imahash '{}' \;

Fix filesystem selectively using find command:

find / \( -fstype rootfs -o -fstype ext4 \) -exec sh -c "< '{}'" \;

Initialize Ima/Evm at Early Boot

IMA/EVM initialization should be normally done from initial RAM file system before mounting root filesystem.

Here is Ubuntu initramfs example script (/etc/initramfs-tools/scripts/local-top/ima.sh)

# mount securityfs if not mounted
SECFS=/sys/kernel/security
grep -q  $SECFS /proc/mounts || mount -n -t securityfs securityfs $SECFS
# search for IMA trusted keyring, then for untrusted
ima_id="`awk '/\.ima/ { printf "%d", "0x"$1; }' /proc/keys`"
if [ -z "$ima_id" ]; then
    ima_id=`keyctl search @u keyring _ima 2>/dev/null`
    if [ -z "$ima_id" ]; then
        ima_id=`keyctl newring _ima @u`
    fi
fi
# import IMA X509 certificate
evmctl import /etc/keys/x509_ima.der $ima_id
# search for EVM keyring
evm_id=`keyctl search @u keyring _evm 2>/dev/null`
if [ -z "$evm_id" ]; then
    evm_id=`keyctl newring _evm @u`
fi
# import EVM X509 certificate
evmctl import /etc/keys/x509_evm.der $evm_id
# a) import EVM encrypted key
cat /etc/keys/kmk | keyctl padd user kmk @u
keyctl add encrypted evm-key "load `cat /etc/keys/evm-key`" @u
# OR
# b) import EVM trusted key
keyctl add trusted kmk "load `cat /etc/keys/kmk`" @u
keyctl add encrypted evm-key "load `cat /etc/keys/evm-key`" @u
# enable EVM
echo "1" > /sys/kernel/security/evm

Optionally it is possible also to forbid adding, removing of new public keys and certificates into keyrings and revoking keys using keyctl setperm command:

# protect EVM keyring
keyctl setperm $evm_id 0x0b0b0000
# protect IMA keyring
keyctl setperm $ima_id 0x0b0b0000
# protecting IMA key from revoking (against DoS)
ima_key=`evmctl import /etc/keys/x509_ima.der $ima_id`
keyctl setperm $ima_key 0x0b0b0000

When using plain RSA public keys in PEM format, use evmctl import --rsa for importing keys:

evmctl import --rsa /etc/keys/pubkey_evm.pem $evm_id

Latest version of keyctl allows to import X509 public key certificates:

cat /etc/keys/x509_ima.der | keyctl padd asymmetric '' $ima_id

Files

Examples of scripts to generate X509 public key certificates:

/usr/share/doc/ima-evm-utils/ima-genkey-self.sh
/usr/share/doc/ima-evm-utils/ima-genkey.sh
/usr/share/doc/ima-evm-utils/ima-gen-local-ca.sh

Author

Written by Dmitry Kasatkin, <dmitry.kasatkin at gmail.com> and others.

Resources

http://sourceforge.net/p/linux-ima/wiki/Home
http://sourceforge.net/p/linux-ima/ima-evm-utils

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Copyright (C) 2012 - 2014 Linux Integrity Project. Free use of this software is granted under the terms of the GNU Public License (GPL).

Info

11/20/2017