Provided by: ima-evm-utils_1.1-0ubuntu2_amd64 
NAME
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 ] [--portable] [--key key] [--pass password] file
verify file
ima_sign [--sigfile] [--key key] [--pass password] file
ima_verify file
ima_hash file
ima_measurement [--key "key1, key2, ..."] [--list] 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)
-o, --portable generate portable EVM signatures
-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. Providing the --portable
option will disable usage of the fs uuid and also the inode number and generation.
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 64" @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:
• Creation of local IMA certification authority (CA). It consist of private and public
key certificate which are used to sign and verify other keys.
• Build Linux kernel with embedded local IMA CA X509 certificate. It is used to verify
other keys added to the .ima trusted keyring
• Generate IMA private signing key and verification public key certificate, which is
signed using local IMA CA private key.
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
COPYING
Copyright (C) 2012 - 2014 Linux Integrity Project. Free use of this software is granted
under the terms of the GNU Public License (GPL).
11/23/2021 EVMCTL(1)