EVMCTL(1) EVMCTL(1)
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_boot_aggregate [--pcrs hash-algorithm,file] [TPM 1.2 BIOS event log]
ima_sign [--sigfile] [--key key] [--pass password] file
ima_verify file
ima_hash file
ima_measurement [--ignore-violations] [--verify-sig [--key "key1, key2, ..."]] [--pcrs [hash-algorithm,]file [--pcrs hash-algorithm,file] ...] 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
--xattr-user store xattrs in user namespace (for testing purposes)
--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
--engine e preload OpenSSL engine e (such as: gost)
--pcrs file containing TPM pcrs, one per hash-algorithm/bank
--ignore-violations ignore ToMToU measurement violations
--verify-sig verify the file signature based on the file hash, both
stored in the template data.
-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 AT 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:
o Creation of local IMA certification authority (CA). It consist of
private and public key certificate which are used to sign and
verify other keys.
o Build Linux kernel with embedded local IMA CA X509 certificate. It
is used to verify other keys added to the .ima trusted keyring
o 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).
04/08/2021 EVMCTL(1)
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