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 (deprecated)] 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_setxattr [--sigfile file]
ima_hash file
ima_measurement [--ignore-violations] [--verify-sig [--key "key1, key2, ..."]]
[--pcrs [hash-algorithm,]file [--pcrs hash-algorithm,file] ...]
[--verify-bank hash-algorithm] file
ima_boot_aggregate [--pcrs hash-algorithm,file] [TPM 1.2 BIOS event log]
[--hwtpm]
ima_fix [-t fdsxm] path
ima_clear [-t fdsxm] path
sign_hash [--veritysig] [--key key] [--pass=<password>]
hmac [--imahash | --imasig] [--hmackey key] file
OPTIONS
-------
-a, --hashalgo sha1, sha224, sha256, sha384, sha512, streebog256, streebog512 (default: sha256)
-s, --imasig make IMA signature
--veritysig sign an fs-verity file digest hash
-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 (deprecated)
-k, --key path to signing key (default: /etc/keys/{privkey,pubkey}_evm.pem)
or a pkcs11 URI
--keyid n overwrite signature keyid with a 32-bit value in hex (for signing)
--keyid-from-cert file
read keyid value from SKID of a x509 cert file
-o, --portable generate portable EVM signatures
-p, --pass password for encrypted signing key; use -p<password>
-r, --recursive recurse into directories (sign)
-t, --type file types to fix 'fxm' (f: file)
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
--ino use custom inode for EVM
--uid use custom UID for EVM
--gid use custom GID for EVM
--mode use custom Mode for EVM
--generation use custom Generation for EVM(unspecified: from FS, empty: use 0)
--ima use custom IMA signature for EVM
--selinux use custom Selinux label for EVM
--caps use custom Capabilities for EVM(unspecified: from FS, empty: do not use)
--pcrs file containing TPM pcrs, one per hash-algorithm/bank
--verify-sig verify the file signature based on the file hash, both
stored in the template data.
--engine e preload OpenSSL engine e (such as: gost) is deprecated
--provider p preload OpenSSL provider (such as: pkcs11)
--ignore-violations ignore ToMToU measurement violations
--hmackey path to symmetric key (default: /etc/keys/evm-key-plain)
-v increase verbosity level
-h, --help display this help and exit
Environment variables:
EVMCTL_KEY_PASSWORD : Private key password to use; do not use --pass option
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).
For v2 signatures x509 certificate (containing the public key) could be appended to the
private key (they both are in PEM format) to automatically extract keyid from its Subject
Key Identifier (SKID).
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 2048
Generate encrypted private key:
openssl genrsa -des3 -out privkey_evm.pem 2048
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 -sha256 -days 36500 -batch \
-x509 -config x509_evm.genkey \
-outform DER -out x509_evm.der -keyout privkey_evm.pem
Create an elliptic curve (EC) key (supported since Linux v5.13)
openssl ecparam -name prime256v1 -genkey -out privkey_evm.pem
Generate self-signed x509 EC public key certificate and private key for using
kernel asymmetric key support (supported since Linux v5.13):
openssl req -new -nodes -utf8 -sha1 -days 36500 -batch \
-x509 -config x509_evm.genkey \
-outform DER -out x509_evm.der -keyout privkey_evm.pem \
-newkey ec -pkeyopt ec_paramgen_curve:prime256v1
Configuration file x509_evm.genkey:
# Beginning of the file
[ req ]
default_bits = 2048
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
extendedKeyUsage=critical,codeSigning
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid
# EOF
Generate public key for using RSA key format:
openssl rsa -pubout -in privkey_evm.pem -out pubkey_evm.pem
Similarly generate public EC key:
openssl ec -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:
# Beginning 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
Note: To generated elliptic curve keys add the following parameters to
the 'req' commands below (supported since Linux v5.13):
-newkey ec -pkeyopt ec_paramgen_curve:prime256v1
Generate private key and X509 public key certificate:
openssl req -new -x509 -utf8 -sha256 -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:
# Beginning of the file
[ req ]
default_bits = 2048
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 -sha256 -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
/usr/share/doc/ima-evm-utils/ima-genkey-self-ecc.sh
/usr/share/doc/ima-evm-utils/ima-genkey-ecc.sh
/usr/share/doc/ima-evm-utils/ima-gen-local-ca-ecc.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).