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Asymmetric keys were introduced in linux-3.7 to verify the signature on signed kernel modules. The asymmetric keys infrastructure abstracts the signature verification from the crypto details. Asymmetric crypto keys support allows to import X509 public key certificate in a DER format into the kernel keyring. Asymmetric keys require a new signature format. 'evmctl -x' or 'evmctl --x509' option can be used to utilize new signature format. Using of key filename after the file name for signing and verification commands is a bit odd. This patch add '--key' parameter to specify non-default key file. Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
146 lines
3.9 KiB
Plaintext
146 lines
3.9 KiB
Plaintext
ima-evm-utils - IMA/EVM signing utility
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=========================================
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Contents:
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1. Key and signature formats
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2. Key generation
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3. Initialization
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4. Signing
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Key and signature formats
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-------------------------
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EVM support (v2) in latest version of the kernel adds the file system UUID to
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the HMAC calculation. It is controlled by the CONFIG_EVM_HMAC_VERSION and
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version 2 is enabled by default. To include the UUID to the signature calculation,
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it is necessary to provide '--uuid -' or '-u -' parameter to the 'sign' command.
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Latest kernel got IMA/EVM support for using X509 certificates and asymmetric key
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support for verifying digital signatures. The new command line parameter
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'-x' or '--x509' was added to the evmctl to enable using of X509 certificates
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and new signature format.
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Key generation
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--------------
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Generate private key in plain text format
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$ openssl genrsa -out privkey_evm.pem 1024
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Generate encrypted private key
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$ openssl genrsa -des3 -out privkey_evm.pem 1024
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Make encrypted private key from unencrypted
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$ openssl rsa -in /etc/keys/privkey_evm.pem -out privkey_evm_enc.pem -des3
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Generate self-signed X509 certificate and private key for using kernel asymmetric
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keys support
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$ openssl req -new -nodes -utf8 -sha1 -days 36500 -batch \
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-x509 -config x509_evm.genkey \
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-outform DER -out x509_evm.der -keyout privkey_evm.pem
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Configuration file x509_evm.genkey:
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# Begining of the file
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[ req ]
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default_bits = 1024
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distinguished_name = req_distinguished_name
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prompt = no
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string_mask = utf8only
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x509_extensions = myexts
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[ req_distinguished_name ]
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O = Magrathea
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CN = Glacier signing key
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emailAddress = slartibartfast@magrathea.h2g2
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[ myexts ]
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basicConstraints=critical,CA:FALSE
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keyUsage=digitalSignature
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subjectKeyIdentifier=hash
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authorityKeyIdentifier=keyid
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# EOF
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Get public key
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$ openssl rsa -pubout -in privkey_evm.pem -out pubkey_evm.pem
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Copy keys to /etc/keys
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$ cp pubkey_evm.pem /etc/keys
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$ scp pubkey_evm.pem target:/etc/keys
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or
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$ cp x509_evm.pem /etc/keys
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$ scp x509_evm.pem target:/etc/keys
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Initialization
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--------------
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IMA/EVM initialization should be normally done from initial RAM file system
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before mounting root filesystem.
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Here is an example script /etc/initramfs-tools/scripts/local-top/ima.sh
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# import EVM HMAC key
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keyctl clear @u
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keyctl add user kmk "testing123" @u
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keyctl add encrypted evm-key "load `cat /etc/keys/evm-key`" @u
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# import IMA public key
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ima_id=`keyctl newring _ima @u`
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evmctl import /etc/keys/pubkey_evm.pem $ima_id
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# import EVM public key
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evm_id=`keyctl newring _evm @u`
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evmctl import /etc/keys/pubkey_evm.pem $evm_id
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# enable EVM
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echo "1" > /sys/kernel/security/evm
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Import X509 certificate into the kernel keyring (since kernel 3.9?)
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$ evmctl -x import /etc/keys/x509_evm.der `keyctl search @u keyring _ima`
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$ evmctl -x import /etc/keys/x509_evm.der `keyctl search @u keyring _evm`
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Signing
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-------
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Default public key: /etc/keys/pubkey_evm.pem
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Default private key: /etc/keys/privkey_evm.pem
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Default X509 certificate: /etc/keys/x509_evm.der
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Signing for using X509 certificates is done using '-x' or '--x509' parameter.
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Signing for using new the EVM HMAC format is done using '-u -' or '--uuid -' parameter.
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Sign file with EVM signature and use hash value for IMA - common case
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$ evmctl sign [-u -] [-x] --imahash test.txt
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Sign file with both IMA and EVM signatures - for immutable files
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$ evmctl sign [-u -] [-x] --imasig test.txt
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Sign file with IMA signature - for immutable files
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$ evmctl ima_sign [-x] test.txt
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Label whole filesystem with EVM signatures
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$ find / \( -fstype rootfs -o -fstype ext4 \) -exec evmctl sign [-u -] [-x] --imahash '{}' \;
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Label filesystem in fix mode - kernel sets correct values to IMA and EVM xattrs
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$ find / \( -fstype rootfs -o -fstype ext4 \) -exec sh -c "< '{}'" \;
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