mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-18 01:34:14 +08:00
b8612e517c
Signing a module should only make it trusted by the specific kernel it was built for, not anything else. If a module signing key is used for multiple ABI-incompatible kernels, the modules need to include enough version information to distinguish them. Signed-off-by: Ben Hutchings <ben@decadent.org.uk> Cc: stable@vger.kernel.org Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
280 lines
11 KiB
Plaintext
280 lines
11 KiB
Plaintext
==============================
|
|
KERNEL MODULE SIGNING FACILITY
|
|
==============================
|
|
|
|
CONTENTS
|
|
|
|
- Overview.
|
|
- Configuring module signing.
|
|
- Generating signing keys.
|
|
- Public keys in the kernel.
|
|
- Manually signing modules.
|
|
- Signed modules and stripping.
|
|
- Loading signed modules.
|
|
- Non-valid signatures and unsigned modules.
|
|
- Administering/protecting the private key.
|
|
|
|
|
|
========
|
|
OVERVIEW
|
|
========
|
|
|
|
The kernel module signing facility cryptographically signs modules during
|
|
installation and then checks the signature upon loading the module. This
|
|
allows increased kernel security by disallowing the loading of unsigned modules
|
|
or modules signed with an invalid key. Module signing increases security by
|
|
making it harder to load a malicious module into the kernel. The module
|
|
signature checking is done by the kernel so that it is not necessary to have
|
|
trusted userspace bits.
|
|
|
|
This facility uses X.509 ITU-T standard certificates to encode the public keys
|
|
involved. The signatures are not themselves encoded in any industrial standard
|
|
type. The facility currently only supports the RSA public key encryption
|
|
standard (though it is pluggable and permits others to be used). The possible
|
|
hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
|
|
SHA-512 (the algorithm is selected by data in the signature).
|
|
|
|
|
|
==========================
|
|
CONFIGURING MODULE SIGNING
|
|
==========================
|
|
|
|
The module signing facility is enabled by going to the "Enable Loadable Module
|
|
Support" section of the kernel configuration and turning on
|
|
|
|
CONFIG_MODULE_SIG "Module signature verification"
|
|
|
|
This has a number of options available:
|
|
|
|
(1) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE)
|
|
|
|
This specifies how the kernel should deal with a module that has a
|
|
signature for which the key is not known or a module that is unsigned.
|
|
|
|
If this is off (ie. "permissive"), then modules for which the key is not
|
|
available and modules that are unsigned are permitted, but the kernel will
|
|
be marked as being tainted, and the concerned modules will be marked as
|
|
tainted, shown with the character 'E'.
|
|
|
|
If this is on (ie. "restrictive"), only modules that have a valid
|
|
signature that can be verified by a public key in the kernel's possession
|
|
will be loaded. All other modules will generate an error.
|
|
|
|
Irrespective of the setting here, if the module has a signature block that
|
|
cannot be parsed, it will be rejected out of hand.
|
|
|
|
|
|
(2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
|
|
|
|
If this is on then modules will be automatically signed during the
|
|
modules_install phase of a build. If this is off, then the modules must
|
|
be signed manually using:
|
|
|
|
scripts/sign-file
|
|
|
|
|
|
(3) "Which hash algorithm should modules be signed with?"
|
|
|
|
This presents a choice of which hash algorithm the installation phase will
|
|
sign the modules with:
|
|
|
|
CONFIG_MODULE_SIG_SHA1 "Sign modules with SHA-1"
|
|
CONFIG_MODULE_SIG_SHA224 "Sign modules with SHA-224"
|
|
CONFIG_MODULE_SIG_SHA256 "Sign modules with SHA-256"
|
|
CONFIG_MODULE_SIG_SHA384 "Sign modules with SHA-384"
|
|
CONFIG_MODULE_SIG_SHA512 "Sign modules with SHA-512"
|
|
|
|
The algorithm selected here will also be built into the kernel (rather
|
|
than being a module) so that modules signed with that algorithm can have
|
|
their signatures checked without causing a dependency loop.
|
|
|
|
|
|
(4) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY)
|
|
|
|
Setting this option to something other than its default of
|
|
"certs/signing_key.pem" will disable the autogeneration of signing keys
|
|
and allow the kernel modules to be signed with a key of your choosing.
|
|
The string provided should identify a file containing both a private key
|
|
and its corresponding X.509 certificate in PEM form, or — on systems where
|
|
the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
|
|
RFC7512. In the latter case, the PKCS#11 URI should reference both a
|
|
certificate and a private key.
|
|
|
|
If the PEM file containing the private key is encrypted, or if the
|
|
PKCS#11 token requries a PIN, this can be provided at build time by
|
|
means of the KBUILD_SIGN_PIN variable.
|
|
|
|
|
|
(5) "Additional X.509 keys for default system keyring" (CONFIG_SYSTEM_TRUSTED_KEYS)
|
|
|
|
This option can be set to the filename of a PEM-encoded file containing
|
|
additional certificates which will be included in the system keyring by
|
|
default.
|
|
|
|
Note that enabling module signing adds a dependency on the OpenSSL devel
|
|
packages to the kernel build processes for the tool that does the signing.
|
|
|
|
|
|
=======================
|
|
GENERATING SIGNING KEYS
|
|
=======================
|
|
|
|
Cryptographic keypairs are required to generate and check signatures. A
|
|
private key is used to generate a signature and the corresponding public key is
|
|
used to check it. The private key is only needed during the build, after which
|
|
it can be deleted or stored securely. The public key gets built into the
|
|
kernel so that it can be used to check the signatures as the modules are
|
|
loaded.
|
|
|
|
Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its
|
|
default, the kernel build will automatically generate a new keypair using
|
|
openssl if one does not exist in the file:
|
|
|
|
certs/signing_key.pem
|
|
|
|
during the building of vmlinux (the public part of the key needs to be built
|
|
into vmlinux) using parameters in the:
|
|
|
|
certs/x509.genkey
|
|
|
|
file (which is also generated if it does not already exist).
|
|
|
|
It is strongly recommended that you provide your own x509.genkey file.
|
|
|
|
Most notably, in the x509.genkey file, the req_distinguished_name section
|
|
should be altered from the default:
|
|
|
|
[ req_distinguished_name ]
|
|
#O = Unspecified company
|
|
CN = Build time autogenerated kernel key
|
|
#emailAddress = unspecified.user@unspecified.company
|
|
|
|
The generated RSA key size can also be set with:
|
|
|
|
[ req ]
|
|
default_bits = 4096
|
|
|
|
|
|
It is also possible to manually generate the key private/public files using the
|
|
x509.genkey key generation configuration file in the root node of the Linux
|
|
kernel sources tree and the openssl command. The following is an example to
|
|
generate the public/private key files:
|
|
|
|
openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
|
|
-config x509.genkey -outform PEM -out kernel_key.pem \
|
|
-keyout kernel_key.pem
|
|
|
|
The full pathname for the resulting kernel_key.pem file can then be specified
|
|
in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will
|
|
be used instead of an autogenerated keypair.
|
|
|
|
|
|
=========================
|
|
PUBLIC KEYS IN THE KERNEL
|
|
=========================
|
|
|
|
The kernel contains a ring of public keys that can be viewed by root. They're
|
|
in a keyring called ".system_keyring" that can be seen by:
|
|
|
|
[root@deneb ~]# cat /proc/keys
|
|
...
|
|
223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 1
|
|
302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
|
|
...
|
|
|
|
Beyond the public key generated specifically for module signing, additional
|
|
trusted certificates can be provided in a PEM-encoded file referenced by the
|
|
CONFIG_SYSTEM_TRUSTED_KEYS configuration option.
|
|
|
|
Further, the architecture code may take public keys from a hardware store and
|
|
add those in also (e.g. from the UEFI key database).
|
|
|
|
Finally, it is possible to add additional public keys by doing:
|
|
|
|
keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
|
|
|
|
e.g.:
|
|
|
|
keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
|
|
|
|
Note, however, that the kernel will only permit keys to be added to
|
|
.system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
|
|
that is already resident in the .system_keyring at the time the key was added.
|
|
|
|
|
|
=========================
|
|
MANUALLY SIGNING MODULES
|
|
=========================
|
|
|
|
To manually sign a module, use the scripts/sign-file tool available in
|
|
the Linux kernel source tree. The script requires 4 arguments:
|
|
|
|
1. The hash algorithm (e.g., sha256)
|
|
2. The private key filename or PKCS#11 URI
|
|
3. The public key filename
|
|
4. The kernel module to be signed
|
|
|
|
The following is an example to sign a kernel module:
|
|
|
|
scripts/sign-file sha512 kernel-signkey.priv \
|
|
kernel-signkey.x509 module.ko
|
|
|
|
The hash algorithm used does not have to match the one configured, but if it
|
|
doesn't, you should make sure that hash algorithm is either built into the
|
|
kernel or can be loaded without requiring itself.
|
|
|
|
If the private key requires a passphrase or PIN, it can be provided in the
|
|
$KBUILD_SIGN_PIN environment variable.
|
|
|
|
|
|
============================
|
|
SIGNED MODULES AND STRIPPING
|
|
============================
|
|
|
|
A signed module has a digital signature simply appended at the end. The string
|
|
"~Module signature appended~." at the end of the module's file confirms that a
|
|
signature is present but it does not confirm that the signature is valid!
|
|
|
|
Signed modules are BRITTLE as the signature is outside of the defined ELF
|
|
container. Thus they MAY NOT be stripped once the signature is computed and
|
|
attached. Note the entire module is the signed payload, including any and all
|
|
debug information present at the time of signing.
|
|
|
|
|
|
======================
|
|
LOADING SIGNED MODULES
|
|
======================
|
|
|
|
Modules are loaded with insmod, modprobe, init_module() or finit_module(),
|
|
exactly as for unsigned modules as no processing is done in userspace. The
|
|
signature checking is all done within the kernel.
|
|
|
|
|
|
=========================================
|
|
NON-VALID SIGNATURES AND UNSIGNED MODULES
|
|
=========================================
|
|
|
|
If CONFIG_MODULE_SIG_FORCE is enabled or module.sig_enforce=1 is supplied on
|
|
the kernel command line, the kernel will only load validly signed modules
|
|
for which it has a public key. Otherwise, it will also load modules that are
|
|
unsigned. Any module for which the kernel has a key, but which proves to have
|
|
a signature mismatch will not be permitted to load.
|
|
|
|
Any module that has an unparseable signature will be rejected.
|
|
|
|
|
|
=========================================
|
|
ADMINISTERING/PROTECTING THE PRIVATE KEY
|
|
=========================================
|
|
|
|
Since the private key is used to sign modules, viruses and malware could use
|
|
the private key to sign modules and compromise the operating system. The
|
|
private key must be either destroyed or moved to a secure location and not kept
|
|
in the root node of the kernel source tree.
|
|
|
|
If you use the same private key to sign modules for multiple kernel
|
|
configurations, you must ensure that the module version information is
|
|
sufficient to prevent loading a module into a different kernel. Either
|
|
set CONFIG_MODVERSIONS=y or ensure that each configuration has a different
|
|
kernel release string by changing EXTRAVERSION or CONFIG_LOCALVERSION.
|