| 1 |
swift 12/10/01 20:22:35 |
| 2 |
|
| 3 |
Added: ima-guide.xml |
| 4 |
Log: |
| 5 |
Initial draft for IMA guide |
| 6 |
|
| 7 |
Revision Changes Path |
| 8 |
1.1 xml/htdocs/proj/en/hardened/integrity/docs/ima-guide.xml |
| 9 |
|
| 10 |
file : http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/proj/en/hardened/integrity/docs/ima-guide.xml?rev=1.1&view=markup |
| 11 |
plain: http://sources.gentoo.org/viewvc.cgi/gentoo/xml/htdocs/proj/en/hardened/integrity/docs/ima-guide.xml?rev=1.1&content-type=text/plain |
| 12 |
|
| 13 |
Index: ima-guide.xml |
| 14 |
=================================================================== |
| 15 |
<?xml version='1.0' encoding='UTF-8'?> |
| 16 |
<!DOCTYPE guide SYSTEM "/dtd/guide.dtd"> |
| 17 |
<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/proj/en/hardened/integrity/docs/ima-guide.xml,v 1.1 2012/10/01 20:22:35 swift Exp $ --> |
| 18 |
|
| 19 |
<guide lang="en"> |
| 20 |
<title>Using Integrity Measurement Architecture in Gentoo</title> |
| 21 |
|
| 22 |
<author title="Author"> |
| 23 |
<mail link="swift"/> |
| 24 |
</author> |
| 25 |
|
| 26 |
<abstract> |
| 27 |
The Linux IMA subsystem is responsible for calculating the hashes |
| 28 |
of files and programs before they are loaded, and supports reporting |
| 29 |
on the hashes and validate if they adhere to a predefined list. In this |
| 30 |
guide, we introduce this technology and how it can be enabled in Gentoo |
| 31 |
Linux. |
| 32 |
</abstract> |
| 33 |
|
| 34 |
<!-- The content of this document is licensed under the CC-BY-SA license --> |
| 35 |
<!-- See http://creativecommons.org/licenses/by-sa/3.0 --> |
| 36 |
<license version="3.0" /> |
| 37 |
|
| 38 |
<version>1</version> |
| 39 |
<date>2012-10-01</date> |
| 40 |
|
| 41 |
<chapter> |
| 42 |
<title>Purpose of IMA</title> |
| 43 |
<section> |
| 44 |
<title>Introduction</title> |
| 45 |
<body> |
| 46 |
|
| 47 |
<p> |
| 48 |
The Linux IMA subsystem introduces hooks within the Linux kernel to |
| 49 |
support measuring the integrity of files that are loaded (including |
| 50 |
application code) before it is executed or mmap()ed to memory. The |
| 51 |
measured value (hash) is then registered in a log that can be consulted |
| 52 |
by administrators. |
| 53 |
</p> |
| 54 |
|
| 55 |
<p> |
| 56 |
To support proven integrity of the files, the IMA subsystem can interact |
| 57 |
with the TPM chip within the system to protect the registered hashes from |
| 58 |
tampering by a rogue administrator or application. |
| 59 |
The IMA subsystem, as already supported by the Linux kernel, supports |
| 60 |
<e>reporting</e> on the hashes of files and commands ran by privileged |
| 61 |
accounts (and more if you create your own measurement policies). |
| 62 |
</p> |
| 63 |
|
| 64 |
<p> |
| 65 |
With a pending patch, called the <e>IMA appraisal patch</e>, |
| 66 |
the IMA subsystem can even register the measured |
| 67 |
value as an extended attribute, and after subsequent measurement(s) |
| 68 |
validate this extended attribute against the measured value and refuse |
| 69 |
to load the file (or execute the application) if the hash does not match. |
| 70 |
In that case, the IMA subsystem allows files and applications to be loaded |
| 71 |
if the hashes match (and will save the updated hash if the file is modified) |
| 72 |
but refuse to load it if it doesn't. This provides some protection against |
| 73 |
offline tampering of the files. |
| 74 |
</p> |
| 75 |
|
| 76 |
<p> |
| 77 |
Protection of the extended attribute itself is handled by EVM, which is |
| 78 |
discussed in a separate document. |
| 79 |
</p> |
| 80 |
|
| 81 |
</body> |
| 82 |
</section> |
| 83 |
<section> |
| 84 |
<title>Trusted Computing Base</title> |
| 85 |
<body> |
| 86 |
|
| 87 |
<p> |
| 88 |
The Trusted Computing Base defines a set of rules that a properly, |
| 89 |
integrity-protected system should adhere to. The Linux IMA subsystem supports |
| 90 |
this set of rules through the <e>ima_tcb</e> policy. This policy roughly states |
| 91 |
that all files read by root (applications, but also regular files, libraries, |
| 92 |
...) need to be measured and registered against the security log, as well as |
| 93 |
application binaries when ran (or mmap()ed for execution) by any user. |
| 94 |
</p> |
| 95 |
|
| 96 |
</body> |
| 97 |
</section> |
| 98 |
</chapter> |
| 99 |
|
| 100 |
<chapter> |
| 101 |
<title>Setting up IMA</title> |
| 102 |
<section> |
| 103 |
<title>Kernel configuration</title> |
| 104 |
<body> |
| 105 |
|
| 106 |
<p> |
| 107 |
First of all, enable the IMA subsystem in the Linux kernel configuration. |
| 108 |
IMA is supported in the main tree since 2.6.30, and it is expected that |
| 109 |
the IMA appraisal patch (needed when you want the system to stop if it |
| 110 |
detects an off-line modified file) will hit the main tree with Linux |
| 111 |
3.7. |
| 112 |
</p> |
| 113 |
|
| 114 |
<pre caption="Linux kernel configuration for IMA"> |
| 115 |
CONFIG_INTEGRITY=y |
| 116 |
CONFIG_IMA=y |
| 117 |
CONFIG_IMA_MEASURE_PCR_IDX=10 |
| 118 |
CONFIG_IMA_AUDIT=y |
| 119 |
CONFIG_IMA_LSM_RULES=y |
| 120 |
|
| 121 |
<comment># Only if the IMA appraisal patch is available:</comment> |
| 122 |
CONFIG_INTEGRITY_SIGNATURE=y |
| 123 |
CONFIG_IMA_APPRAISE=y |
| 124 |
</pre> |
| 125 |
|
| 126 |
</body> |
| 127 |
</section> |
| 128 |
<section> |
| 129 |
<title>Bootloader configuration</title> |
| 130 |
<body> |
| 131 |
|
| 132 |
<p> |
| 133 |
Next, configure the bootloader to enable the TCB policy: |
| 134 |
</p> |
| 135 |
|
| 136 |
<pre caption="Bootloader configuration to enable IMA TCB policy"> |
| 137 |
kernel /boot/vmlinuz root=/dev/vda1 <i>ima_tcb</i> |
| 138 |
|
| 139 |
<comment># Only if IMA appraisal patch is enabled:</comment> |
| 140 |
kernel /boot/vmlinuz root=/dev/vda1 <i>ima_tcb ima_appraise=enforce ima_appraise_tcb</i> |
| 141 |
</pre> |
| 142 |
|
| 143 |
<p> |
| 144 |
However, at the first boot, you will need to set <c>ima_appraise=fix</c> |
| 145 |
instead. Otherwise, your system will surely refuse to boot as no preregistered |
| 146 |
values are available. |
| 147 |
</p> |
| 148 |
|
| 149 |
</body> |
| 150 |
</section> |
| 151 |
<section> |
| 152 |
<title>Enable security file system</title> |
| 153 |
<body> |
| 154 |
|
| 155 |
<p> |
| 156 |
Finally, have the security file system mounted (if this is not already the |
| 157 |
case): |
| 158 |
</p> |
| 159 |
|
| 160 |
<pre caption="Mounting the security file system"> |
| 161 |
~# <i>mount | grep securityfs</i> |
| 162 |
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime) |
| 163 |
</pre> |
| 164 |
|
| 165 |
</body> |
| 166 |
</section> |
| 167 |
<section> |
| 168 |
<title>Enable i_version mount option</title> |
| 169 |
<body> |
| 170 |
|
| 171 |
<p> |
| 172 |
Mount all file systems with the <e>i_version</e> support (which, sadly, means |
| 173 |
you need to mount it with <c>iversion</c> mount option - without the |
| 174 |
underscore). This is needed for IMA to detect changes on files, allowing it to |
| 175 |
remeasure the hash of the file. |
| 176 |
</p> |
| 177 |
|
| 178 |
<pre caption="Enabling i_version on all mounts"> |
| 179 |
<comment># Example for a single partition, in /etc/fstab:</comment> |
| 180 |
/dev/vda1 / ext4 noatime<i>,iversion</i> 1 2 |
| 181 |
</pre> |
| 182 |
|
| 183 |
</body> |
| 184 |
</section> |
| 185 |
</chapter> |
| 186 |
|
| 187 |
<chapter> |
| 188 |
<title>Using the IMA subsystem</title> |
| 189 |
<section> |
| 190 |
<title>Reading the integrity log</title> |
| 191 |
<body> |
| 192 |
|
| 193 |
<p> |
| 194 |
To read the integrity log as registered by the IMA subsystem, look at the |
| 195 |
<path>/sys/kernel/security/ima/ascii_runtime_measurements</path> file: |
| 196 |
</p> |
| 197 |
|
| 198 |
<pre caption="Reading the ascii_runtime_measurements file"> |
| 199 |
~# <i>head /sys/kernel/security/ima/ascii_runtime_measurements</i> |
| 200 |
10 0000000000000000000000000000000000000000 ima 0000000000000000000000000000000000000000 boot_aggregate |
| 201 |
10 dc99efa590c706a43792618dde88c590a6942ec7 ima fe932380326d7c51d17bac45f5d1c9f576d19f6c /sbin/init |
| 202 |
10 fcaa7505fae70096cb9b6a8ec06ec6400b756aa2 ima 0ddd922ae7f5a6dcf788438db1fe47e9a0641e6d ld-2.15.so |
| 203 |
10 501975777299919e49aac14c262d6388eae38e79 ima 8d848950517879e0dd77dc9602cad294b454b05a ld.so.cache |
| 204 |
10 195830b88844db79ff994c57022e94da416c486c ima 28c4c3a750f5679b9092b2bb2f98c5f745e422f7 libselinux.so.1 |
| 205 |
10 770cd9400624a5678da388545df1297e182ccd10 ima 03db374e3cedeaf987db096a034bccb5c5bcf3d0 libc-2.15.so |
| 206 |
10 82d48ec5fc4344a18a9d17ec1bf1bd8511f99fe6 ima e801e50a5f3ce7acc6e39b1133bce04120c46c35 libpcre.so.1.0.1 |
| 207 |
10 81ee4b0bbf4f5b464135e3e3d79b2777bceaa236 ima 869231d2fe1afe45ab284adc0efe5a237509bc7f libdl-2.15.so |
| 208 |
10 67f5923749dfa266721ee0d6ad038102297c1170 ima e5f8003967fd31f295a115e1d682dd0169b34592 config |
| 209 |
10 24894f13a9def8dd2f18838f04fde4becc184fc3 ima 032663452ea268aa1528bd466dda3738bb59a8f2 libsepol.so.1 |
| 210 |
|
| 211 |
# <i>tail /sys/kernel/security/ima/ascii_runtime_measurements</i> |
| 212 |
10 d326b31af052ccb96366e4eceaf34f72450dad4b ima def5be24075b501b43ad864bd7141e9db1c55611 /bin/getfattr |
| 213 |
10 3059f6b0f617256e42617c9e09ad13f6a1eb8f24 ima 7bb939b0ef153dce06098fd5460e31e6052af2a8 libattr.so.1.1.0 |
| 214 |
10 8c4a121f6e06255e5a8cf3cb94f526412273d661 ima 66abbade137338df340767b0f54a5da6b132e4c6 /usr/bin/touch |
| 215 |
10 a8cf6c5215be6e6a2de12f50f09b8dbb12af04c8 ima f0cf26d5095d4b19134d8aaf71763bc8f53cd6a9 /usr/bin/zgrep |
| 216 |
10 60fd90cb2d9581feb241a165f2f83e03abe46c24 ima 8c72350a7d0774c25f8b36b49327d874dec19ea5 test.te |
| 217 |
10 e359faf89a81e105d3d470d6a513b3ee201376de ima 6b4aa6f895bcdd34ace49879f9a25c1c12197317 /usr/bin/tail |
| 218 |
10 4a91a89db743704b7afe599528156823b48f192b ima 34228b5c748b13b6d51168c88b0ddf10467a5979 mtab |
| 219 |
10 5c4c4ffec7487c7fb07a9946238efc3c2957075e ima 41ec2f49dd9c025bcf2810f17bf69de80d45bec1 user_u |
| 220 |
10 0000000000000000000000000000000000000000 ima 0000000000000000000000000000000000000000 utmp |
| 221 |
10 6582662a747e7bcf051a6b83367c69c854d5d414 ima 38ccbf9330666831e0b4d9dafa2d5364a3449e1d /usr/bin/du |
| 222 |
</pre> |
| 223 |
|
| 224 |
<p> |
| 225 |
What you see here are: |
| 226 |
</p> |
| 227 |
|
| 228 |
<ul> |
| 229 |
<li> |
| 230 |
The PCR (Process Control Register) in which the values are registered. This |
| 231 |
only makes sense if you have a TPM chip in use. |
| 232 |
</li> |
| 233 |
<li> |
| 234 |
The hash of the hash of the file + filename (padded with null characters to |
| 235 |
the length of 255 bytes) |
| 236 |
</li> |
| 237 |
<li> |
| 238 |
The subsystem that registered the integrity value (ima in our case) |
| 239 |
</li> |
| 240 |
<li> |
| 241 |
The hash of the file content |
| 242 |
</li> |
| 243 |
</ul> |
| 244 |
|
| 245 |
<p> |
| 246 |
Unless you have asked for a different hashing algorithm, the default one will be |
| 247 |
used which is the SHA-1 hashing algorithm. |
| 248 |
</p> |
| 249 |
|
| 250 |
</body> |
| 251 |
</section> |
| 252 |
<section> |
| 253 |
<title>Registering the file hashes for the system</title> |
| 254 |
<body> |
| 255 |
|
| 256 |
<note> |
| 257 |
This is only applicable when IMA appraisal patch is enabled. |
| 258 |
</note> |
| 259 |
|
| 260 |
<p> |
| 261 |
First boot with the <c>ima_appraise=fix</c> boot option. This will allow the |
| 262 |
system to boot up even when no (or wrong) hashes are registered. |
| 263 |
</p> |
| 264 |
|
| 265 |
<p> |
| 266 |
Next, have all files on the file system checked and their value stored as an |
| 267 |
extended attribute. This is done by reading all files that the default appraisal |
| 268 |
policy will check and take action on (which is all root-owned files). |
| 269 |
</p> |
| 270 |
|
| 271 |
<pre caption="Registering all files"> |
| 272 |
~# <i>find / \( -fstype rootfs -o ext4 -type f \) -uid 0 -exec head -n 1 '{}' > /dev/null \;</i> |
| 273 |
</pre> |
| 274 |
|
| 275 |
<p> |
| 276 |
When done, you should be able to see the registered hash value as an extended |
| 277 |
attribute: |
| 278 |
</p> |
| 279 |
|
| 280 |
<pre caption="Viewing the extended attributes of a file"> |
| 281 |
~# <i>getfattr -m . -d /boot/grub/grub.conf</i> |
| 282 |
# file: grub.conf |
| 283 |
security.selinux="root:object_r:boot_t" |
| 284 |
security.ima="76a0d787be14d84dfe3cf3930ac3da38bb389464" |
| 285 |
</pre> |
| 286 |
|
| 287 |
<p> |
| 288 |
Finally, reboot with <c>ima_appraise=enforce</c>. The system should now run with |
| 289 |
appraisal enabled, causing the system to validate the hash against the stored |
| 290 |
value before using it. If it doesn't match, then the file is not loaded. If it |
| 291 |
does match, and afterwards the file is modified, then the new hash is stored as |
| 292 |
extended attribute. |
| 293 |
</p> |
| 294 |
|
| 295 |
<p> |
| 296 |
You can check if this works by booting with <c>ima_appraise=off</c> and changing |
| 297 |
the contents of a root-owned file (or the value of the extended attribute) and |
| 298 |
reboot with <c>ima_appraise=enforce</c>. |
| 299 |
</p> |
| 300 |
|
| 301 |
</body> |
| 302 |
</section> |
| 303 |
<section> |
| 304 |
<title>Using immutable files</title> |
| 305 |
<body> |
| 306 |
|
| 307 |
<p> |
| 308 |
The IMA appraisal code also supports immutable files. In this case, an RSA-key |
| 309 |
based signature is taken of the file and stored in the extended attribute. The |
| 310 |
private key is used to sign the files, whereas the public key is used to verify |
| 311 |
the signature. This provides additional protection against tampering as the |
| 312 |
private key does not need to be available on the system while its running (only |
| 313 |
during the initial marking). |
| 314 |
</p> |
| 315 |
|
| 316 |
<p> |
| 317 |
To sign such immutable files (like kernel modules and application code), you |
| 318 |
need to use the <c>evmctl</c> command provided by the |
| 319 |
<path>sys-admin/ima-evm-utils</path> package: |
| 320 |
</p> |
| 321 |
|
| 322 |
<pre caption="Signing files to mark them as immutable"> |
| 323 |
~# <i>find /lib/modules -name "*.ko" -type f -uid 0 -exec \ |
| 324 |
evmctl sign --imasig '{}' /path/to/rsa_private.pem \;</i> |
| 325 |
</pre> |
| 326 |
|
| 327 |
<p> |
| 328 |
This private key can be generated first using <c>openssl</c>: |
| 329 |
</p> |
| 330 |
|
| 331 |
<pre caption="Generating a public/private key pair"> |
| 332 |
<comment># Unencrypted private key (non-protected):</comment> |
| 333 |
~# <i>openssl genrsa -out rsa_private.pem 1024</i> |
| 334 |
|
| 335 |
<comment># Or encrypted private key (password-protected):</comment> |
| 336 |
~# <i>openssl genrsa -des3 -out rsa_private.pem 1024</i> |
| 337 |
|
| 338 |
<comment># Public key:</comment> |
| 339 |
~# <i>openssl rsa -pubout -in rsa_private.pem -out rsa_public.pem</i> |
| 340 |
</pre> |
| 341 |
|
| 342 |
<p> |
| 343 |
To allow the IMA subsystem to validate the signature, you will need to load the |
| 344 |
public key onto the IMA keyring. You will need to do this every time the system |
| 345 |
boots, so it makes sense to do so within an initramfs (early in the boot |
| 346 |
process): |
| 347 |
</p> |
| 348 |
|
| 349 |
<pre caption="Loading the public key in the IMA keyring"> |
| 350 |
~# <i>ima_id=`keyctl newring _ima @u`</i> |
| 351 |
~# <i>evmctl import /path/to/rsa_public.pem $ima_id</i> |
| 352 |
</pre> |
| 353 |
|
| 354 |
</body> |
| 355 |
</section> |
| 356 |
</chapter> |
| 357 |
|
| 358 |
<chapter> |
| 359 |
<title>Working with a TPM</title> |
| 360 |
<section> |
| 361 |
<title>Introduction</title> |
| 362 |
<body> |
| 363 |
|
| 364 |
<p> |
| 365 |
When your system supports TPM, the measured hash values can be registered |
| 366 |
against the TPM to provide protection against tampering. In that case, the list |
| 367 |
provided by <path>ascii_runtime_measurements</path> can be validated against the |
| 368 |
TPM chip. This is done by <e>extending</e> a register inside the TPM chip with |
| 369 |
the measured value. |
| 370 |
</p> |
| 371 |
|
| 372 |
<p> |
| 373 |
When a register is extended, its value (which is a hash) is appended with the |
| 374 |
value that it is extended with. Then, this new code is hashed again and stored |
| 375 |
in the register. Because of this operation, the entire measured list can be |
| 376 |
replayed by a third party and the final result validated against the value in |
| 377 |
the TPM chip. If it matches, then the list surely comes from this system, and |
| 378 |
hasn't been tampered by a rogue administrator or application. |
| 379 |
</p> |
| 380 |
|
| 381 |
</body> |
| 382 |
</section> |
| 383 |
<section> |
| 384 |
<title>Taking ownership</title> |
| 385 |
<body> |
| 386 |
|
| 387 |
<p> |
| 388 |
Before you can use the TPM on your device, you need to take ownership. This will |
| 389 |
allow you to set a specific password (to manage the TPM) and is needed for the |
| 390 |
TPM to generate specific keys it'll use later. |
| 391 |
</p> |
| 392 |
|
| 393 |
<p> |
| 394 |
To take ownership, first install the <path>app-crypt/tpm-tools</path> package. |
| 395 |
Then start the <e>tcsd</e> service and execute <c>tpm_takeownership -u -z</c>. |
| 396 |
</p> |
| 397 |
|
| 398 |
<pre caption="Taking ownership of the TPM"> |
| 399 |
~# <i>emerge app-crypt/tpm-tools</i> |
| 400 |
~# <i>rc-service tcsd start</i> |
| 401 |
~# <i>tpm_takeownership -u -z</i> |
| 402 |
</pre> |
| 403 |
|
| 404 |
</body> |
| 405 |
</section> |
| 406 |
<section> |
| 407 |
<title>Validating the measured list</title> |
| 408 |
<body> |
| 409 |
|
| 410 |
<p> |
| 411 |
$$$TODO |
| 412 |
</p> |
| 413 |
|
| 414 |
</body> |
| 415 |
</section> |
| 416 |
</chapter> |
| 417 |
|
| 418 |
</guide> |
Archives