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Simon Glass3e569a62013-06-13 15:10:00 -07001U-Boot FIT Signature Verification
2=================================
3
4Introduction
5------------
6FIT supports hashing of images so that these hashes can be checked on
7loading. This protects against corruption of the image. However it does not
8prevent the substitution of one image for another.
9
10The signature feature allows the hash to be signed with a private key such
11that it can be verified using a public key later. Provided that the private
12key is kept secret and the public key is stored in a non-volatile place,
13any image can be verified in this way.
14
15See verified-boot.txt for more general information on verified boot.
16
17
18Concepts
19--------
20Some familiarity with public key cryptography is assumed in this section.
21
22The procedure for signing is as follows:
23
24 - hash an image in the FIT
25 - sign the hash with a private key to produce a signature
26 - store the resulting signature in the FIT
27
28The procedure for verification is:
29
30 - read the FIT
31 - obtain the public key
32 - extract the signature from the FIT
33 - hash the image from the FIT
34 - verify (with the public key) that the extracted signature matches the
35 hash
36
37The signing is generally performed by mkimage, as part of making a firmware
38image for the device. The verification is normally done in U-Boot on the
39device.
40
41
42Algorithms
43----------
44In principle any suitable algorithm can be used to sign and verify a hash.
45At present only one class of algorithms is supported: SHA1 hashing with RSA.
46This works by hashing the image to produce a 20-byte hash.
47
48While it is acceptable to bring in large cryptographic libraries such as
49openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
50For the run-time verification side, it is important to keep code and data
51size as small as possible.
52
53For this reason the RSA image verification uses pre-processed public keys
54which can be used with a very small amount of code - just some extraction
55of data from the FDT and exponentiation mod n. Code size impact is a little
56under 5KB on Tegra Seaboard, for example.
57
58It is relatively straightforward to add new algorithms if required. If
59another RSA variant is needed, then it can be added to the table in
60image-sig.c. If another algorithm is needed (such as DSA) then it can be
61placed alongside rsa.c, and its functions added to the table in image-sig.c
62also.
63
64
Andreas Dannenberg4c1d5c22016-03-23 18:24:10 -050065Creating an RSA key pair and certificate
66----------------------------------------
67To create a new public/private key pair, size 2048 bits:
Simon Glass3e569a62013-06-13 15:10:00 -070068
Michael van der Westhuizene0f2f152014-07-02 10:17:26 +020069$ openssl genpkey -algorithm RSA -out keys/dev.key \
70 -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
Simon Glass3e569a62013-06-13 15:10:00 -070071
Andreas Dannenberg4c1d5c22016-03-23 18:24:10 -050072To create a certificate for this containing the public key:
Simon Glass3e569a62013-06-13 15:10:00 -070073
74$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
75
76If you like you can look at the public key also:
77
78$ openssl rsa -in keys/dev.key -pubout
79
80
81Device Tree Bindings
82--------------------
83The following properties are required in the FIT's signature node(s) to
84allow thes signer to operate. These should be added to the .its file.
85Signature nodes sit at the same level as hash nodes and are called
86signature@1, signature@2, etc.
87
88- algo: Algorithm name (e.g. "sha1,rs2048")
89
90- key-name-hint: Name of key to use for signing. The keys will normally be in
91a single directory (parameter -k to mkimage). For a given key <name>, its
92private key is stored in <name>.key and the certificate is stored in
93<name>.crt.
94
95When the image is signed, the following properties are added (mandatory):
96
97- value: The signature data (e.g. 256 bytes for 2048-bit RSA)
98
99When the image is signed, the following properties are optional:
100
101- timestamp: Time when image was signed (standard Unix time_t format)
102
103- signer-name: Name of the signer (e.g. "mkimage")
104
105- signer-version: Version string of the signer (e.g. "2013.01")
106
107- comment: Additional information about the signer or image
108
Simon Glass4d098522013-06-13 15:10:09 -0700109For config bindings (see Signed Configurations below), the following
110additional properties are optional:
Simon Glass3e569a62013-06-13 15:10:00 -0700111
Simon Glass4d098522013-06-13 15:10:09 -0700112- sign-images: A list of images to sign, each being a property of the conf
113node that contains then. The default is "kernel,fdt" which means that these
114two images will be looked up in the config and signed if present.
115
116For config bindings, these properties are added by the signer:
117
118- hashed-nodes: A list of nodes which were hashed by the signer. Each is
119 a string - the full path to node. A typical value might be:
120
121 hashed-nodes = "/", "/configurations/conf@1", "/images/kernel@1",
122 "/images/kernel@1/hash@1", "/images/fdt@1",
123 "/images/fdt@1/hash@1";
124
125- hashed-strings: The start and size of the string region of the FIT that
126 was hashed
127
128Example: See sign-images.its for an example image tree source file and
129sign-configs.its for config signing.
Simon Glass3e569a62013-06-13 15:10:00 -0700130
131
132Public Key Storage
133------------------
134In order to verify an image that has been signed with a public key we need to
135have a trusted public key. This cannot be stored in the signed image, since
136it would be easy to alter. For this implementation we choose to store the
137public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
138
139Public keys should be stored as sub-nodes in a /signature node. Required
140properties are:
141
142- algo: Algorithm name (e.g. "sha1,rs2048")
143
144Optional properties are:
145
146- key-name-hint: Name of key used for signing. This is only a hint since it
147is possible for the name to be changed. Verification can proceed by checking
148all available signing keys until one matches.
149
150- required: If present this indicates that the key must be verified for the
151image / configuration to be considered valid. Only required keys are
152normally verified by the FIT image booting algorithm. Valid values are
153"image" to force verification of all images, and "conf" to force verfication
154of the selected configuration (which then relies on hashes in the images to
155verify those).
156
157Each signing algorithm has its own additional properties.
158
159For RSA the following are mandatory:
160
161- rsa,num-bits: Number of key bits (e.g. 2048)
162- rsa,modulus: Modulus (N) as a big-endian multi-word integer
Michael van der Westhuizene0f2f152014-07-02 10:17:26 +0200163- rsa,exponent: Public exponent (E) as a 64 bit unsigned integer
Simon Glass3e569a62013-06-13 15:10:00 -0700164- rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
165- rsa,n0-inverse: -1 / modulus[0] mod 2^32
166
167
Simon Glass4d098522013-06-13 15:10:09 -0700168Signed Configurations
169---------------------
170While signing images is useful, it does not provide complete protection
171against several types of attack. For example, it it possible to create a
172FIT with the same signed images, but with the configuration changed such
173that a different one is selected (mix and match attack). It is also possible
174to substitute a signed image from an older FIT version into a newer FIT
175(roll-back attack).
176
177As an example, consider this FIT:
178
179/ {
180 images {
181 kernel@1 {
182 data = <data for kernel1>
183 signature@1 {
184 algo = "sha1,rsa2048";
185 value = <...kernel signature 1...>
186 };
187 };
188 kernel@2 {
189 data = <data for kernel2>
190 signature@1 {
191 algo = "sha1,rsa2048";
192 value = <...kernel signature 2...>
193 };
194 };
195 fdt@1 {
196 data = <data for fdt1>;
197 signature@1 {
198 algo = "sha1,rsa2048";
199 vaue = <...fdt signature 1...>
200 };
201 };
202 fdt@2 {
203 data = <data for fdt2>;
204 signature@1 {
205 algo = "sha1,rsa2048";
206 vaue = <...fdt signature 2...>
207 };
208 };
209 };
210 configurations {
211 default = "conf@1";
212 conf@1 {
213 kernel = "kernel@1";
214 fdt = "fdt@1";
215 };
216 conf@1 {
217 kernel = "kernel@2";
218 fdt = "fdt@2";
219 };
220 };
221};
222
223Since both kernels are signed it is easy for an attacker to add a new
224configuration 3 with kernel 1 and fdt 2:
225
226 configurations {
227 default = "conf@1";
228 conf@1 {
229 kernel = "kernel@1";
230 fdt = "fdt@1";
231 };
232 conf@1 {
233 kernel = "kernel@2";
234 fdt = "fdt@2";
235 };
236 conf@3 {
237 kernel = "kernel@1";
238 fdt = "fdt@2";
239 };
240 };
241
242With signed images, nothing protects against this. Whether it gains an
243advantage for the attacker is debatable, but it is not secure.
244
245To solved this problem, we support signed configurations. In this case it
246is the configurations that are signed, not the image. Each image has its
247own hash, and we include the hash in the configuration signature.
248
249So the above example is adjusted to look like this:
250
251/ {
252 images {
253 kernel@1 {
254 data = <data for kernel1>
255 hash@1 {
256 algo = "sha1";
257 value = <...kernel hash 1...>
258 };
259 };
260 kernel@2 {
261 data = <data for kernel2>
262 hash@1 {
263 algo = "sha1";
264 value = <...kernel hash 2...>
265 };
266 };
267 fdt@1 {
268 data = <data for fdt1>;
269 hash@1 {
270 algo = "sha1";
271 value = <...fdt hash 1...>
272 };
273 };
274 fdt@2 {
275 data = <data for fdt2>;
276 hash@1 {
277 algo = "sha1";
278 value = <...fdt hash 2...>
279 };
280 };
281 };
282 configurations {
283 default = "conf@1";
284 conf@1 {
285 kernel = "kernel@1";
286 fdt = "fdt@1";
287 signature@1 {
288 algo = "sha1,rsa2048";
289 value = <...conf 1 signature...>;
290 };
291 };
292 conf@2 {
293 kernel = "kernel@2";
294 fdt = "fdt@2";
295 signature@1 {
296 algo = "sha1,rsa2048";
297 value = <...conf 1 signature...>;
298 };
299 };
300 };
301};
302
303
304You can see that we have added hashes for all images (since they are no
305longer signed), and a signature to each configuration. In the above example,
306mkimage will sign configurations/conf@1, the kernel and fdt that are
307pointed to by the configuration (/images/kernel@1, /images/kernel@1/hash@1,
308/images/fdt@1, /images/fdt@1/hash@1) and the root structure of the image
309(so that it isn't possible to add or remove root nodes). The signature is
310written into /configurations/conf@1/signature@1/value. It can easily be
311verified later even if the FIT has been signed with other keys in the
312meantime.
313
314
Simon Glass3e569a62013-06-13 15:10:00 -0700315Verification
316------------
317FITs are verified when loaded. After the configuration is selected a list
318of required images is produced. If there are 'required' public keys, then
319each image must be verified against those keys. This means that every image
320that might be used by the target needs to be signed with 'required' keys.
321
322This happens automatically as part of a bootm command when FITs are used.
323
324
325Enabling FIT Verification
326-------------------------
327In addition to the options to enable FIT itself, the following CONFIGs must
328be enabled:
329
330CONFIG_FIT_SIGNATURE - enable signing and verfication in FITs
331CONFIG_RSA - enable RSA algorithm for signing
332
Heiko Schocher21d29f72014-05-28 11:33:33 +0200333WARNING: When relying on signed FIT images with required signature check
334the legacy image format is default disabled by not defining
335CONFIG_IMAGE_FORMAT_LEGACY
Simon Glass3e569a62013-06-13 15:10:00 -0700336
337Testing
338-------
339An easy way to test signing and verfication is to use the test script
340provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
341of U-Boot which runs under Linux) to show the operation of a 'bootm'
342command loading and verifying images.
343
344A sample run is show below:
345
346$ make O=sandbox sandbox_config
347$ make O=sandbox
348$ O=sandbox ./test/vboot/vboot_test.sh
349Simple Verified Boot Test
350=========================
351
352Please see doc/uImage.FIT/verified-boot.txt for more information
353
Heiko Schocher646257d2014-03-03 12:19:26 +0100354/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
Simon Glass3e569a62013-06-13 15:10:00 -0700355Build keys
Heiko Schocher646257d2014-03-03 12:19:26 +0100356do sha1 test
Simon Glass3e569a62013-06-13 15:10:00 -0700357Build FIT with signed images
358Test Verified Boot Run: unsigned signatures:: OK
359Sign images
360Test Verified Boot Run: signed images: OK
361Build FIT with signed configuration
362Test Verified Boot Run: unsigned config: OK
363Sign images
364Test Verified Boot Run: signed config: OK
Heiko Schocher29a23f92014-03-03 12:19:30 +0100365check signed config on the host
Simon Glassce1400f2014-06-12 07:24:53 -0600366Signature check OK
Heiko Schocher29a23f92014-03-03 12:19:30 +0100367OK
368Test Verified Boot Run: signed config: OK
Heiko Schocher646257d2014-03-03 12:19:26 +0100369Test Verified Boot Run: signed config with bad hash: OK
370do sha256 test
371Build FIT with signed images
372Test Verified Boot Run: unsigned signatures:: OK
373Sign images
374Test Verified Boot Run: signed images: OK
375Build FIT with signed configuration
376Test Verified Boot Run: unsigned config: OK
377Sign images
378Test Verified Boot Run: signed config: OK
Heiko Schocher29a23f92014-03-03 12:19:30 +0100379check signed config on the host
Simon Glassce1400f2014-06-12 07:24:53 -0600380Signature check OK
Heiko Schocher29a23f92014-03-03 12:19:30 +0100381OK
382Test Verified Boot Run: signed config: OK
Heiko Schocher646257d2014-03-03 12:19:26 +0100383Test Verified Boot Run: signed config with bad hash: OK
Simon Glass3e569a62013-06-13 15:10:00 -0700384
385Test passed
386
Simon Glassce1400f2014-06-12 07:24:53 -0600387
Simon Glass3e569a62013-06-13 15:10:00 -0700388Future Work
389-----------
390- Roll-back protection using a TPM is done using the tpm command. This can
391be scripted, but we might consider a default way of doing this, built into
392bootm.
393
394
395Possible Future Work
396--------------------
397- Add support for other RSA/SHA variants, such as rsa4096,sha512.
398- Other algorithms besides RSA
399- More sandbox tests for failure modes
400- Passwords for keys/certificates
401- Perhaps implement OAEP
402- Enhance bootm to permit scripted signature verification (so that a script
403can verify an image but not actually boot it)
404
405
406Simon Glass
407sjg@chromium.org
4081-1-13