Success Case
acer-wmi.ko (from kernel-default-3.12.28-4.6.x86_64.rpm):STEP 1. Find out the "Subject Key Identifier"
linux-aiip:/lib/modules/3.12.28-4-default # modinfo acer-wmi filename: /lib/modules/3.12.28-4-default/kernel/drivers/platform/x86/acer-wmi.ko [...snip] signer: SUSE Linux Enterprise Secure Boot Signkey sig_key: 3F:B0:77:B6:CE:BC:6F:F2:52:2E:1C:14:8C:57:C7:77:C7:88:E3:E7 sig_hashalgo: sha256
The sig_key match with "Subject Key Identifier" in SUSE Linux Enterprise Secure Boot
Signkey, so the public key can decrypt the signature that it is attached on acer-wmi.ko.
STEP 2. Extract the signature from acer-wmi.ko
Signature is attached behind the key name and Subject Key Identifier number.It's a 256 bytes block because hashed by SHA256. In this case, the Subject Key
Identifier is 3F B0 77 B6...E3 E7, finding out the position from acer-wmi.ko:
0000eb10 00 00 00 00 00 00 00 00 53 55 53 45 20 4c 69 6e |........SUSE Lin| 0000eb20 75 78 20 45 6e 74 65 72 70 72 69 73 65 20 53 65 |ux Enterprise Se| 0000eb30 63 75 72 65 20 42 6f 6f 74 20 53 69 67 6e 6b 65 |cure Boot Signke| 0000eb40 79 3f b0 77 b6 ce bc 6f f2 52 2e 1c 14 8c 57 c7 |y?.w...o.R....W.| <=== 3F B0 77 B6... 0000eb50 77 c7 88 e3 e7 01 00 9b eb 31 d2 cd f7 3a 65 92 |w........1...:e.| <=== ...E3 E7 01 00 signature /* 60240 *//* 60240 + 7 = 60247 */ 0000eb60 30 ee 2e d4 97 d2 7b 15 a0 e0 08 1f db 2d a7 9e |0.....{......-..| 0000eb70 7f 0a 5f 25 ed 04 6e 95 2d 98 85 cc 98 5a 4f 08 |.._%..n.-....ZO.|
The signature is always leading by "01 00" head, we don't need the head, so
the start of signature is 60240 + 7 = 60247. Use dd to extract signature to
another file:
dd skip=60247 count=256 bs=1 if=./acer-wmi.ko of=./acer-wmi.ko.sig
> hexdump -C acer-wmi.ko.sig
00000000 9b eb 31 d2 cd f7 3a 65 92 30 ee 2e d4 97 d2 7b |..1...:e.0.....{|
00000010 15 a0 e0 08 1f db 2d a7 9e 7f 0a 5f 25 ed 04 6e |......-...._%..n|
00000020 95 2d 98 85 cc 98 5a 4f 08 b5 c6 b5 1b 2f 87 52 |.-....ZO...../.R|
00000030 00 28 bb f0 6b bd f1 60 8e 58 be 18 0e 30 e7 dd |.(..k..`.X...0..|
00000040 87 91 ce 1f 84 71 f8 83 f3 ba f7 07 68 c4 35 6d |.....q......h.5m|
00000050 c4 3d 87 e7 ff 4c b2 20 ae b9 65 52 0f 56 38 38 |.=...L. ..eR.V88|
STEP 3. Verify signature by the public key in SLES certificate
First, extract public key from certificate file:openssl x509 -in SLES-UEFI-SIGN-Certificate.der -inform DER -pubkey -noout > SLES-UEFI-SIGN-Certificate.pub
Then, decrypting signature by public key. If the signature didn't encrypt by right
key, then this step will be fail:
openssl rsautl -verify -inkey SLES-UEFI-SIGN-Certificate.pub -pubin -in acer-wmi.ko.sig > acer-wmi.ko.sha
The acer-wmi.ko.sha file is the decrypted signature. It's leaded by ASN.1 format
and a SHA256 hash result:
> hexdump -C acer-wmi.ko.sha 00000000 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 |010...`.H.e.....| <== ASN1. format head 00000010 00 04 20 ae 4a 31 b2 46 2b 1d e6 01 26 aa 38 2e |.. .J1.F+...&.8.| <=== should match with the result of modhash 00000020 9d 3d ab 08 78 1a c2 85 b3 2f 87 96 3e 7f 15 7a |.=..x..../..>..z| 00000030 31 b7 8c |1..| 00000033
The attached perl script, modhash, is developed by Gary Lin for calculating
signature from a signed ko file. Comparing the result from modhash and the above decrypted
hash value to confirm the hash value matched:
> perl modhash -v acer-wmi.ko
Hash algorithm: sha256
acer-wmi.ko: ae4a31b2462b1de60126aa382e9d3dab08781ac285b32f87963e7f157a31b78c
The same procedure, following is a failed case.
Failed Case
sample.ko:
It's also claimed signed by SUSE Linux Enterprise Secure Boot Signkey:
linux-aiip:~ # modinfo sample
filename: /lib/modules/3.12.28-4-default/updates/sample.ko
description: xxxxxxxxxxxxxx
[...snip]
signer: SUSE Linux Enterprise Secure Boot Signkey
sig_key: 3F:B0:77:B6:CE:BC:6F:F2:52:2E:1C:14:8C:57:C7:77:C7:88:E3:E7
sig_hashalgo: sha256
Found the Subject Key Identifier in .ko file, looks OK:
00061790 70 61 67 65 73 5f 63 75 72 72 65 6e 74 00 53 55 |pages_current.SU| 000617a0 53 45 20 4c 69 6e 75 78 20 45 6e 74 65 72 70 72 |SE Linux Enterpr| 000617b0 69 73 65 20 53 65 63 75 72 65 20 42 6f 6f 74 20 |ise Secure Boot | 000617c0 53 69 67 6e 6b 65 79 3f b0 77 b6 ce bc 6f f2 52 |Signkey?.w...o.R| <=== 3F B0 77... 000617d0 2e 1c 14 8c 57 c7 77 c7 88 e3 e7 01 00 c6 12 1d |....W.w.........| <=== ...88 E3 E7 01 00 signature /* 399312 */ /* 399325 */ 000617e0 ba 45 3a b3 b1 99 fb 55 1b fc d3 90 6a ea 92 64 |.E:....U....j..d|
Then extract signature:
dd skip=399325 count=256 bs=1 if=./sample.ko of=./sample.ko.sig > hexdump -C sample.ko.sig 00000000 c6 12 1d ba 45 3a b3 b1 99 fb 55 1b fc d3 90 6a |....E:....U....j| 00000010 ea 92 64 8a 04 04 f9 22 a7 74 35 98 05 d7 e6 85 |..d....".t5.....| 00000020 8c 5f 32 e6 6c 71 f7 ba 1c 0a 0f 8a 95 f3 ec c7 |._2.lq..........| 00000030 88 b2 11 71 27 28 ca b8 b8 55 ae df 56 38 c6 b4 |...q'(...U..V8..|
Unfortunately the signature can not be decrypted by public key success:
openssl rsautl -verify -inkey SLES-UEFI-SIGN-Certificate.pub -pubin -in sample.ko.sig > sample.ko.sha RSA operation error 139755852273296:error:0407006A:rsa routines:RSA_padding_check_PKCS1_type_1:block type is not 01:rsa_pk1.c:100: 139755852273296:error:04067072:rsa routines:RSA_EAY_PUBLIC_DECRYPT:padding check failed:rsa_eay.c:721:
Got problem when decrypting the blob by public key. So, looks signature didn't encrypted by appropriate key.
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