MFCI Structures¶

Policy¶

MFCI provides 64 bits of configuration policy, 32 bits to be defined by Microsoft, 32 bits reserved for OEM definition. Each 32-bit range is split into two 16-bit ranges, one for state that persists across reboots for the duration a policy is installed, and one for 1-time actions that are triggered once at policy installation. Example actions include Microsoft-defined Secure Boot clear and TPM clear. An example state might be "OEM Manufacturing Mode foo", which enables modification of secure configuration.

Structure Definition

Policy Blob¶

An MFCI policy blob is a structure that contains the aforementioned 64-bit configuration policy, along with device targeting information including OEM, model, serial number, 2 OEM-defined fields, & a security nonce.

To prevent tampering and provide authentication & authorization, MFCI policy blobs are digitally signed using the attached, embedded PKCS7 format (not the detached format as is used elsewhere in UEFI). Authentication of MFCI blobs leverages both a trust anchor public certificate that must be present in the blob's signing chain, and an Enhanced Key Usage (EKU) that must be present on the leaf certificate signer. By default, MfciPkg trusts a provided test certificate chain when SHIP_MODE==FALSE, otherwise the certificate and EKU specified by the Microsoft MFCI cloud signing service.

At runtime, a UEFI implementor may consider the policy blobs to be opaque, as MfciPkg's private parsing lib handles the task of authenticating, verifying targeting, and parsing the blobs down to the 64-bit policies.

For the purpose of testing, a python library and commandline wrappers are provided to facilitate creation of unsigned MFCI policies. After the policies are created, they will still need to be signed.

Targeting Information¶

Policies are targeted at specific devices via Make (OEM), Model, Serial Number, Nonce, and 2 optional, OEM-defined fields. The Nonce is a randomly-generated 64-bit integer, and the remaining fields are unescaped, WIDE NULL-terminated UTF-16LE strings.

Digital Signature¶

The PKCS7 digital signature format familiar to the UEFI ecosystem is used with 2 key differences. First, a full PKCS7 is used, not the SignedData subset used by authenticated variables. Second, the P7 is embedded, not detached. The policy targeting and flavor information are embedded in a PKCS 7 Data inside the full PKCS 7 Signed object.

Signing Key¶

To digitally sign a policy blob, a public/private key combination is required. The scripts used to generate test signing keys are included in this repo for reference. Please note that the EKU needs to include OID 1.3.6.1.5.5.7.3.3 as well as the user designed leaf EKU that is used as an additional test. This second EKU must be set in pcd PcdMfciPkcs7RequiredLeafEKU.

Example signing¶

Signtool is available in Windows Kits and can be used to sign the policy blob.

After the policy blob is created, sign tool can be called, as in the below example. The signed policy blob has a .p7 extension, and this is the file that should be place into the MfciNext variable.

    c:\\Program Files (x86)\\Windows Kits\\10\\bin\\10.0.17763.0\\x64\\signtool.exe
    sign 
    /fd SHA256 
    /p7 .
    /p7co 1.2.840.113549.1.7.1
    /p7ce Embedded
    /f <pfx file of leaf key>.pfx
    /v /debug 
    /p <password used to secure the leaf key pfx file>
    <input policy>.bin

Signed Packet Example¶

Output of running certutil.exe on a signed MFCI Policy packet (signed). certutil.exe -asn

0000: 30 82 0e 6c                               ; SEQUENCE (e6c Bytes)
0004:    06 09                                  ; OBJECT_ID (9 Bytes)
0006:    |  2a 86 48 86 f7 0d 01 07  02
         |     ; 1.2.840.113549.1.7.2 PKCS 7 Signed
000f:    a0 82 0e 5d                            ; OPTIONAL[0] (e5d Bytes)
0013:       30 82 0e 59                         ; SEQUENCE (e59 Bytes)
0017:          02 01                            ; INTEGER (1 Bytes)
0019:          |  01
001a:          31 0f                            ; SET (f Bytes)
001c:          |  30 0d                         ; SEQUENCE (d Bytes)
001e:          |     06 09                      ; OBJECT_ID (9 Bytes)
0020:          |     |  60 86 48 01 65 03 04 02  01
               |     |     ; 2.16.840.1.101.3.4.2.1 sha256 (sha256NoSign)
0029:          |     05 00                      ; NULL (0 Bytes)
002b:          30 82 02 17                      ; SEQUENCE (217 Bytes)
002f:          |  06 09                         ; OBJECT_ID (9 Bytes)
0031:          |  |  2a 86 48 86 f7 0d 01 07  01
               |  |     ; 1.2.840.113549.1.7.1 PKCS 7 Data
003a:          |  a0 82 02 08                   ; OPTIONAL[0] (208 Bytes)
003e:          |     04 82 02 04                ; OCTET_STRING (204 Bytes)

[policy payload starts here]

0042:          |        02 00 01 00 00 00 08 f8  e6 5a 84 83 b9 4e a2 3a  ; .........Z...N.:
0052:          |        0c cc 10 93 e3 dd 00 00  00 00 00 00 00 00 07 00  ; ................
0062:          |        00 00 10 ef 00 00 00 00  0e 00 00 00 28 00 00 00  ; ............(...
0072:          |        00 00 10 ef 58 00 00 00  66 00 00 00 76 00 00 00  ; ....X...f...v...
0082:          |        00 00 10 ef 8e 00 00 00  9c 00 00 00 b6 00 00 00  ; ................
0092:          |        00 00 10 ef e0 00 00 00  ee 00 00 00 fc 00 00 00  ; ................
00a2:          |        00 00 10 ef 0e 01 00 00  1c 01 00 00 2a 01 00 00  ; ............*...
00b2:          |        00 00 10 ef 2e 01 00 00  3c 01 00 00 48 01 00 00  ; ........<...H...
00c2:          |        00 00 10 ef 52 01 00 00  5c 01 00 00 6a 01 00 00  ; ....R...\...j...
00d2:          |        0c 00 54 00 61 00 72 00  67 00 65 00 74 00 18 00  ; ..T.a.r.g.e.t...
00e2:          |        4d 00 61 00 6e 00 75 00  66 00 61 00 63 00 74 00  ; M.a.n.u.f.a.c.t.
00f2:          |        75 00 72 00 65 00 72 00  00 00 2c 00 43 00 6f 00  ; u.r.e.r...,.C.o.
0102:          |        6e 00 74 00 6f 00 73 00  6f 00 20 00 43 00 6f 00  ; n.t.o.s.o. .C.o.
0112:          |        6d 00 70 00 75 00 74 00  65 00 72 00 73 00 2c 00  ; m.p.u.t.e.r.s.,.
0122:          |        20 00 4c 00 4c 00 43 00  0c 00 54 00 61 00 72 00  ;  .L.L.C...T.a.r.
0132:          |        67 00 65 00 74 00 0e 00  50 00 72 00 6f 00 64 00  ; g.e.t...P.r.o.d.
0142:          |        75 00 63 00 74 00 00 00  14 00 4c 00 61 00 70 00  ; u.c.t.....L.a.p.
0152:          |        74 00 6f 00 70 00 20 00  46 00 6f 00 6f 00 0c 00  ; t.o.p. .F.o.o...
0162:          |        54 00 61 00 72 00 67 00  65 00 74 00 18 00 53 00  ; T.a.r.g.e.t...S.
0172:          |        65 00 72 00 69 00 61 00  6c 00 4e 00 75 00 6d 00  ; e.r.i.a.l.N.u.m.
0182:          |        62 00 65 00 72 00 00 00  26 00 46 00 30 00 30 00  ; b.e.r...&.F.0.0.
0192:          |        31 00 33 00 2d 00 30 00  30 00 30 00 32 00 34 00  ; 1.3.-.0.0.0.2.4.
01a2:          |        33 00 35 00 34 00 36 00  2d 00 58 00 30 00 32 00  ; 3.5.4.6.-.X.0.2.
01b2:          |        0c 00 54 00 61 00 72 00  67 00 65 00 74 00 0c 00  ; ..T.a.r.g.e.t...
01c2:          |        4f 00 45 00 4d 00 5f 00  30 00 31 00 00 00 0e 00  ; O.E.M._.0.1.....
01d2:          |        4f 00 44 00 4d 00 20 00  46 00 6f 00 6f 00 0c 00  ; O.D.M. .F.o.o...
01e2:          |        54 00 61 00 72 00 67 00  65 00 74 00 0c 00 4f 00  ; T.a.r.g.e.t...O.
01f2:          |        45 00 4d 00 5f 00 30 00  32 00 00 00 00 00 0c 00  ; E.M._.0.2.......
0202:          |        54 00 61 00 72 00 67 00  65 00 74 00 0a 00 4e 00  ; T.a.r.g.e.t...N.
0212:          |        6f 00 6e 00 63 00 65 00  05 00 ef cd ab 89 67 45  ; o.n.c.e.......gE
0222:          |        23 01 08 00 55 00 45 00  46 00 49 00 0c 00 50 00  ; #...U.E.F.I...P.
0232:          |        6f 00 6c 00 69 00 63 00  79 00 05 00 03 00 00 00  ; o.l.i.c.y.......
0242:          |        00 00 00 00                                       ; ....

[policy payload ends here]

[digital signature continues but is not shown here]