when I said some things about Secure Boot in June of 2014. I …">

The Uncoöperative Organization

Programming and other human stuff.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

The UEFI Security Databases

A little background

When we talk about UEFI Secure Boot, a lot of times we talk about what we call the whitelist and the blacklist. These databases go by many names—formally EFI_IMAGE_SECURITY_DATABASE and EFI_IMAGE_SECURITY_DATABASE1, sometimes d719b2cb-3d3a-4596-a3bc-dad00e67656f:db and d719b2cb-3d3a-4596-a3bc-dad00e67656f:dbx, but most often just db and dbx. These two1 databases, stored in UEFI Authenticated Variables2, constitute lists of which binaries can and cannot be executed within a UEFI environment when Secure Boot is enabled.

When a UEFI binary is loaded, the system first checks to see if any revocations in dbx are applicable to that binary. A dbx entry may apply in three ways—it may contain the hash of a specific binary, an X.509 certificate, or the hash of a certificate3. If any of these matches the binary in question, UEFI raises the error EFI_SECURITY_VIOLATION, and your binary will not go to space today.

If a binary successfully passes that hurdle, then the same verification method is processed with db, the whitelist, but with the opposite policy: if any entry describes the binary in question, verification has succeeded. If no entry does, EFI_SECURITY_VIOLATION is raised.

The need for updates

When a UEFI binary is discovered to have a serious security flaw which would allow a malicious user to circumvent Secure Boot, it becomes necessary to prevent it from running on machines. When this happens, the UEFI CA issues a dbx update. The mechanism for the update is a file that’s structured as a UEFI Authenticated Variable append. This file gets distributed as part of an OS update, and when the update is applied, the UEFI variable is updated.

One mechanism for doing this in Linux is via dbxtool. In Fedora, the dbxtool package includes the current dbx updates in /usr/share/dbxtool/DBXUpdate-2014-04-13-22-14-00.bin, as well as a systemd service to apply them during boot4. In the special case of Fedora’s shim being added to a blacklist, we would include a dependency on the fixed version of shim in the dbxtool package so that systems would remain bootable.

The structure of a `dbx` update

The dbx variable is composed of an array of EFI_SIGNATURE_LIST structures, each themselves containing an array of EFI_SIGNATURE_DATA entries. A dbx update is that same structure, but wrapped in a EFI_VARIABLE_AUTHENTICATION_2 structure to authenticate it. The definitions look like this5:

wincert.h
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typedef struct {
        efi_guid_t      SignatureOwner;         // who owns this entry
        uint8_t         SignatureData[0];       // the data we want to
                                                // fish out of this thing
} EFI_SIGNATURE_DATA;

typedef struct {
        efi_guid_t      SignatureType;       // type of structure in
                                             // EFI_SIGNATURE_DATA.SignatureData
        uint32_t        SignatureListSize;   // Total size of the signature
                                             // list, including this header.
        uint32_t        SignatureHeaderSize; // Size of type-specific header
        uint32_t        SignatureSize;       // The size of each individual
                                             // EFI_SIGNATURE_DATA.SignatureData
                                             // in this list.
        // uint8_t      SignatureHeader[SignatureHeaderSize]
                                             // this is a header defined by
                                             // and for each specific
                                             // signature type.  Of course
                                             // none of them actually define
                                             // a header.
        // EFI_SIGNATURE_DATA[...][SignatureSize] // actual signature data
} EFI_SIGNATURE_LIST;

typedef struct {
        efi_guid_t        HashType;
        uint8_t           PublicKey[256];
        uint8_t           Signature[256];
} EFI_CERT_BLOCK_RSA_2048_SHA256;

typedef struct {
        uint32_t        dwLength;         // Length of this structure
        uint16_t        wRevision;        // Revision of this structure (2)
        uint16_t        wCertificateType; // The kind of signature this is
        //uint16_t      bCertificate[0];  // The signature data itself. This
                                          // is actually, and not the least
                                          // bit confusingly, the rest of
                                          // the WIN_CERTIFICATE_EFI_GUID
                                          // structure wrapping this one.
} WIN_CERTIFICATE;

#define WIN_CERT_TYPE_PKCS_SIGNED_DATA  0x0002
#define WIN_CERT_TYPE_EFI_PKCS115       0x0ef0
#define WIN_CERT_TYPE_EFI_GUID          0x0ef1

typedef struct {
        WIN_CERTIFICATE   Hdr;         // Info about which structure this is
        efi_guid_t        CertType;    // Type of certificate in CertData
        uint8_t           CertData[0]; // A certificate of some kind
} WIN_CERTIFICATE_EFI_GUID;

typedef struct {
        EFI_TIME                 TimeStamp; // monotonically increasing
                                            // timestamp to prevent replay
                                            // attacks.
        WIN_CERTIFICATE_EFI_GUID AuthInfo;  // Information about how to
                                            // authenticate this variable
                                            // against some KEK entry
} EFI_VARIABLE_AUTHENTICATION_2;

Conceptually, this means the structure we’ve got is:

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[ dbx update file:
  [ Authentication structure:
    [ monotonic number | timestamp ]
    [ WIN_CERTIFICATE Header ]
    [ Cert Type ]
    [ Certificate Data ] ]
  [ EFI_SIGNATURE_LIST:
    [ EFI_SIGNATURE_DATA ]
    [ EFI_SIGNATURE_DATA ]
    [ EFI_SIGNATURE_DATA ] ]
  [ EFI_SIGNATURE_LIST:
    [ EFI_SIGNATURE_DATA ]
    ...
    [ EFI_SIGNATURE_DATA ] ]
  ... ]

So a full update looks something like this:

auth2.TimeStamp
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00000000  da 07 03 06 13 11 15 00  00 00 00 00 00 00 00 00  |................|

2010-03-06 19:17:21 GMT+0000

auth2.AuthInfo.Hdr.DwLength
1
00000010  bd 0c 00 00                                       |....            |

It is 0x00000cbd bytes long6.

auth2.AuthInfo.Hdr.wRevision
1
00000010              00 02                                 |    ..          |

It’s revision is 2. It is always revision 2.

auth2.AuthInfo.Hdr.wCertificateType
1
00000010                    f1 0e                           |      ..        |

0x0ef1, which as we see above is WIN_CERT_TYPE_EFI_GUID. The interesting bit here is that .bCertificate isn’t quite a real thing. This is actually describing that auth2.AuthInfo is a WIN_CERTIFICATE_EFI_GUID, and .bCertificate is actually the fields other than auth2.AuthInfo.Hdrauth2.AuthInfo.CertType and auth2.AuthInfo.CertData. Again, this shouldn’t confuse you at all. As a result, next, in the place of .bCertificate, we have:

auth2.AuthInfo.CertType
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00000010                           9d d2 af 4a df 68 ee 49  |        ...J.h.I|
00000020  8a a9 34 7d 37 56 65 a7                           |..4}7Ve.        |

4aafd29d-68df-49ee-8aa9-347d375665a7, aka EFI_GUID_PKCS7_CERT, which means that .CertData is verified against EFI_SIGNATURE_DATA entries in the Key Exchange Keys database (KEK), but only those entries which are contained in EFI_SIGNATURE_LIST structures with .SignatureType of EFI_CERT_X509_GUID. Whew.

auth2.AuthInfo.CertData
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00000020                           30 82 0c a1 02 01 01 31  |        0......1|
00000030  0f 30 0d 06 09 60 86 48  01 65 03 04 02 01 05 00  |.0...`.H.e......|
[ what an X.509 certificate looks like is left as an exercise for the reader ]
00000cb0  b6 2b 89 02 73 c4 86 57  83 6f 28 57 e0 12 cb 05  |.+..s..W.o(W....|
00000cc0  6d d0 3e 60 8f 85 9f dd  fc 46 ac 54 44           |m.>`.....F.TD   |

Yep, that’s an ASN.1 DER encoding of a PKCS-7 Certificate.
That’s the end of the EFI_VARIABLE_AUTHENTICATION_2 structure, and so on to the actual data:

EFI_SIGNATURE_LIST.SignatureType
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00000cc0                                          26 16 c4  |             &..|
00000cd0  c1 4c 50 92 40 ac a9 41  f9 36 93 43 28           |.LP.@..A.6.C(   |

That’s c1c41626-504c-4092-aca9-41f936934328, aka EFI_GUID_SHA256

EFI_SIGNATURE_LIST.SignatureListSize
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00000cd0                                          cc 01 00  |             ...|
00000ce0  00                                                |.               |

The list size is 0x00001cc bytes

EFI_SIGNATURE_LIST.SignatureHeaderSize:
1
00000ce0     00 00 00 00                                    | ....           |

This is actually always 0.

EFI_SIGNATURE_LIST.SignatureSize
1
00000ce0                 30 00 00  00                       |     0...       |

0x00000030 bytes. That’s 16 bytes for the efi_guid_t, and since .SignatureType was EFI_GUID_SHA256, 32 bytes of SHA-256 data.

EFI_SIGNATURE_DATA[0].SignatureOwner
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00000ce0                              bd 9a fa 77 59 03 32  |.....0......wY.2|
00000cf0  4d bd 60 28 f4 e7 8f 78  4b                       |M.`(...xK       |

77fa9abd-0359-4d32-bd60-28f4e78f784b , which is the GUID Microsoft uses to identify themselves7

EFI_SIGNATURE_DATA[0].SignatureData
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00000cf0                              80 b4 d9 69 31 bf 0d  |         ...i1..|
00000d00  02 fd 91 a6 1e 19 d1 4f  1d a4 52 e6 6d b2 40 8c  |.......O..R.m.@.|
00000d10  a8 60 4d 41 1f 92 65 9f  0a                       |.`MA..e..       |

This is the actual SHA-256 data. It goes on like this:

EFI_SIGNATURE_DATA[1..8]
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00000d10                              bd 9a fa 77 59 03 32  |         ...wY.2|
00000d20  4d bd 60 28 f4 e7 8f 78  4b f5 2f 83 a3 fa 9c fb  |M.`(...xK./.....|
00000d30  d6 92 0f 72 28 24 db e4  03 45 34 d2 5b 85 07 24  |...r($...E4.[..$|
00000d40  6b 3b 95 7d ac 6e 1b ce  7a bd 9a fa 77 59 03 32  |k;.}.n..z...wY.2|
00000d50  4d bd 60 28 f4 e7 8f 78  4b c5 d9 d8 a1 86 e2 c8  |M.`(...xK.......|
00000d60  2d 09 af aa 2a 6f 7f 2e  73 87 0d 3e 64 f7 2c 4e  |-...*o..s..>d.,N|
00000d70  08 ef 67 79 6a 84 0f 0f  bd                       |..gyj....       |
...
00000e60                              bd 9a fa 77 59 03 32  |         ...wY.2|
00000e70  4d bd 60 28 f4 e7 8f 78  4b 53 91 c3 a2 fb 11 21  |M.`(...xKS.....!|
00000e80  02 a6 aa 1e dc 25 ae 77  e1 9f 5d 6f 09 cd 09 ee  |.....%.w..]o....|
00000e90  b2 50 99 22 bf cd 59 92  ea                       |.P."..Y..|

How the structure is used

When you apply a database update, you’re basically doing a SetVariable() call to UEFI with a couple of flags set:

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  rc = efi_set_variable(EFI_IMAGE_SECURITY_DATABASE_GUID, "dbx", data, len,
                        EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS |
                        EFI_VARIABLE_APPEND_WRITE |
                        EFI_VARIABLE_BOOTSERVICE_ACCESS |
                        EFI_VARIABLE_RUNTIME_ACCESS |
                        EFI_VARIABLE_NON_VOLATILE);

These flags tell the firmware some crucial things – that this variable is authenticated with the EFI_VARIABLE_AUTHENTICATION_2 structure, that this is an append, that both Boot Services (i.e. the firmware) and Runtime Services (i.e. the OS) should have access to it, and that it should persist across a reboot. As a special case in the spec, an append has a special meaning for the UEFI security databases:

For variables with the GUID EFI_IMAGE_SECURITY_DATABASE_GUID (i.e. where the data buffer is formatted as EFI_SIGNATURE_LIST), the driver shallnot perform an append of EFI_SIGNATURE_DATA values that are already part of the existing variable value.
Note: This situation is not considered an error, and shall in itself not cause a status code other than EFI_SUCCESS to be returned or the timestamp associated with the variable not to be updated.

UEFI Specification section 7.2.1 Revision 2.4

As a result, what happens here is that the first time you write to dbx, any EFI_SIGNATURE_LIST structures and the EFI_SIGNATURE_DATA entries they contain get added to the variable, but not the EFI_VARIABLE_AUTHENTICATION_2 structure. Then when later dbx updates are issued, they contain a superset of the previous ones. When you apply them, the firmware only appends the difference to the variable.

Tools

Obviously a system this complex needs some tools. To manage these databases on linux, I’ve written a tool called dbxtool, which may be available in your linux distribution of choice. It can be used to apply dbx changes8, to list the contents of the UEFI Security Databases, and to list the contents of updates files:

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fenchurch:~$ dbxtool -l
   1: {microsoft} {sha256} 80b4d96931bf0d02fd91a61e19d14f1da452e66db2408ca8604d411f92659f0a
   2: {microsoft} {sha256} f52f83a3fa9cfbd6920f722824dbe4034534d25b8507246b3b957dac6e1bce7a
   3: {microsoft} {sha256} c5d9d8a186e2c82d09afaa2a6f7f2e73870d3e64f72c4e08ef67796a840f0fbd
   4: {microsoft} {sha256} 363384d14d1f2e0b7815626484c459ad57a318ef4396266048d058c5a19bbf76
   5: {microsoft} {sha256} 1aec84b84b6c65a51220a9be7181965230210d62d6d33c48999c6b295a2b0a06
   6: {microsoft} {sha256} e6ca68e94146629af03f69c2f86e6bef62f930b37c6fbcc878b78df98c0334e5
   7: {microsoft} {sha256} c3a99a460da464a057c3586d83cef5f4ae08b7103979ed8932742df0ed530c66
   8: {microsoft} {sha256} 58fb941aef95a25943b3fb5f2510a0df3fe44c58c95e0ab80487297568ab9771
   9: {microsoft} {sha256} 5391c3a2fb112102a6aa1edc25ae77e19f5d6f09cd09eeb2509922bfcd5992ea
  10: {microsoft} {sha256} d626157e1d6a718bc124ab8da27cbb65072ca03a7b6b257dbdcbbd60f65ef3d1
  11: {microsoft} {sha256} d063ec28f67eba53f1642dbf7dff33c6a32add869f6013fe162e2c32f1cbe56d
  12: {microsoft} {sha256} 29c6eb52b43c3aa18b2cd8ed6ea8607cef3cfae1bafe1165755cf2e614844a44
  13: {microsoft} {sha256} 90fbe70e69d633408d3e170c6832dbb2d209e0272527dfb63d49d29572a6f44c

As usual, there are a couple of places where vendors have not gotten everything quite right, and sometimes things fail to work correctly—but that’s for another post.


  1. There is actually a third in this set, dbt, which can be used in revocation processing.

  2. Strictly speaking these are only an analog to Authenticated Variables, which can only be appended to, replaced, or deleted by updates signed with the key that created them. Key database updates are instead controlled by a list of keys stored in another variable called KEK – the Key Exchange Keys. Otherwise the mechanism is the same.

  3. That is, the digest of the certificate’s TBSCertificate as defined in RFC 5280 section 4.1.1.1, using the digest specified in the database entry itself.

  4. This is currently disabled by default in Fedora. I’m looking at enabling this as an F22 feature. Getting these things right is important, and it takes time.

  5. I have left out the definitions of EFI_TIME and efi_guid_t; they are quite boring.

  6. Here dwLength includes the size of Hdr itself (that is, the size of dwLength, wRevision, and wCertificateType) as well as the data following it (bCertificate). Because we live in the best of all possible worlds, Authenticode signatures—the signatures on binaries themselves—use the same structure but only include the size of Hdr.bCertificate. This hasn’t ever confused anybody.

  7. Big congratulations to Acer, who used 55555555-5555-5555-5555-555555555555 for this in one of their db entries. Not only did they win the random number generator lottery to get that, but also experienced a minimum of 3 single bit errors, since the closest valid GUID to that is 55555555-5555-4555-9555-555555555555.

  8. Also theoretically db updates, dbt updates, and KEK updates, but those are much more rare.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

You probably remember when I said some things about Secure Boot in June of 2014. I said there’d be more along those lines, and there is.

So there’s another statement about that here.

This message fixes a small grammar error from the previous one and adds a section about CentOS and Secure Boot.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.

Secure Boot — Fedora, RHEL, and Shim Upstream Maintenance: Government Involvement or Lack Thereof

After the big kerfuffle with TrueCrypt, it seems clear to me that I need to make some statements about Secure Boot and any interaction with governments whose regimes I fall under.

So there’s a statement about that here.

I’m going to try to remember to post a message like this once per month or so. If I miss one, keep an eye out, but maybe don’t get terribly suspicious unless I miss several in a row.

Note that there are parts of this chain I’m not a part of, and obviously linux distributions I’m not involved in that support Secure Boot. I encourage other maintainers to offer similar statements for their respective involvement.