macOS Code Signing

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Taarifa Msingi

Mach O Entitlements And Ipsw Indexing

Mach-o binaries zina load command inayoitwa LC_CODE_SIGNATURE ambayo inaonyesha offset na size ya signatures ndani ya binary. Kwa kutumia zana ya GUI MachOView, inawezekana kupata mwishoni mwa binary sehemu inayoitwa Code Signature yenye taarifa hizi:

The magic header of the Code Signature is 0xFADE0CC0. Then you have information such as the length and the number of blobs of the superBlob that contains them.
It's possible to find this information in the source code here:

/*
* Structure of an embedded-signature SuperBlob
*/

typedef struct __BlobIndex {
uint32_t type;                                  /* type of entry */
uint32_t offset;                                /* offset of entry */
} CS_BlobIndex
__attribute__ ((aligned(1)));

typedef struct __SC_SuperBlob {
uint32_t magic;                                 /* magic number */
uint32_t length;                                /* total length of SuperBlob */
uint32_t count;                                 /* number of index entries following */
CS_BlobIndex index[];                   /* (count) entries */
/* followed by Blobs in no particular order as indicated by offsets in index */
} CS_SuperBlob
__attribute__ ((aligned(1)));

#define KERNEL_HAVE_CS_GENERICBLOB 1
typedef struct __SC_GenericBlob {
uint32_t magic;                                 /* magic number */
uint32_t length;                                /* total length of blob */
char data[];
} CS_GenericBlob
__attribute__ ((aligned(1)));

Blob za kawaida zinazojumuishwa ni Code Directory, Requirements na Entitlements na Cryptographic Message Syntax (CMS).
Zaidi ya hayo, angalia jinsi data iliyoko ndani ya blobs imekodwa kwa Big Endian.

Zaidi ya hayo, saini zinaweza kuachwa (detached) kutoka kwa binaries na kuhifadhiwa katika /var/db/DetachedSignatures (inayotumika na iOS).

Code Directory Blob

Inawezekana kupata tamko la Code Directory Blob in the code:

typedef struct __CodeDirectory {
uint32_t magic;                                 /* magic number (CSMAGIC_CODEDIRECTORY) */
uint32_t length;                                /* total length of CodeDirectory blob */
uint32_t version;                               /* compatibility version */
uint32_t flags;                                 /* setup and mode flags */
uint32_t hashOffset;                    /* offset of hash slot element at index zero */
uint32_t identOffset;                   /* offset of identifier string */
uint32_t nSpecialSlots;                 /* number of special hash slots */
uint32_t nCodeSlots;                    /* number of ordinary (code) hash slots */
uint32_t codeLimit;                             /* limit to main image signature range */
uint8_t hashSize;                               /* size of each hash in bytes */
uint8_t hashType;                               /* type of hash (cdHashType* constants) */
uint8_t platform;                               /* platform identifier; zero if not platform binary */
uint8_t pageSize;                               /* log2(page size in bytes); 0 => infinite */
uint32_t spare2;                                /* unused (must be zero) */

char end_earliest[0];

/* Version 0x20100 */
uint32_t scatterOffset;                 /* offset of optional scatter vector */
char end_withScatter[0];

/* Version 0x20200 */
uint32_t teamOffset;                    /* offset of optional team identifier */
char end_withTeam[0];

/* Version 0x20300 */
uint32_t spare3;                                /* unused (must be zero) */
uint64_t codeLimit64;                   /* limit to main image signature range, 64 bits */
char end_withCodeLimit64[0];

/* Version 0x20400 */
uint64_t execSegBase;                   /* offset of executable segment */
uint64_t execSegLimit;                  /* limit of executable segment */
uint64_t execSegFlags;                  /* executable segment flags */
char end_withExecSeg[0];

/* Version 0x20500 */
uint32_t runtime;
uint32_t preEncryptOffset;
char end_withPreEncryptOffset[0];

/* Version 0x20600 */
uint8_t linkageHashType;
uint8_t linkageApplicationType;
uint16_t linkageApplicationSubType;
uint32_t linkageOffset;
uint32_t linkageSize;
char end_withLinkage[0];

/* followed by dynamic content as located by offset fields above */
} CS_CodeDirectory
__attribute__ ((aligned(1)));

Kumbuka kwamba kuna matoleo tofauti ya struct hii ambapo zile za zamani zinaweza kuwa zina taarifa chache.

Kusaini Kurasa za Code

Hashing the full binary ingekuwa isiyofaa na hata isiyo na maana ikiwa binary inapakiawa kwenye memory kwa sehemu tu. Therefore, the code signature is actually a hash of hashes where each binary page is hashed individually.
Kwa kweli, katika Code Directory code ya awali unaweza kuona kwamba page size is specified katika moja ya fields zake. Zaidi ya hayo, ikiwa ukubwa wa binary sio mara kamili ya ukubwa wa page, field CodeLimit inaonyesha ni wapi mwisho wa signature.

# Get all hashes of /bin/ps
codesign -d -vvvvvv /bin/ps
[...]
CandidateCDHash sha256=c46e56e9490d93fe35a76199bdb367b3463c91dc
CandidateCDHashFull sha256=c46e56e9490d93fe35a76199bdb367b3463c91dcdb3c46403ab8ba1c2d13fd86
Hash choices=sha256
CMSDigest=c46e56e9490d93fe35a76199bdb367b3463c91dcdb3c46403ab8ba1c2d13fd86
CMSDigestType=2
Executable Segment base=0
Executable Segment limit=32768
Executable Segment flags=0x1
Page size=4096
-7=a542b4dcbc134fbd950c230ed9ddb99a343262a2df8e0c847caee2b6d3b41cc8
-6=0000000000000000000000000000000000000000000000000000000000000000
-5=2bb2de519f43b8e116c7eeea8adc6811a276fb134c55c9c2e9dcbd3047f80c7d
-4=0000000000000000000000000000000000000000000000000000000000000000
-3=0000000000000000000000000000000000000000000000000000000000000000
-2=4ca453dc8908dc7f6e637d6159c8761124ae56d080a4a550ad050c27ead273b3
-1=0000000000000000000000000000000000000000000000000000000000000000
0=a5e6478f89812c0c09f123524cad560a9bf758d16014b586089ddc93f004e39c
1=ad7facb2586fc6e966c004d7d1d16b024f5805ff7cb47c7a85dabd8b48892ca7
2=93d476eeace15a5ad14c0fb56169fd080a04b99582b4c7a01e1afcbc58688f
[...]

# Calculate the hasehs of each page manually
BINARY=/bin/ps
SIZE=`stat -f "%Z" $BINARY`
PAGESIZE=4096 # From the previous output
PAGES=`expr $SIZE / $PAGESIZE`
for i in `seq 0 $PAGES`; do
dd if=$BINARY of=/tmp/`basename $BINARY`.page.$i bs=$PAGESIZE skip=$i count=1
done
openssl sha256 /tmp/*.page.*

Blob ya Entitlements

Kumbuka kwamba programu zinaweza pia kuwa na entitlement blob ambapo entitlements zote zimefafanuliwa. Zaidi ya hayo, baadhi ya binaries za iOS zinaweza kuwa na entitlements zao maalum katika special slot -7 (badala ya katika -5 entitlements special slot).

Slots Maalum

Programu za MacOS hazina kila kitu wanachohitaji ili zitekelezwe ndani ya binary; pia zinatumia external resources (kwa kawaida ndani ya programu bundle). Kwa hiyo, kuna baadhi ya slots ndani ya binary zitakazokuwa na hashes za baadhi ya external resources za kuvutia ili kuangalia hazijabadilishwa.

Kwa kweli, inawezekana kuona katika Code Directory structs paramita iitwayo nSpecialSlots inayoonyesha idadi ya slots maalum. Hakuna special slot 0 na za kawaida zaidi (kutoka -1 hadi -6) ni:

• Hash ya info.plist (au ile ndani ya __TEXT.__info__plist).
• Hash ya Requirements
• Hash ya Resource Directory (hash ya faili _CodeSignature/CodeResources ndani ya bundle).
• Maalum kwa programu (haijatumika)
• Hash ya entitlements
• DMG code signatures tu
• DER Entitlements

Bendera za Code Signing

Kila mchakato una bitmask inayojulikana kama status ambayo huanzishwa na kernel, na baadhi ya bendera hizo zinaweza kueuzwa na code signature. Bendera hizi ambazo zinaweza kujumuishwa katika code signing zimetajwa defined in the code:

/* code signing attributes of a process */
#define CS_VALID                    0x00000001  /* dynamically valid */
#define CS_ADHOC                    0x00000002  /* ad hoc signed */
#define CS_GET_TASK_ALLOW           0x00000004  /* has get-task-allow entitlement */
#define CS_INSTALLER                0x00000008  /* has installer entitlement */

#define CS_FORCED_LV                0x00000010  /* Library Validation required by Hardened System Policy */
#define CS_INVALID_ALLOWED          0x00000020  /* (macOS Only) Page invalidation allowed by task port policy */

#define CS_HARD                     0x00000100  /* don't load invalid pages */
#define CS_KILL                     0x00000200  /* kill process if it becomes invalid */
#define CS_CHECK_EXPIRATION         0x00000400  /* force expiration checking */
#define CS_RESTRICT                 0x00000800  /* tell dyld to treat restricted */

#define CS_ENFORCEMENT              0x00001000  /* require enforcement */
#define CS_REQUIRE_LV               0x00002000  /* require library validation */
#define CS_ENTITLEMENTS_VALIDATED   0x00004000  /* code signature permits restricted entitlements */
#define CS_NVRAM_UNRESTRICTED       0x00008000  /* has com.apple.rootless.restricted-nvram-variables.heritable entitlement */

#define CS_RUNTIME                  0x00010000  /* Apply hardened runtime policies */
#define CS_LINKER_SIGNED            0x00020000  /* Automatically signed by the linker */

#define CS_ALLOWED_MACHO            (CS_ADHOC | CS_HARD | CS_KILL | CS_CHECK_EXPIRATION | \
CS_RESTRICT | CS_ENFORCEMENT | CS_REQUIRE_LV | CS_RUNTIME | CS_LINKER_SIGNED)

#define CS_EXEC_SET_HARD            0x00100000  /* set CS_HARD on any exec'ed process */
#define CS_EXEC_SET_KILL            0x00200000  /* set CS_KILL on any exec'ed process */
#define CS_EXEC_SET_ENFORCEMENT     0x00400000  /* set CS_ENFORCEMENT on any exec'ed process */
#define CS_EXEC_INHERIT_SIP         0x00800000  /* set CS_INSTALLER on any exec'ed process */

#define CS_KILLED                   0x01000000  /* was killed by kernel for invalidity */
#define CS_NO_UNTRUSTED_HELPERS     0x02000000  /* kernel did not load a non-platform-binary dyld or Rosetta runtime */
#define CS_DYLD_PLATFORM            CS_NO_UNTRUSTED_HELPERS /* old name */
#define CS_PLATFORM_BINARY          0x04000000  /* this is a platform binary */
#define CS_PLATFORM_PATH            0x08000000  /* platform binary by the fact of path (osx only) */

#define CS_DEBUGGED                 0x10000000  /* process is currently or has previously been debugged and allowed to run with invalid pages */
#define CS_SIGNED                   0x20000000  /* process has a signature (may have gone invalid) */
#define CS_DEV_CODE                 0x40000000  /* code is dev signed, cannot be loaded into prod signed code (will go away with rdar://problem/28322552) */
#define CS_DATAVAULT_CONTROLLER     0x80000000  /* has Data Vault controller entitlement */

#define CS_ENTITLEMENT_FLAGS        (CS_GET_TASK_ALLOW | CS_INSTALLER | CS_DATAVAULT_CONTROLLER | CS_NVRAM_UNRESTRICTED)

Kumbuka kuwa kazi ya exec_mach_imgact inaweza pia kuongeza bendera CS_EXEC_* kwa wakati wa kuanza utekelezaji.

Mahitaji ya Saini ya Msimbo

Kila programu huhifadhi baadhi ya mahitaji ambayo lazima iyatimize ili iweze kutekelezwa. Ikiwa mahitaji yaliyomo kwenye programu hayajatimizwa, haitatekelezwa (inawezekana imebadilishwa).

Mahitaji ya binary hutumia sarufi maalum ambayo ni mfululizo wa mielezo na yamefichwa kama blobs zikitumia 0xfade0c00 kama magic ambapo hash yake imehifadhiwa kwenye slot maalum ya code.

Mahitaji ya binary yanaweza kuonekana kwa kuendesha:

codesign -d -r- /bin/ls
Executable=/bin/ls
designated => identifier "com.apple.ls" and anchor apple

codesign -d -r- /Applications/Signal.app/
Executable=/Applications/Signal.app/Contents/MacOS/Signal
designated => identifier "org.whispersystems.signal-desktop" and anchor apple generic and certificate 1[field.1.2.840.113635.100.6.2.6] /* exists */ and certificate leaf[field.1.2.840.113635.100.6.1.13] /* exists */ and certificate leaf[subject.OU] = U68MSDN6DR

Aidha, inawezekana kuunda baadhi ya mahitaji yaliyokompailiwa kwa kutumia chombo csreq:

# Generate compiled requirements
csreq -b /tmp/output.csreq -r='identifier "org.whispersystems.signal-desktop" and anchor apple generic and certificate 1[field.1.2.840.113635.100.6.2.6] /* exists */ and certificate leaf[field.1.2.840.113635.100.6.1.13] /* exists */ and certificate leaf[subject.OU] = U68MSDN6DR'

# Get the compiled bytes
od -A x -t x1 /tmp/output.csreq
0000000    fa  de  0c  00  00  00  00  b0  00  00  00  01  00  00  00  06
0000010    00  00  00  06  00  00  00  06  00  00  00  06  00  00  00  02
0000020    00  00  00  21  6f  72  67  2e  77  68  69  73  70  65  72  73
[...]

Inawezekana kupata taarifa hizi na kuunda au kubadilisha requirements kwa kutumia baadhi ya API kutoka Security.framework kama:

Kukagua Uhalali

• Sec[Static]CodeCheckValidity: Angalia uhalali wa SecCodeRef kulingana na Requirement.
• SecRequirementEvaluate: Thibitisha Requirement katika muktadha wa cheti.
• SecTaskValidateForRequirement: Thibitisha SecTask inayotumika dhidi ya Requirement ya CFString.

Kuunda na Kusimamia Code Requirements

• SecRequirementCreateWithData: Inatengeneza SecRequirementRef kutoka data ya binary inayowakilisha requirement.
• SecRequirementCreateWithString: Inatengeneza SecRequirementRef kutoka kwa mfuatano wa string wa requirement.
• SecRequirementCopy[Data/String]: Inapata uwakilishi wa data ya binary wa SecRequirementRef.
• SecRequirementCreateGroup: Unda requirement kwa uanachama wa app-group

Kupata Taarifa za Code Signing

• SecStaticCodeCreateWithPath: Huanzisha SecStaticCodeRef kutoka kwenye path ya file system kwa kuchunguza code signatures.
• SecCodeCopySigningInformation: Inapata taarifa za saini kutoka SecCodeRef au SecStaticCodeRef.

Kurekebisha Code Requirements

• SecCodeSignerCreate: Inatengeneza SecCodeSignerRef kwa ajili ya kufanya operesheni za code signing.
• SecCodeSignerSetRequirement: Inaweka requirement mpya kwa code signer atakayetekeleza wakati wa kusaini.
• SecCodeSignerAddSignature: Inaongeza signature kwa code inayosainiwa na signer uliotajwa.

Kukagua Msimbo kwa Requirements

• SecStaticCodeCheckValidity: Inathibitisha static code object dhidi ya requirements zilizotajwa.

API Nyingine Zinazofaa

• SecCodeCopy[Internal/Designated]Requirement: Get SecRequirementRef from SecCodeRef
• SecCodeCopyGuestWithAttributes: Inatengeneza SecCodeRef inayowakilisha kitu cha code kulingana na attributes maalum, inayofaa kwa sandboxing.
• SecCodeCopyPath: Inapata path ya file system inayohusiana na SecCodeRef.
• SecCodeCopySigningIdentifier: Inapata signing identifier (mf. Team ID) kutoka SecCodeRef.
• SecCodeGetTypeID: Inarudisha type identifier kwa vitu vya SecCodeRef.
• SecRequirementGetTypeID: Inapata CFTypeID ya SecRequirementRef

Code Signing Flags and Constants

• kSecCSDefaultFlags: Bendera za default zinazotumika katika kazi nyingi za Security.framework kwa operesheni za code signing.
• kSecCSSigningInformation: Bendera inayotumika kuonyesha kwamba taarifa za saini zinapaswa kupatikana.

Utekelezaji wa Code Signature

Kernel ndio inayokagua saini ya msimbo kabla ya kuruhusu msimbo wa app utekelezwe. Zaidi ya hayo, njia moja ya kuandika na kutekeleza msimbo mpya katika memory ni kutumia mabaya JIT ikiwa mprotect inaitwa na bendera ya MAP_JIT. Kumbuka kwamba application inahitaji entitlement maalum ili kufanya hivi.

cs_blobs & cs_blob

cs_blob struct ina taarifa kuhusu entitlement za mchakato unaoendesha juu yake. csb_platform_binary pia inaonyesha kama application ni platform binary (ambayo inakaguliwa kwa nyakati tofauti na OS ili kutekeleza taratibu za usalama kama kulinda haki za SEND kwa task ports za michakato hii).

struct cs_blob {
struct cs_blob  *csb_next;
vnode_t         csb_vnode;
void            *csb_ro_addr;
__xnu_struct_group(cs_cpu_info, csb_cpu_info, {
cpu_type_t      csb_cpu_type;
cpu_subtype_t   csb_cpu_subtype;
});
__xnu_struct_group(cs_signer_info, csb_signer_info, {
unsigned int    csb_flags;
unsigned int    csb_signer_type;
});
off_t           csb_base_offset;        /* Offset of Mach-O binary in fat binary */
off_t           csb_start_offset;       /* Blob coverage area start, from csb_base_offset */
off_t           csb_end_offset;         /* Blob coverage area end, from csb_base_offset */
vm_size_t       csb_mem_size;
vm_offset_t     csb_mem_offset;
void            *csb_mem_kaddr;
unsigned char   csb_cdhash[CS_CDHASH_LEN];
const struct cs_hash  *csb_hashtype;
#if CONFIG_SUPPLEMENTAL_SIGNATURES
unsigned char   csb_linkage[CS_CDHASH_LEN];
const struct cs_hash  *csb_linkage_hashtype;
#endif
int             csb_hash_pageshift;
int             csb_hash_firstlevel_pageshift;   /* First hash this many bytes, then hash the hashes together */
const CS_CodeDirectory *csb_cd;
const char      *csb_teamid;
#if CONFIG_SUPPLEMENTAL_SIGNATURES
char            *csb_supplement_teamid;
#endif
const CS_GenericBlob *csb_entitlements_blob;    /* raw blob, subrange of csb_mem_kaddr */
const CS_GenericBlob *csb_der_entitlements_blob;    /* raw blob, subrange of csb_mem_kaddr */

/*
* OSEntitlements pointer setup by AMFI. This is PAC signed in addition to the
* cs_blob being within RO-memory to prevent modifications on the temporary stack
* variable used to setup the blob.
*/
void *XNU_PTRAUTH_SIGNED_PTR("cs_blob.csb_entitlements") csb_entitlements;

unsigned int    csb_reconstituted;      /* signature has potentially been modified after validation */
__xnu_struct_group(cs_blob_platform_flags, csb_platform_flags, {
/* The following two will be replaced by the csb_signer_type. */
unsigned int    csb_platform_binary:1;
unsigned int    csb_platform_path:1;
});

/* Validation category used for TLE */
unsigned int    csb_validation_category;

#if CODE_SIGNING_MONITOR
void *XNU_PTRAUTH_SIGNED_PTR("cs_blob.csb_csm_obj") csb_csm_obj;
bool csb_csm_managed;
#endif
};

Marejeleo

• *OS Internals Volume III