macOS Potpisivanje koda

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Osnovne informacije

Mach O Entitlements And Ipsw Indexing

Mach-o binarni fajlovi sadrže load command nazvan LC_CODE_SIGNATURE koji označava offset i size potpisa unutar binarnog fajla. Zapravo, koristeći GUI alat MachOView, moguće je pronaći na kraju binarnog fajla sekciju nazvanu Code Signature sa ovim informacijama:

Magijski header Code Signature-a je 0xFADE0CC0. Zatim sadrži informacije kao što su length i broj blobs superBlob-a koji ih sadrži.
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)));

Uobičajeni blobovi koji se nalaze su Code Directory, Requirements i Entitlements i Cryptographic Message Syntax (CMS).
Obratite pažnju da su podaci u blobovima enkodirani u Big Endian.

Takođe, potpisi mogu biti odvojeni od binarnih fajlova i smešteni u /var/db/DetachedSignatures (koristi se na iOS).

Code Directory Blob

Moguće je pronaći deklaraciju Code Directory Blob u kodu:

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)));

Imajte na umu da postoje različite verzije ovog struct-a, pri čemu starije mogu sadržati manje informacija.

Potpisivanje stranica koda

Heširanje celokupnog binarnog fajla bilo bi neefikasno, pa čak i besmisleno ako se u memoriji učitava samo delimično. Zbog toga je potpis koda zapravo heš heševa gde se svaka binarna stranica hešira pojedinačno.
U stvari, u prethodnom Code Directory kodu možete videti da je page size is specified u jednom od njegovih polja. Štaviše, ako veličina binarnog fajla nije višekratnik veličine stranice, polje CodeLimit određuje gde je kraj potpisa.

# 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.*

Entitlements Blob

Napomena: aplikacije takođe mogu sadržati entitlement blob gde su definisana sva entitlements. Pored toga, neki iOS binarni fajlovi mogu imati entitlements u specijalnom slotu -7 (umesto u -5 entitlements special slot).

Special Slots

MacOS applications nemaju sve što im je potrebno za izvršavanje unutar samog binarnog fajla, već koriste i external resources (obično unutar aplikacionog bundle). Zbog toga postoje specijalni slotovi unutar binarnog fajla koji sadrže hešove nekih važnih eksternih resursa kako bi se proverilo da li su izmenjeni.

U Code Directory strukturama postoji parametar nSpecialSlots koji označava broj specijalnih slotova. Ne postoji specijalni slot 0, a najčešći su (od -1 do -6):

• Hash of info.plist (or the one inside __TEXT.__info__plist).
• Hash of the Requirements
• Hash of the Resource Directory (hash of _CodeSignature/CodeResources file inside the bundle).
• Application specific (unused)
• Hash of the entitlements
• DMG code signatures only
• DER Entitlements

Code Signing Flags

Svaki proces ima pridružen bitmasku poznatu kao status koju postavlja kernel, a neki od tih bitova mogu biti prepisani od strane code signature. Ovi flagovi koji se mogu uključiti u code signing su definisani u kodu:

/* 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)

Note that the function exec_mach_imgact can also add the CS_EXEC_* flags dynamically when starting the execution.

Zahtevi potpisa koda

Svaka aplikacija čuva neke zahteve koje mora ispuniti da bi mogla da se izvrši. Ako zahtevi koje aplikacija sadrži nisu ispunjeni, ona se neće izvršiti (verovatno je izmenjena).

Zahtevi binarnog fajla koriste posebnu gramatiku koja je niz izraza i enkodirani su kao blobovi koristeći 0xfade0c00 kao magic, čiji se hash čuva u posebnom code slot-u.

Zahtevi binarnog fajla mogu se videti pokretanjem:

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

Pored toga, moguće je generisati neke kompajlirane zahteve koristeći alat 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
[...]

Moguće je pristupiti ovim informacijama i kreirati ili izmeniti zahteve pomoću nekih API-ja iz Security.framework, kao što su:

Provera validnosti

• Sec[Static]CodeCheckValidity: Proverava validnost SecCodeRef u odnosu na Requirement.
• SecRequirementEvaluate: Validira requirement u kontekstu sertifikata.
• SecTaskValidateForRequirement: Validira pokrenuti SecTask u odnosu na CFString requirement.

Kreiranje i upravljanje zahtevima koda

• SecRequirementCreateWithData: Kreira SecRequirementRef iz binarnih podataka koji predstavljaju zahtev.
• SecRequirementCreateWithString: Kreira SecRequirementRef iz string izraza zahteva.
• SecRequirementCopy[Data/String]: Vraća binarne podatke SecRequirementRef.
• SecRequirementCreateGroup: Kreira zahtev za članstvo u app-group.

Pristup informacijama o potpisivanju koda

• SecStaticCodeCreateWithPath: Inicijalizuje SecStaticCodeRef objekat iz putanje u fajl-sistemu radi inspekcije potpisa koda.
• SecCodeCopySigningInformation: Dobija informacije o potpisivanju iz SecCodeRef ili SecStaticCodeRef.

Izmena zahteva koda

• SecCodeSignerCreate: Kreira SecCodeSignerRef objekat za izvođenje operacija potpisivanja koda.
• SecCodeSignerSetRequirement: Postavlja novi zahtev koji će potpisivač koda primeniti tokom potpisivanja.
• SecCodeSignerAddSignature: Dodaje potpis kodu koji se potpisuje koristeći navedeni potpisivač.

Validacija koda prema zahtevima

• SecStaticCodeCheckValidity: Validira statički objekat koda prema specificiranim zahtevima.

Dodatni korisni API-ji

• SecCodeCopy[Internal/Designated]Requirement: Get SecRequirementRef from SecCodeRef
• SecCodeCopyGuestWithAttributes: Kreira SecCodeRef koji predstavlja objekat koda baziran na određenim atributima, koristan za sandboxing.
• SecCodeCopyPath: Vraća putanju u fajl-sistemu povezanu sa SecCodeRef.
• SecCodeCopySigningIdentifier: Dobija signing identifier (npr. Team ID) iz SecCodeRef.
• SecCodeGetTypeID: Vraća tip identifikator za SecCodeRef objekte.
• SecRequirementGetTypeID: Dobija CFTypeID za SecRequirementRef.

Zastavice i konstante za potpisivanje koda

• kSecCSDefaultFlags: Podrazumevane zastavice korišćene u mnogim funkcijama iz Security.framework za operacije potpisivanja koda.
• kSecCSSigningInformation: Zastavica koja označava da treba dohvatiti informacije o potpisivanju.

Sprovodjenje provere potpisa koda

Jezgro (kernel) je odgovorno za proveru potpisa koda pre nego što dozvoli izvršenje koda aplikacije. Pored toga, jedan način da se može upisivati i izvršavati novi kod u memoriji je zloupotreba JIT-a ako se mprotect pozove sa MAP_JIT zastavicom. Napomena: aplikaciji je potreban poseban entitlement da bi ovo mogla da uradi.

cs_blobs & cs_blob

cs_blob struct sadrži informacije o entitlement-u pokrenutog procesa na njemu. csb_platform_binary takođe informiše da li je aplikacija platform binary (što se proverava u različitim trenucima od strane OS-a kako bi se primenile bezbednosne mere, npr. zaštita SEND prava na task portovima ovih procesa).

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
};

Reference

• *OS Internals Volume III