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macOS Code Signing

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Basic Information

Mach-o binaries contains a load command called LC_CODE_SIGNATURE that indicates the offset and size of the signatures inside the binary. Actually, using the GUI tool MachOView, it's possible to find at the end of the binary a section called Code Signature with this information:

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)));

Common blobs contained are Code Directory, Requirements and Entitlements and a Cryptographic Message Syntax (CMS). Moreover, note how the data encoded in the blobs is encoded in Big Endian.

Moreover, signatures cloud be detached from the binaries and stored in /var/db/DetachedSignatures (used by iOS).

Code Directory Blob

It's possible to find the declaration of the 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)));

Note that there are different versions of this struct where old ones might contain less information.

Signing Code Pages

Hashing the full binary would be inefficient and even useless if when it's only loaded in memory partially. Therefore, the code signature is actually a hash of hashes where each binary page is hashed individually. Actually, in the previous Code Directory code you can see that the page size is specified in one of its fields. Moreover, if the size of the binary is not a multiple of the size of a page, the field CodeLimit specifies where is the end of the 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.*

Entitlements Blob

Note that applications might also contain an entitlement blob where all the entitlements are defined. Moreover, some iOS binaries might have their entitlements specific in the special slot -7 (instead of in the -5 entitlements special slot).

Special Slots

MacOS applications doesn't have everything they need to execute inside the binary but they also use external resources (usually inside the applications bundle). Therefore, there are some slots inside the binary who will be containing the hashes of some interesting external resources to check they weren't modified.

Actually, it's possible to see in the Code Directory structs a parameter called nSpecialSlots indicating the number of the special slots. The there isn't a special slot 0 and the most common ones (from -1 to -6 are):

  • Hash of info.plist (or the one inside __TEXT.__info__plist).

  • Has 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

Every process has related a bitmask known as the status which is started by the kernel and some of them can be overridden by the code signature. These flags that can be included in the code signing are 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)

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

Code Signature Requirements

Each application store some requirements that it must satisfy in order to be able to be executed. If the application contains requirements aren't satisfied by the application, it won't be executed (as it has probably been altered).

The requirements of a binary use a special grammar which is a stream of expressions and are encoded as blobs using 0xfade0c00 as the magic whose hash is stored in a special code slot.

The requirements of a binary can be seen running:

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

Note how this signatures can check things like certification information, TeamID, IDs, entitlements and many other data.

Moreover, it's possible to generate some compiled requirements using the csreq tool:

# 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
[...]

It's possible to access this information and create or modify requirements with some APIs from the Security.framework like:

Checking Validity

  • Sec[Static]CodeCheckValidity: Check the validity of SecCodeRef per Requirement.

  • SecRequirementEvaluate: Validate requirement in certificate context

  • SecTaskValidateForRequirement: Validate a running SecTask against CFString requirement.

Creating and Managing Code Requirements

  • SecRequirementCreateWithData: Creates a SecRequirementRef from binary data representing the requirement.

  • SecRequirementCreateWithString: Creates a SecRequirementRef from a string expression of the requirement.

  • SecRequirementCopy[Data/String]: Retrieves the binary data representation of a SecRequirementRef.

  • SecRequirementCreateGroup: Create a requirement for app-group membership

Accessing Code Signing Information

  • SecStaticCodeCreateWithPath: Initializes a SecStaticCodeRef object from a file system path for inspecting code signatures.

  • SecCodeCopySigningInformation: Obtains signing information from a SecCodeRef or SecStaticCodeRef.

Modifying Code Requirements

  • SecCodeSignerCreate: Creates a SecCodeSignerRef object for performing code signing operations.

  • SecCodeSignerSetRequirement: Sets a new requirement for the code signer to apply during signing.

  • SecCodeSignerAddSignature: Adds a signature to the code being signed with the specified signer.

Validating Code with Requirements

  • SecStaticCodeCheckValidity: Validates a static code object against specified requirements.

Additional Useful APIs

  • SecCodeCopy[Internal/Designated]Requirement: Get SecRequirementRef from SecCodeRef

  • SecCodeCopyGuestWithAttributes: Creates a SecCodeRef representing a code object based on specific attributes, useful for sandboxing.

  • SecCodeCopyPath: Retrieves the file system path associated with a SecCodeRef.

  • SecCodeCopySigningIdentifier: Obtains the signing identifier (e.g., Team ID) from a SecCodeRef.

  • SecCodeGetTypeID: Returns the type identifier for SecCodeRef objects.

  • SecRequirementGetTypeID: Gets a CFTypeID of a SecRequirementRef

Code Signing Flags and Constants

  • kSecCSDefaultFlags: Default flags used in many Security.framework functions for code signing operations.

  • kSecCSSigningInformation: Flag used to specify that signing information should be retrieved.

Code Signature Enforcement

The kernel is the one that checks the code signature before allowing the code of the app to execute. Moreover, one way to be able to write and execute in memory new code is abusing JIT if mprotect is called with MAP_JIT flag. Note that the application needs a special entitlement to be able to do this.

cs_blobs & cs_blob

cs_blob struct contains the information about the entitlement of the running process on it. csb_platform_binary also informs if the application is a platform binary (which is checked in different moments by the OS to apply security mechanisms like to protect the SEND rights to the task ports of these processes).

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
};

References

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