macOS MACF - Mandatory Access Control Framework

Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE) Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)

Support HackTricks

• Check the subscription plans!

• Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.

• Share hacking tricks by submitting PRs to the HackTricks and HackTricks Cloud github repos.

Osnovne informacije

MACF označava Okvir obaveznog pristupa, koji je bezbednosni sistem ugrađen u operativni sistem kako bi pomogao u zaštiti vašeg računara. Funkcioniše tako što postavlja stroga pravila o tome ko ili šta može pristupiti određenim delovima sistema, kao što su datoteke, aplikacije i sistemski resursi. Sprovodeći ova pravila automatski, MACF osigurava da samo ovlašćeni korisnici i procesi mogu izvršavati određene radnje, smanjujući rizik od neovlašćenog pristupa ili zlonamernih aktivnosti.

Napomena: MACF zapravo ne donosi nikakve odluke, već samo presreće radnje, ostavljajući odluke modulima politike (kernel ekstenzijama) koje poziva, kao što su AppleMobileFileIntegrity.kext, Quarantine.kext, Sandbox.kext, TMSafetyNet.kext i mcxalr.kext.

Tok

1. Proces izvršava syscall/mach trap

2. Relevantna funkcija se poziva unutar kernela

3. Funkcija poziva MACF

4. MACF proverava module politike koji su zatražili da se ta funkcija poveže u svojoj politici

5. MACF poziva relevantne politike

6. Politike označavaju da li dozvoljavaju ili odbacuju radnju

Apple je jedini koji može koristiti MAC Framework KPI.

Oznake

MACF koristi oznake koje će zatim politike koristiti da provere da li treba da odobre neki pristup ili ne. Kod deklaracije strukture oznaka može se pronaći ovde, koja se zatim koristi unutar struct ucred u ovde u delu cr_label. Oznaka sadrži zastavice i broj slotova koji se mogu koristiti od strane MACF politika za dodeljivanje pokazivača. Na primer, Sanbox će ukazivati na profil kontejnera.

MACF Politike

MACF politika definiše pravila i uslove koji se primenjuju u određenim operacijama kernela.

Kernel ekstenzija može konfigurisati mac_policy_conf strukturu i zatim je registrovati pozivajući mac_policy_register. Od ovde:

#define mpc_t	struct mac_policy_conf *

/**
@brief Mac policy configuration

This structure specifies the configuration information for a
MAC policy module.  A policy module developer must supply
a short unique policy name, a more descriptive full name, a list of label
namespaces and count, a pointer to the registered enty point operations,
any load time flags, and optionally, a pointer to a label slot identifier.

The Framework will update the runtime flags (mpc_runtime_flags) to
indicate that the module has been registered.

If the label slot identifier (mpc_field_off) is NULL, the Framework
will not provide label storage for the policy.  Otherwise, the
Framework will store the label location (slot) in this field.

The mpc_list field is used by the Framework and should not be
modified by policies.
*/
/* XXX - reorder these for better aligment on 64bit platforms */
struct mac_policy_conf {
const char		*mpc_name;		/** policy name */
const char		*mpc_fullname;		/** full name */
const char		**mpc_labelnames;	/** managed label namespaces */
unsigned int		 mpc_labelname_count;	/** number of managed label namespaces */
struct mac_policy_ops	*mpc_ops;		/** operation vector */
int			 mpc_loadtime_flags;	/** load time flags */
int			*mpc_field_off;		/** label slot */
int			 mpc_runtime_flags;	/** run time flags */
mpc_t			 mpc_list;		/** List reference */
void			*mpc_data;		/** module data */
};

Lako je identifikovati kernel ekstenzije koje konfigurišu ove politike proverom poziva na mac_policy_register. Štaviše, proverom disasembla ekstenzije takođe je moguće pronaći korišćenu mac_policy_conf strukturu.

Napomena da se MACF politike mogu registrovati i deregistrovati takođe dinamički.

Jedno od glavnih polja mac_policy_conf je mpc_ops. Ovo polje specificira koje operacije politika zanima. Napomena da ih ima stotine, tako da je moguće postaviti sve na nulu, a zatim izabrati samo one koje su politici zanimljive. Od ovde:

struct mac_policy_ops {
mpo_audit_check_postselect_t		*mpo_audit_check_postselect;
mpo_audit_check_preselect_t		*mpo_audit_check_preselect;
mpo_bpfdesc_label_associate_t		*mpo_bpfdesc_label_associate;
mpo_bpfdesc_label_destroy_t		*mpo_bpfdesc_label_destroy;
mpo_bpfdesc_label_init_t		*mpo_bpfdesc_label_init;
mpo_bpfdesc_check_receive_t		*mpo_bpfdesc_check_receive;
mpo_cred_check_label_update_execve_t	*mpo_cred_check_label_update_execve;
mpo_cred_check_label_update_t		*mpo_cred_check_label_update;
[...]

Almost all the hooks will be called back by MACF when one of those operations are intercepted. However, mpo_policy_* hooks are an exception because mpo_hook_policy_init() is a callback called upon registration (so after mac_policy_register()) and mpo_hook_policy_initbsd() is called during late registration once the BSD subsystem has initialised properly.

Moreover, the mpo_policy_syscall hook can be registered by any kext to expose a private ioctl style call interface. Then, a user client will be able to call mac_syscall (#381) specifying as parameters the policy name with an integer code and optional arguments. For example, the Sandbox.kext uses this a lot.

Checking the kext's __DATA.__const* is possible to identify the mac_policy_ops structure used when registering the policy. It's possible to find it because its pointer is at an offset inside mpo_policy_conf and also because the amount of NULL pointers that will be in that area.

Moreover, it's also possible to get the list of kexts that have configured a policy by dumping from memory the struct _mac_policy_list which is updated with every policy that is registered.

MACF Initialization

MACF is initialised very soon. It's set up in XNU's bootstrap_thread: after ipc_bootstrap a call to mac_policy_init() which initializes the mac_policy_list and moments later mac_policy_initmach() is called. Among other things, this function will get all the Apple kexts with the AppleSecurityExtension key in their Info.plist like ALF.kext, AppleMobileFileIntegrity.kext, Quarantine.kext, Sandbox.kext and TMSafetyNet.kext and loads them.

MACF Callouts

It's common to find callouts to MACF defined in code like: #if CONFIG_MAC conditional blocks. Moreover, inside these blocks it's possible to find calls to mac_proc_check* which calls MACF to check for permissions to perform certain actions. Moreover, the format of the MACF callouts is: mac_<object>_<opType>_opName.

The object is one of the following: bpfdesc, cred, file, proc, vnode, mount, devfs, ifnet, inpcb, mbuf, ipq, pipe, sysv[msg/msq/shm/sem], posix[shm/sem], socket, kext. The opType is usually check which will be used to allow or deny the action. However, it's also possible to find notify, which will allow the kext to react to the given action.

You can find an example in https://github.com/apple-oss-distributions/xnu/blob/94d3b452840153a99b38a3a9659680b2a006908e/bsd/kern/kern_mman.c#L621:

int
mmap(proc_t p, struct mmap_args *uap, user_addr_t *retval)
{
[...]
#if CONFIG_MACF
			error = mac_file_check_mmap(vfs_context_ucred(ctx),
			    fp->fp_glob, prot, flags, file_pos + pageoff,
&maxprot);
if (error) {
(void)vnode_put(vp);
goto bad;
}
#endif /* MAC */
[...]

Then, it's possible to find the code of mac_file_check_mmap in https://github.com/apple-oss-distributions/xnu/blob/94d3b452840153a99b38a3a9659680b2a006908e/security/mac_file.c#L174

mac_file_check_mmap(struct ucred *cred, struct fileglob *fg, int prot,
int flags, uint64_t offset, int *maxprot)
{
int error;
int maxp;

maxp = *maxprot;
MAC_CHECK(file_check_mmap, cred, fg, NULL, prot, flags, offset, &maxp);
if ((maxp | *maxprot) != *maxprot) {
panic("file_check_mmap increased max protections");
}
*maxprot = maxp;
return error;
}

Koji poziva MAC_CHECK makro, čiji se kod može naći na https://github.com/apple-oss-distributions/xnu/blob/94d3b452840153a99b38a3a9659680b2a006908e/security/mac_internal.h#L261

/*
* MAC_CHECK performs the designated check by walking the policy
* module list and checking with each as to how it feels about the
* request.  Note that it returns its value via 'error' in the scope
* of the caller.
*/
#define MAC_CHECK(check, args...) do {                              \
error = 0;                                                      \
MAC_POLICY_ITERATE({                                            \
if (mpc->mpc_ops->mpo_ ## check != NULL) {              \
DTRACE_MACF3(mac__call__ ## check, void *, mpc, int, error, int, MAC_ITERATE_CHECK); \
int __step_err = mpc->mpc_ops->mpo_ ## check (args); \
DTRACE_MACF2(mac__rslt__ ## check, void *, mpc, int, __step_err); \
error = mac_error_select(__step_err, error);         \
}                                                           \
});                                                             \
} while (0)

Koji će proći kroz sve registrovane mac politike pozivajući njihove funkcije i čuvajući izlaz unutar promenljive greške, koja će biti prepisiva samo od strane mac_error_select pomoću kodova uspeha, tako da ako bilo koja provera ne uspe, cela provera će pasti i akcija neće biti dozvoljena.

Međutim, zapamtite da se ne koriste svi MACF pozivi samo za odbijanje akcija. Na primer, mac_priv_grant poziva makro MAC_GRANT, koji će odobriti traženu privilegiju ako bilo koja politika odgovori sa 0:

/*
* MAC_GRANT performs the designated check by walking the policy
* module list and checking with each as to how it feels about the
* request.  Unlike MAC_CHECK, it grants if any policies return '0',
* and otherwise returns EPERM.  Note that it returns its value via
* 'error' in the scope of the caller.
*/
#define MAC_GRANT(check, args...) do {                              \
error = EPERM;                                                  \
MAC_POLICY_ITERATE({                                            \
if (mpc->mpc_ops->mpo_ ## check != NULL) {                  \
DTRACE_MACF3(mac__call__ ## check, void *, mpc, int, error, int, MAC_ITERATE_GRANT); \
int __step_res = mpc->mpc_ops->mpo_ ## check (args); \
if (__step_res == 0) {                              \
error = 0;                                  \
}                                                   \
DTRACE_MACF2(mac__rslt__ ## check, void *, mpc, int, __step_res); \
}                                                           \
});                                                             \
} while (0)

priv_check & priv_grant

Ove funkcije su namenjene za proveru i dodeljivanje (desetina) privilegija definisanih u bsd/sys/priv.h. Neki kernel kod bi pozvao priv_check_cred() iz bsd/kern/kern_priv.c sa KAuth kredencijalima procesa i jednim od kodova privilegija koji će pozvati mac_priv_check da vidi da li neka politika odbija dodeljivanje privilegije, a zatim poziva mac_priv_grant da vidi da li neka politika dodeljuje privilegiju.

proc_check_syscall_unix

Ova funkcija omogućava presretanje svih sistemskih poziva. U bsd/dev/[i386|arm]/systemcalls.c moguće je videti deklarisanu funkciju unix_syscall, koja sadrži ovaj kod:

#if CONFIG_MACF
if (__improbable(proc_syscall_filter_mask(proc) != NULL && !bitstr_test(proc_syscall_filter_mask(proc), syscode))) {
error = mac_proc_check_syscall_unix(proc, syscode);
if (error) {
goto skip_syscall;
}
}
#endif /* CONFIG_MACF */

Koji će proveriti u pozivnom procesu bitmasku da li trenutni syscall treba da pozove mac_proc_check_syscall_unix. To je zato što se syscalls pozivaju tako često da je zanimljivo izbeći pozivanje mac_proc_check_syscall_unix svaki put.

Napomena da funkcija proc_set_syscall_filter_mask(), koja postavlja bitmasku syscalls u procesu, se poziva od strane Sandbox-a da postavi maske na sandboxed procesima.

Izloženi MACF syscalls

Moguće je interagovati sa MACF kroz neke syscalls definisane u security/mac.h:

/*
* Extended non-POSIX.1e interfaces that offer additional services
* available from the userland and kernel MAC frameworks.
*/
#ifdef __APPLE_API_PRIVATE
__BEGIN_DECLS
int      __mac_execve(char *fname, char **argv, char **envv, mac_t _label);
int      __mac_get_fd(int _fd, mac_t _label);
int      __mac_get_file(const char *_path, mac_t _label);
int      __mac_get_link(const char *_path, mac_t _label);
int      __mac_get_pid(pid_t _pid, mac_t _label);
int      __mac_get_proc(mac_t _label);
int      __mac_set_fd(int _fildes, const mac_t _label);
int      __mac_set_file(const char *_path, mac_t _label);
int      __mac_set_link(const char *_path, mac_t _label);
int      __mac_mount(const char *type, const char *path, int flags, void *data,
struct mac *label);
int      __mac_get_mount(const char *path, struct mac *label);
int      __mac_set_proc(const mac_t _label);
int      __mac_syscall(const char *_policyname, int _call, void *_arg);
__END_DECLS
#endif /*__APPLE_API_PRIVATE*/

Reference

• *OS Internals Volume III

Učite i vežbajte AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE) Učite i vežbajte GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)

Podržite HackTricks

• Proverite planove pretplate!

• Pridružite se 💬 Discord grupi ili telegram grupi ili pratite nas na Twitteru 🐦 @hacktricks_live.

• Podelite hakerske trikove slanjem PR-ova na HackTricks i HackTricks Cloud github repozitorijume.