macOS MACF - Mandatory Access Control Framework

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Basic Information

MACF inamaanisha Mandatory Access Control Framework, ambayo ni mfumo wa usalama uliojengwa ndani ya mfumo wa uendeshaji kusaidia kulinda kompyuta yako. Inafanya kazi kwa kuweka kanuni kali kuhusu nani au nini kinaweza kufikia sehemu fulani za mfumo, kama vile faili, programu, na rasilimali za mfumo. Kwa kutekeleza kanuni hizi kiotomatiki, MACF inahakikisha kwamba ni watumiaji na michakato walioidhinishwa pekee wanaweza kufanya vitendo maalum, kupunguza hatari ya ufikiaji usioidhinishwa au shughuli mbaya.

Kumbuka kwamba MACF haifanyi maamuzi yoyote kwani inachukua tu hatua, inawaachia maamuzi moduli za sera (kernel extensions) inazopiga simu kama AppleMobileFileIntegrity.kext, Quarantine.kext, Sandbox.kext, TMSafetyNet.kext na mcxalr.kext.

Flow

1. Mchakato unafanya syscall/mach trap

2. Kazi husika inaitwa ndani ya kernel

3. Kazi inaita MACF

4. MACF inakagua moduli za sera ambazo zilitaka kuunganisha kazi hiyo katika sera zao

5. MACF inaita sera husika

6. Sera zinaonyesha kama zinaruhusu au kukataa hatua hiyo

Apple ndiye pekee anayeweza kutumia MAC Framework KPI.

Labels

MACF inatumia labels ambazo kisha sera zinakagua kama zinapaswa kutoa ufikiaji fulani au la. Kanuni ya kutangaza muundo wa labels inaweza kupatikana hapa, ambayo kisha inatumika ndani ya struct ucred hapa katika sehemu ya cr_label. Label ina bendera na nambari ya slots ambazo zinaweza kutumika na sera za MACF kutoa viashiria. Kwa mfano, Sanbox itakuwa na kiashiria cha wasifu wa kontena.

MACF Policies

Sera ya MACF inafafanua kanuni na masharti yanayopaswa kutumika katika operesheni fulani za kernel.

Kupanua kernel kunaweza kuunda muundo wa mac_policy_conf kisha kujiandikisha kwa kuita mac_policy_register. Kutoka hapa:

#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 */
};

Ni rahisi kubaini nyongeza za kernel zinazokamilisha sera hizi kwa kuangalia simu za mac_policy_register. Zaidi ya hayo, kuangalia disassemble ya nyongeza pia inawezekana kupata muundo wa mac_policy_conf unaotumika.

Kumbuka kwamba sera za MACF zinaweza kuandikishwa na kufutwa pia kitaalamu.

Moja ya maeneo makuu ya mac_policy_conf ni mpc_ops. Huu ni uwanja unaoeleza ni shughuli zipi sera inavutiwa nazo. Kumbuka kwamba kuna mamia yao, hivyo inawezekana kuweka sifuri zote kisha kuchagua zile tu ambazo sera inavutiwa nazo. Kutoka hapa:

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 inaanzishwa mapema sana. Inapangwa katika bootstrap_thread ya XNU: baada ya ipc_bootstrap wito wa mac_policy_init() ambayo inaanzisha mac_policy_list na muda mfupi baadaye mac_policy_initmach() inaitwa. Miongoni mwa mambo mengine, kazi hii itapata kexts zote za Apple zenye ufunguo wa AppleSecurityExtension katika Info.plist yao kama vile ALF.kext, AppleMobileFileIntegrity.kext, Quarantine.kext, Sandbox.kext na TMSafetyNet.kext na kuzipakia.

MACF Callouts

Ni kawaida kupata wito kwa MACF ulioainishwa katika msimbo kama: #if CONFIG_MAC vizuizi vya masharti. Aidha, ndani ya vizuizi hivi inawezekana kupata wito kwa mac_proc_check* ambayo inaita MACF ili kuangalia ruhusa za kutekeleza vitendo fulani. Aidha, muundo wa wito wa MACF ni: mac_<object>_<opType>_opName.

Kitu ni kimoja kati ya yafuatayo: bpfdesc, cred, file, proc, vnode, mount, devfs, ifnet, inpcb, mbuf, ipq, pipe, sysv[msg/msq/shm/sem], posix[shm/sem], socket, kext. opType kwa kawaida ni check ambayo itatumika kuruhusu au kukataa kitendo. Hata hivyo, pia inawezekana kupata notify, ambayo itaruhusu kext kujibu kitendo kilichotolewa.

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;
}

Ambayo inaita macro ya MAC_CHECK, ambayo msimbo wake unaweza kupatikana katika 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)

Ambayo itapitia sera zote za mac zilizorekodiwa ikitumia kazi zao na kuhifadhi matokeo ndani ya mabadiliko ya makosa, ambayo yanaweza kubadilishwa tu na mac_error_select kwa nambari za mafanikio hivyo ikiwa ukaguzi wowote unashindwa ukaguzi kamili utashindwa na hatua haitaruhusiwa.

Hata hivyo, kumbuka kwamba si kila kito cha MACF kinatumika tu kukataa hatua. Kwa mfano, mac_priv_grant inaita macro MAC_GRANT, ambayo itatoa kibali kilichohitajika ikiwa sera yoyote itajibu kwa 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

Hizi callas zinakusudia kuangalia na kutoa (mifumo ya) privileges zilizofafanuliwa katika bsd/sys/priv.h. Baadhi ya msimbo wa kernel utaita priv_check_cred() kutoka bsd/kern/kern_priv.c kwa kutumia KAuth credentials za mchakato na moja ya msimbo wa privileges ambayo itaita mac_priv_check ili kuona kama sera yoyote inasitisha kutoa ile privilege na kisha inaita mac_priv_grant ili kuona kama sera yoyote inatoa privilege.

proc_check_syscall_unix

Hii hook inaruhusu kukamata simu zote za mfumo. Katika bsd/dev/[i386|arm]/systemcalls.c inawezekana kuona kazi iliyoelezwa unix_syscall, ambayo ina msimbo huu:

#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 */

Ambayo itakagua katika mchakato unaoitwa bitmask ikiwa syscall ya sasa inapaswa kuita mac_proc_check_syscall_unix. Hii ni kwa sababu syscalls zinaitwa mara nyingi sana kwamba ni muhimu kuepuka kuita mac_proc_check_syscall_unix kila wakati.

Kumbuka kwamba kazi proc_set_syscall_filter_mask(), ambayo huweka bitmask syscalls katika mchakato inaitwa na Sandbox kuweka masks kwenye mchakato zilizowekwa kwenye sandbox.

Syscalls za MACF zilizofichuliwa

Inawezekana kuingiliana na MACF kupitia baadhi ya syscalls zilizofafanuliwa katika 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*/

References

• *OS Internals Volume III

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