IOS PAC 实现详解
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作者： wzt
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内核pac key初始化
common_start
mrs x0, S3_4_C15_C0_4
and x1, x0, #0x2
cbz x1, 0xfffffff0081384d0
orr x0, x0, #0x1
orr x0, x0, #0x4
msr S3_4_C15_C0_4, x0
Isb首先探测下S3_4_C15_C0_4寄存器的第2个bit是否为0， 如果为0， 则一直等待下去，否则把其第1个和第3个比特位置1，结合后面对_ml_set_kernelkey_enabled的分析，可以推测出S3_4_C15_C0_4寄存器是苹果公司为增强pac而加入的控制寄存器。
LDR X0, =0xFEEDFACEFEEDFACF
MSR #0, c2, c1, #2, X0 ; APIBKeyLo_EL1
ADD X0, X0, #1
MSR #0, c2, c1, #3, X0 ; APIBKeyHi_EL1
ADD X0, X0, #1
MSR #0, c2, c2, #2, X0 ; APDBKeyLo_EL1
ADD X0, X0, #1
MSR #0, c2, c2, #3, X0 ; APDBKeyHi_EL1
ADD X0, X0, #1
MSR #4, c15, c1, #0, X0; ???
ADD X0, X0, #1
MSR #4, c15, c1, #1, X0; ???
ADD X0, X0, #1
MSR #0, c2, c1, #0, X0 ; APIAKeyLo_EL1
ADD X0, X0, #1
MSR #0, c2, c1, #1, X0 ; APIAKeyHi_EL1
ADD X0, X0, #1
MSR #0, c2, c2, #0, X0 ; APDAKeyLo_EL1
ADD X0, X0, #1
MSR #0, c2, c2, #1, X0 ; APDAKeyHi_EL1
ADD X0, X0, #1
MSR #0, c2, c3, #0, X0 ; APGAKeyLo_EL1
ADD X0, X0, #1
MSR #0, c2, c3, #1, X0 ; APGAKeyHi_EL1Iphone12使用固定值0xFEEDFACEFEEDFACF依次初始化APIAKey_EL1、APIBKey_EL1、APDAKey_EL1、APDBKey_EL1、APGAKey_EL1寄存器。高版本的Iphone内核是否也使用固定值有待确认。
MOVK X0, #0,LSL#48
MOVK X0, #0,LSL#32
MOVK X0, #0x3454,LSL#16
MOVK X0, #0x593D ; 0x3454593d
ORR X0, X0, #0x40000000 ; 0x7454593d
MSR #0, c1, c0, #0, X0 ; SCTLR_EL1很奇怪这里sctlr_el1只设置了EnIB位（笔者翻遍了代码，也没发现对sctlr_el1其他pac比特位的使用）。
用户进程pac key初始化
通过execve执行一个binary时，XNU内核会进行新进程pac key的初始化，XNU将每个进程的pac key保存在一个叫做shared region的进程内存区域，这个区域可以被其他进程共享代码和数据，用来加快进程的启动速度，通常情况下一个进程只有一个shared region区域，但是在支持pac的情况下，shared region根据不同的shared_region_id会有不同的shared region区域，通过一个队列_shared_region_jop_key_queue链接起来，每个节点是一个struct shared_region_jop_key_map类型的结构体，xnu source code有如下定义：
typedef struct shared_region_jop_key_map {
queue_chain_t srk_queue;
char *srk_shared_region_id;
uint64_t srk_jop_key;
os_refcnt_t srk_ref_count; /* count of tasks active with this shared_region_id */
} *shared_region_jop_key_map_t;成员srk_jop_key保存的是这个region所使用的pac key。下面我们来看下pac key是如何被初始化的。
/*
void
shared_region_key_alloc(char *shared_region_id, bool inherit, uint64_t inherited_key)
*/
exec_mach_imgact->_shared_region_key_alloc
LDR X8, [X24] ; _shared_region_jop_key_queue
CMP X8, X24
B. EQ loc_FFFFFFF007B33338 ;遍历_shared_region_jop_key_queue，如果队列为空，跳转到后面去申请一个新的shared_region节点。
MOV X9, X8
LDR X10, [X9,#0x10]
MOV X11, X22
LDRB W12, [X10]
LDRB W13, [X11]
CMP W12, W13
B.NE loc_FFFFFFF007B3332C
ADD X11, X11, #1
ADD X10, X10, #1
CBNZ W12, loc_FFFFFFF007B3330C
B loc_FFFFFFF007B33450 ; srk_ref_count
LDR X9, [X9]
CMP X9, X24
C. NE loc_FFFFFFF007B33304循环遍历每个shared_region节点，如果节点名与第一个参数相同，证明存在一个要匹配的节点。
__TEXT_EXEC:__text:FFFFFFF007B33450
E ADD X0, X9, #0x20 ; ' ' ; srk_ref_count
MOV W8, #1__TEXT_EXEC:__text:FFFFFFF007B33458 LDADD W8, W8, [X0] ; srk_ref_count++将引用计数srk_ref_count加1.
CBZ W8, loc_FFFFFFF007B33554
MOV W10, #0xFFFFFFF
CMP W8, W10
B. CS loc_FFFFFFF007B33558判断引用计数是否溢出
MOV X22, X21 ; x21 == 0
MOV X21, X9
CBZ W20, loc_FFFFFFF007B33488 ; inherit == 0
LDR X8, [X21,#0x18] ; inherit == 1 -> srk_jop_key
LDR X9, [SP,#0x70+var_60]
CMP X8, X9 ; arg3: inherited_key
C. NE loc_FFFFFFF007B3356C如果第二个参数inherit为1，并且此shared_region保存的pac key与第三个参数不相同，则panic，否则直接返回，不需要更新pac key。
下面看下队列如果为空，或者没有找到要匹配到的shared_region_id的后续处理流程。
ADRL X0, _KHEAP_DEFAULT
MOV W1, #0x28 ; '('
MOV W2, #0
ADRL X3, _shared_region_key_alloc.site
BL _kalloc_ext分配一个新的struct shared_region_jop_key_map结构体
MOV X21, X0 ; new struct shared_region_jop_key_map
MOV X0, X22 ; __s
BL _strlen ; shared_region_id计算参数1shared_region_id的长度
ADD W28, W0, #1
ADRL X0, _KHEAP_DATA_BUFFERS
MOV X1, X28
MOV W2, #0
ADRL X3, _shared_region_key_alloc.site.3
BL _kalloc_ext ; alloc shared_region_id buffer分配shared_region_id内存
STR X0, [X21,#0x10] ; set srk_shared_region_id保存shared_region_id到struct shared_region_jop_key_map对应成员。
MOV X1, X22 ; __source
MOV X2, X28 ; __size
LDR X23, [SP,#0x70+var_60]
BL _strlcpy ; strlcpy(srk_shared_region_id, shared_region_id, size);拷贝shared_region_id。
MOV W8, #1
STR W8, [X21,#0x20] ; srk_ref_count = 1引用计数srk_ref_count初始化为1。
ADRP X8, #_diversify_user_jop@PAGE
LDR W9, [X8,#_diversify_user_jop@PAGEOFF]
CMP W9, #0
CSET W8, NE
TST W8, W20 ; arg2: inherit
MOV X8, #0xFEEDFACEFEEDFAD5
CSEL X8, X23, X8, NE ; x23: arg3 inherit如果diversify_user_jop为1并且第2个参数inherit也为1，x8被设置为第三个参数inherit，否则设置为0xFEEDFACEFEEDFAD5。
ADRL X24, _shared_region_jop_key_queue
CBZ W9, loc_FFFFFFF007B33444
TBNZ W20, #0, loc_FFFFFFF007B33444
MOV X0, X22 ; __s
BL _strlen
CBZ X0, loc_FFFFFFF007B33434 ; strlen(shared_region_id) == 0如果shared_region_id长度为0， x8设置为0xFEEDFACEFEEDFAD5。我们看到用户进程的pac key居然和内核的pac key0xFEEDFACEFEEDFACF值很接近！
__TEXT_EXEC:__text:FFFFFFF007B333FC
LDR X8, [X19]
LDR X0, [X26,#_prng_ctx@PAGEOFF]
BLRAA X8, X28
LDR X8, [X19,#(qword_FFFFFFF0076E7760 - 0xFFFFFFF0076E7758)]
LDR X0, [X26,#_prng_ctx@PAGEOFF]
MRS X9, #0, c13, c0, #4 ; TPIDR_EL1
LDR X9, [X9,#0x4F8]
LDRH W1, [X9]
ADD X3, SP, #0x70+var_58
MOV W2, #8
BLRAA X8, X25
LDR X8, [SP,#0x70+var_58]
CBZ X8, loc_FFFFFFF007B333FC获取一个随机数，赋值给x8。
STR X8, [X21,#0x18]X8为要保存的pac key值。
总结以下，一个进程可以继承父进程的pac key，可以是个随机值，还可以是固定值0xFEEDFACEFEEDFAD5。
这里还存在另一个安全问题，虽然把生成随机数的两个函数指针都使用pac签名过，但是pac计算过程中使用的context值居然是个固定的4个字节值：0x2ABE和0x9BF6。这会弱化随机数生成函数的安全性。
进程之间切换pac
由于每个进程的pac key可能不同，所以在进程切换的时候，也需要切换pac key。
_Switch_context
STR XZR, [X3,#0x78] ; SS64_KERNEL_PC
MOV W4, #0x100004 ; PSR64_KERNEL_POISON
STR W4, [X3,#0x80] ; SS64_KERNEL_CPSR
STP X0, X1, [SP,#var_10]!
STP X2, X3, [SP,#0x10+var_20]!
STP X4, X5, [SP,#0x20+var_30]!
MOV X0, X3
MOV X1, #0
MOV W2, W4
MOV X3, X30
MOV X4, X16
MOV X5, X17
BL _ml_sign_kernel_thread_state ; compute old thread pac code.在进程切换前，首先计算处当前进程的thread state pac值。linux内核也提供了pac服务，但是没有对thread状态做校验。
__TEXT_EXEC:__text:FFFFFFF008139A8C _ml_sign_kernel_thread_state ; CODE PACGA X1, X1, X0
AND X2, X2, #0xFFFFFFFFDFFFFFFF
PACGA X1, X2, X1
PACGA X1, X3, X1
PACGA X1, X4, X1
PACGA X1, X5, X1
STR X1, [X0,#0x88] ; struct arm_kernel_saved_state->jophash
RETPacga指令用于计算大块的内存区域，可以看到XNU使用pacga指令依次对x0: The ARM context pointer、x1: PC value to sign、x2: CPSR value to sign、x3: LR to sign、x16、x17做计算生成pac。
X0是一个struct arm_kernel_save_state的数据结构：
struct arm_kernel_saved_state {
uint64_t x[12]; /* General purpose registers x16-x28 */
uint64_t fp; /* Frame pointer x29 */
uint64_t lr; /* Link register x30 */
uint64_t sp; /* Stack pointer x31 */
uint64_t pc; /* Program counter */
uint32_t cpsr; /* Current program status register */
uint64_t jophash;
} __attribute__((aligned(16)));在生成当前进程的thread state pac后，开始校验下要切换的进程thread state状态，如果校验不过，证明要切换的进程状态被篡改过，系统直接panic。
LDR X3, [X2,#0x498] ; new TH_KSTACKPTR
MOV X20, X0
MOV X21, X1
MOV X22, X2
MOV X0, X3
LDR W2, [X0,#0x80] ; new SS64_KERNEL_CPSR
DMB LD
LDR X1, [X0,#0x78] ; new SS64_KERNEL_PC
LDP X16, X17, [X0]
MOV X25, X3
MOV X26, X4
MOV X27, X5
MOV X23, X1
MOV X24, X2
LDR X3, [X0,#0x68]
MOV X4, X16
MOV X5, X17
BL _ml_check_kernel_signed_state ; check new thread pac code.
__TEXT_EXEC:__text:FFFFFFF008139AEC _ml_check_kernel_signed_state ; CODE PACGA X1, X1, X0
AND X2, X2, #0xFFFFFFFFDFFFFFFF
PACGA X1, X2, X1
PACGA X1, X3, X1
PACGA X1, X4, X1
PACGA X1, X5, X1
LDR X2, [X0,#0x88]从struct arm_kernel_saved_state结构体提取pac code。
CMP X1, X2
B.NE loc_FFFFFFF008139B14
RET相同返回上层函数。
__TEXT_EXEC:__text:FFFFFFF008139B14 ;
PACIBSP
STP X29, X30, [SP,#var_10]!
MOV X29, SP
BL _panic不相同则panic系统。
LDR X5, [X2,#0x4F8]
MOV W6, #0
LDR X3, [X2,#0x508]
LDR X4, [X5,#0x200]
CMP X3, X4
B.EQ loc_FFFFFFF008138B48
STR X3, [X5,#0x200]
MSR #0, c2, c1, #2, X3 ; APIBKeyLo_EL1
ADD X3, X3, #1
MSR #0, c2, c1, #3, X3 ; APIBKeyHi_EL1
ADD X3, X3, #1
MSR #0, c2, c2, #2, X3 ; APDBKeyLo_EL1
ADD X3, X3, #1
MSR #0, c2, c2, #3, X3 ; APDBKeyHi_EL1加载新进程的pac key到对应的系统寄存器。
内核pac与用户进程pac切换
进程每次通过系统调用进入内核或者发生错误进入内核异常处理流程时，需要把进程的pac key切换为内核的pac key。
__TEXT_EXEC:__text:FFFFFFF008131410 fleh_dispatch64
MOVK X2, #0xFEED,LSL#48
MOVK X2, #0xFACE,LSL#32
MOVK X2, #0xFEED,LSL#16
MOVK X2, #0xFAD5 ; 0xFEEDFACEFEEDFAD5
MRS X3, #0, c13, c0, #4 ; TPIDR_EL1
LDR X3, [X3,#0x4F8]
LDR X4, [X3,#0x208]
CMP X2, X4
B.EQ loc_FFFFFFF0081314F0
MSR #0, c2, c1, #0, X2 ; APIAKeyLo_EL1
ADD X4, X2, #1
MSR #0, c2, c1, #1, X4 ; APIAKeyHi_EL1
ADD X4, X4, #1
MSR #0, c2, c2, #0, X4 ; APDAKeyLo_EL1
ADD X4, X4, #1
MSR #0, c2, c2, #1, X4 ; APDAKeyHi_EL1
STR X2, [X3,#0x208]在进入内核时，首先要更新APIAKey_EL1和APDAKey_EL1为固定值0xFEEDFACEFEEDFAD5极其增值。
MOV X21, X1
MOV X20, X30
MOV X1, X22
MOV W2, W23
MOV X3, X20
MOV X4, X16
MOV X5, X17
BL _ml_sign_thread_state更新完pac key值，马上对当前进程的thread state进行签名。
__TEXT_EXEC:__text:FFFFFFF00813197C exception_return_unint_tpidr_x3_dont_trash_x18
MOV X0, SP
LDR W2, [X0,#arg_110]
LDR X1, [X0,#arg_108]
LDP X16, X17, [X0,#arg_88]
MOV X22, X3
MOV X23, X4
MOV X24, X5
MOV X20, X1
MOV X21, X2
LDR X3, [X0,#arg_F8]
MOV X4, X16
MOV X5, X17
BL _ml_check_signed_state在退出内核到用户态前，需要再次校验下当前进程的thread state，如果发生错误，则panic系统。
LDR X1, [X2,#0x510]
LDR X2, [X2,#0x4F8]
LDR X3, [X2,#0x208]
CMP X1, X3
B.EQ loc_FFFFFFF008131A3C
MSR #0, c2, c1, #0, X1 ; APIAKeyLo_EL1
ADD X3, X1, #1
MSR #0, c2, c1, #1, X3 ; APIAKeyHi_EL1
ADD X3, X3, #1
MSR #0, c2, c2, #0, X3 ; APDAKeyLo_EL1
ADD X3, X3, #1
MSR #0, c2, c2, #1, X3 ; APDAKeyHi_EL1
STR X1, [X2,#0x208]恢复进程使用的pac key。
进程thread state pac的初始化
前面提到在进程切换时，需要验证进程的thread state pac，那么它是在何时初始化的呢？
答案在进程初始化stack时进行thread state的pac计算。
_machine_stack_attach
STR X1, [X0,#0x78] ; struct arm_kernel_saved_state->pc
MOV X2, #0x100004
STR W2, [X0,#0x80] ; struct arm_kernel_saved_state->cpsr
ADRL X3, _thread_continue
STR X3, [X0,#0x68] ; struct arm_kernel_saved_state->lr
MOV X4, XZR
MOV X5, XZR
STP X4, X5, [X0]
MOV X6, X30
BL _ml_sign_kernel_thread_state ; sign thread state.Ppl相关
Ppl提供了两个服务用来给用户进程数据进行签名与验签。
_pmap_sign_user_ptr
__TEXT_EXEC:__text:FFFFFFF007B609A0 _pmap_sign_user_ptr
ADD X29, SP, #0x30
MOV X19, X0
MRS X23, #3, c4, c2, #1 ; DAIFSet
TBNZ W23, #7, loc_FFFFFFF007B609D0 ; TPIDR_EL1
MSR #6, #7关闭DAIF中断
MRS X8, #0, c13, c0, #4 ; TPIDR_EL1
LDR X8, [X8,#0x4F8]
LDR X20, [X8,#0x208]
MOV W0, #0
MOV X1, X3
BL _ml_set_kernelkey_enabled将0作为参数传递给_ml_set_kernelkey_enabled。
__TEXT_EXEC:__text:FFFFFFF008139374 _ml_set_kernelkey_enabled
MRS X2, #0, c13, c0, #4 ; TPIDR_EL1
LDR X2, [X2,#0x4F8]
LDR X3, [X2,#0x208]
CMP X1, X3
B. EQ loc_FFFFFFF0081393A8 ; S3_4_C15_C0_4如果当前进程的pac key和用户传递进来的pac key相等，则直接跳转到后面。
MSR #0, c2, c1, #0, X1 ; APIAKeyLo_EL1
ADD X3, X1, #1
MSR #0, c2, c1, #1, X3 ; APIAKeyHi_EL1
ADD X3, X3, #1
MSR #0, c2, c2, #0, X3 ; APDAKeyLo_EL1
ADD X3, X3, #1
MSR #0, c2, c2, #1, X3 ; APDAKeyHi_EL1
STR X1, [X2,#0x208]依次切换系统寄存器APIAKey_EL1、APDAKey_EL1，可以看到ppl只用key A来签名用户代码或数据。
MRS X1, #4, c15, c0, #4 ; S3_4_C15_C0_4
ORR X3, X1, #4
AND X2, X1, #0xFFFFFFFFFFFFFFFB
CMP W0, #0
CSEL X1, X2, X3, EQ
MSR #4, c15, c0, #4, X1 ; S3_4_C15_C0_4
ISB
RET如果传递给_ml_set_kernelkey_enabled函数的第一个参数为0，那么将S3_4_C15_C0_4的第3个bit置0，否则置1。
回到_pmap_sign_user_ptr
CMP W22, #2
B.EQ loc_FFFFFFF007B609FC
CBNZ W22, loc_FFFFFFF007B60A34
PACIA X19, X21
B loc_FFFFFFF007B60A00
PACDA X19, X21如果第2个参数为2，则使用pacda对第1个参数签名，如果为0，使用pacia进行签名，如果不是0，也不是2， 则panic系统。
MOV W0, #1
MOV X1, X20
BL _ml_set_kernelkey_enabled将1作为参数传递给_ml_set_kernelkey_enabled。
_ml_set_kernelkey_enabled(0, pac_key)
...
Pacia/pacib
...
_ml_set_kernelkey_enabled(1, pac_key)因此我们可以推测出S3_4_C15_C0_4的第2个bit为上锁功能， 0代表上锁，1代表解锁。
_pmap_auth_user_ptr
AUTIA X19, X21
B loc_FFFFFFF007B60AD4
AUTDA X19, X21
B loc_FFFFFFF007B60AD4
AUTDB X19, X21
B loc_FFFFFFF007B60AD4
AUTIB X19, X21这里我们看到_pmap_auth_user_ptr在验签的时候还使用了key b，而_ml_set_kernelkey_enabled只设置了key a， 说明了key a是进程相关的，而key b是进程不相关的， XNU源码里定义了以下几种类型的pac key：
EXTERNAL_HEADERS/ptrauth.h
typedef enum {
ptrauth_key_asia = 0,
ptrauth_key_asib = 1,
ptrauth_key_asda = 2,
ptrauth_key_asdb = 3,
/* A process-independent key which can be used to sign code pointers.
Signing and authenticating with this key is a no-op in processes
which disable ABI pointer authentication. */
ptrauth_key_process_independent_code = ptrauth_key_asia,
/* A process-specific key which can be used to sign code pointers.
Signing and authenticating with this key is enforced even in processes
which disable ABI pointer authentication. */
ptrauth_key_process_dependent_code = ptrauth_key_asib,
/* A process-independent key which can be used to sign data pointers.
Signing and authenticating with this key is a no-op in processes
which disable ABI pointer authentication. */
ptrauth_key_process_independent_data = ptrauth_key_asda,
/* A process-specific key which can be used to sign data pointers.
Signing and authenticating with this key is a no-op in processes
which disable ABI pointer authentication. */
ptrauth_key_process_dependent_data = ptrauth_key_asdb,
/* The key used to sign C function pointers.
The extra data is always 0. */
ptrauth_key_function_pointer = ptrauth_key_process_independent_code,
/* The key used to sign return addresses on the stack.
The extra data is based on the storage address of the return address.
On ARM64, that is always the storage address of the return address plus 8
(or, in other words, the value of the stack pointer on function entry) */
ptrauth_key_return_address = ptrauth_key_process_dependent_code,
/* The key used to sign frame pointers on the stack.
The extra data is based on the storage address of the frame pointer.
On ARM64, that is always the storage address of the frame pointer plus 16
(or, in other words, the value of the stack pointer on function entry) */
ptrauth_key_frame_pointer = ptrauth_key_process_dependent_data,
/* The key used to sign block function pointers, including:
invocation functions,
block object copy functions,
block object destroy functions,
__block variable copy functions, and
__block variable destroy functions.
The extra data is always the address at which the function pointer
is stored.
Note that block object pointers themselves (i.e. the direct
representations of values of block-pointer type) are not signed. */
ptrauth_key_block_function = ptrauth_key_asia,
/* The key used to sign C++ v-table pointers.
The extra data is always 0. */
ptrauth_key_cxx_vtable_pointer = ptrauth_key_asda,
/* Other pointers signed under the ABI use private ABI rules. */
} ptrauth_key;
本文由 Seebug Paper 发布，如需转载请注明来源。本文地址：https://paper.seebug.org/1777/
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