出于应用安全防护的目的，安全开发人员为防止函数被HOOK(inlinehook/fishhook)，一般会对敏感函数做SVC调用，如access/stat/open等函数。做SVC调用的方式明显由于直接调用和动态dlsym调用，隐蔽性得到增强，且Hook难度增加。具体实施形式可分为为裸函数或内联汇编。裸函数方式，即为一个独立函数，其他函数通过传递系统调用号和参数调用该函数内联汇编方式，会将SVC内联到每个调用函数中。这两者相比较，各有优势。裸函数方式，如果攻击者对此函数进行hook，可以很方便的通过内存校验检测出问题(如果直接用变量访问这段内存，也是很容易逆向分析，因此实际操作时可以使用一些间接引用技巧)。但无论如何，因为iOS上非越狱环境无法动态创建并执行代码，因此SVC这条汇编指令一定是存在于内存中，最常见的就是svc 0x80和svc 0x50(注意这个指令的立即数是默认忽略的，因此svc 0或者svc 0xffff效果都是一样的)，对应机器码为01 10 00 D4，通常在逆向分析时，直接搜此二进制即可直达检测的关键处。为了对抗svc指令的检测和修改，缓解的方式也很多，比如:
本文提出一种方式，可以更好的隐藏svc调用。libsystem_kernel.dylib是具体实现系统调用的动态库，通过逆向可以发现其包含大量的svc指令，那取巧的办法就是获取到这里svc的地址，并包装成函数进行调用，即可神不知鬼不觉达到隐藏svc并进行系统调用的目的。
arm64上具体实现代码如下，此代码通过获取libsystem_kernel模块地址，并爆搜指令的方式获取svc，并实现裸函数和内联函数两种调用方式，通过逆向可以发现，程序中并没有svc指令，完美达到目的！:
需要注意的是上述代码没有进行错误处理，这个实现是通过BCC指令，具体可以参照libsystem_kernel的反汇编。
__text:00000002382DA4AC EXPORT ___gettimeofday
__text:00000002382DA4AC ___gettimeofday ; CODE XREF: _prepare_times_array_and_attrs+74↑p
__text:00000002382DA4AC
__text:00000002382DA4AC var_10 = -0x10
__text:00000002382DA4AC
__text:00000002382DA4AC 02 00 80 D2 MOV X2,
__text:00000002382DA4B0 90 0E 80 D2 MOV X16,
__text:00000002382DA4B4 01 10 00 D4 SVC 0x80
__text:00000002382DA4B8 C3 00 00 54 B.CC locret_2382DA4D0
__text:00000002382DA4BC FD 7B BF A9 STP X29, X30, [SP,
__text:00000002382DA4C0 FD 03 00 91 MOV X29, SP
__text:00000002382DA4C4 5E D9 FF 97 BL _cerror_nocancel
__text:00000002382DA4C8 BF 03 00 91 MOV SP, X29
__text:00000002382DA4CC FD 7B C1 A8 LDP X29, X30, [SP+0x10+var_10],
__text:00000002382DA4D0
__text:00000002382DA4D0 locret_2382DA4D0 ; CODE XREF: ___gettimeofday+C↑j
__text:00000002382DA4D0 C0 03 5F D6 RET
__text:00000002382DA4AC EXPORT ___gettimeofday
__text:00000002382DA4AC ___gettimeofday ; CODE XREF: _prepare_times_array_and_attrs+74↑p
__text:00000002382DA4AC
__text:00000002382DA4AC var_10 = -0x10
__text:00000002382DA4AC
__text:00000002382DA4AC 02 00 80 D2 MOV X2,
__text:00000002382DA4B0 90 0E 80 D2 MOV X16,
__text:00000002382DA4B4 01 10 00 D4 SVC 0x80
__text:00000002382DA4B8 C3 00 00 54 B.CC locret_2382DA4D0
__text:00000002382DA4BC FD 7B BF A9 STP X29, X30, [SP,
__text:00000002382DA4C0 FD 03 00 91 MOV X29, SP
__text:00000002382DA4C4 5E D9 FF 97 BL _cerror_nocancel
__text:00000002382DA4C8 BF 03 00 91 MOV SP, X29
__text:00000002382DA4CC FD 7B C1 A8 LDP X29, X30, [SP+0x10+var_10],
__text:00000002382DA4D0
__text:00000002382DA4D0 locret_2382DA4D0 ; CODE XREF: ___gettimeofday+C↑j
__text:00000002382DA4D0 C0 03 5F D6 RET
intptr_t get_text_range(void* base, intptr_t off, uint32_t* psize) {
mach_header_t* pmh = (mach_header_t*)base;
load_command* plc = (load_command*)(pmh + 1);
for (int i = 0; i < pmh->ncmds; i++) {
if (plc->cmd != LC_SEGMENT_CURARCH) {
plc = (struct load_command*)((unsigned char*)plc + plc->cmdsize);
continue;
}
segment_command_t* psc = (segment_command_t*)plc;
if (0 == strcmp(psc->segname, "__TEXT")) {
if (psize) *psize = (uint32_t)psc->vmsize;
return (intptr_t)(psc->vmaddr + off);
}
plc = (struct load_command*)((unsigned char*)plc + plc->cmdsize);
}
return -1;
}
static void* find_svc() {
for (int i = 0; i < _dyld_image_count(); i++) {
const char* path = _dyld_get_image_name(i);
void* base = (void*)_dyld_get_image_header(i);
intptr_t slide = _dyld_get_image_vmaddr_slide(i);
if (0 != strcmp(path, "/usr/lib/system/libsystem_kernel.dylib")) {
continue;
}
NSLog(@"%s", path);
intptr_t text_base = 0;
uint32_t text_size = 0;
text_base = get_text_range(base, slide, &text_size);
for (int i = 0; i < text_size / 4; i += 4) {
uint32_t* addr = (uint32_t*)(text_base + i * 4);
if (*addr == 0xd4001001) { // for arm64
return (void*)addr;
}
}
}
return 0;
}
static void* svc_addr = 0;
naked int asm_getpid1() {
__asm("mov x16, 0x14 \n"
"mov x8, %[svc_addr] \n"
"br x8 \n"
::[svc_addr]"r"(svc_addr):
);
}
finline int asm_getpid2() {
int ret = 0;
__asm("mov x16, 0x14 \n"
"mov x8, %[svc_addr] \n"
"blr x8 \n"
"mov %[ret], x0 \n"
:[ret]"=r"(ret):[svc_addr]"r"(svc_addr):
);
return ret;
}
int main(int argc, char * argv[]) {
svc_addr = find_svc();
NSLog(@"asm_pid1=%d, asm_pid2=%d, real_pid=%d", asm_getpid1(), asm_getpid2(), getpid());
NSString * appDelegateClassName;
@autoreleasepool {
appDelegateClassName = NSStringFromClass([AppDelegate class]);
}
return UIApplicationMain(argc, argv, nil, appDelegateClassName);
}
intptr_t get_text_range(void* base, intptr_t off, uint32_t* psize) {
mach_header_t* pmh = (mach_header_t*)base;
load_command* plc = (load_command*)(pmh + 1);
for (int i = 0; i < pmh->ncmds; i++) {
if (plc->cmd != LC_SEGMENT_CURARCH) {
plc = (struct load_command*)((unsigned char*)plc + plc->cmdsize);
continue;
}
segment_command_t* psc = (segment_command_t*)plc;
if (0 == strcmp(psc->segname, "__TEXT")) {
if (psize) *psize = (uint32_t)psc->vmsize;
return (intptr_t)(psc->vmaddr + off);
}
plc = (struct load_command*)((unsigned char*)plc + plc->cmdsize);
}
return -1;
}
static void* find_svc() {
for (int i = 0; i < _dyld_image_count(); i++) {
const char* path = _dyld_get_image_name(i);
void* base = (void*)_dyld_get_image_header(i);
intptr_t slide = _dyld_get_image_vmaddr_slide(i);
if (0 != strcmp(path, "/usr/lib/system/libsystem_kernel.dylib")) {
continue;
}
NSLog(@"%s", path);
intptr_t text_base = 0;
uint32_t text_size = 0;
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最后于 2022-7-5 19:07
被lichaolich编辑
，原因：
