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看雪论坛作者ID：xiaohang
一
ptrace反调试的原理与实现
由于厂商对于app安全方面的认识不断提升，当前iOS上的调试对抗愈演愈烈。而ptrace attach deny作为比较常用的反调试手段，其原理是将相关进程proc的p_lflag加上一个P_LNOATTACH标识位，当外部调试器想要再加载进程时，会返回一个Segmentation fault: 11 的错误标识：
iPhone8k:/usr/local root# debugserver 127.0.0.1:6666 -a Xxxx[email protected](#)PROGRAM:LLDB PROJECT:lldb-900.3.104for arm64.Attaching to process Xxxx...Segmentation fault: 11
ptrace源码，摘自xnu-6153.101.6/bsd/kern/mach_process.c
intptrace(struct proc *p, struct ptrace_args *uap, int32_t *retval){//....if (uap->req == PT_DENY_ATTACH) {//....proc_lock(p);if (ISSET(p->p_lflag, P_LTRACED)) {proc_unlock(p);//...exit1(p, W_EXITCODE(ENOTSUP, 0), retval);thread_exception_return();/* NOTREACHED */}SET(p->p_lflag, P_LNOATTACH);//p_lflag |=0x00001000proc_unlock(p);return 0;}....}
厂商为了防止API hook使其失效，开始大量使用基于svc 0x80的服务调用方式，并伴随着代码混淆以及代码膨胀，使得想要快速定位svc 0x80调用并将其patch掉也变得难以实现。
使用svc方式调用ptrace attach deny
__asm__("mov X0, #31""mov X1, #0""mov X2, #0""mov X3, #0""mov X16, #26""svc #0x80");
以上是ptrace反调试的简单介绍，如有疑问可参考下面的文章：
https://blog.it-securityguard.com/itunes-exploit-development/
https://cardaci.xyz/blog/2018/02/12/a-macos-anti-debug-technique-using-ptrace/
二
对抗方案
ptrace实现PT_DENY_ATTACH，就是对相关进程proc的p_lflag加上P_LNOATTACH标示位。那么要想使得进程和被调试器加载，只需要取消这个标志位。现在的问题是，proc链表结构，是位于iOS内核中，所以我们必须要拥有读写iOS内核的能力，要获取这个能力，第一个想到的办法是对iOS的漏洞利用，毕竟，iOS越狱也是基于这些漏洞，对特定内核位置进行读写。
所幸的是，当前一些越狱工具，提供了tfp0(task for pid 0)接口，可供我们读写iOS内核。
那什么是tfp0呢？theiphonewiki上给出的说明如下：
In the XNU kernel, task_for_pid is a function that allows a (privileged) process to get the task port of another process on the same host, except the kernel task (process ID 0). A tfp0 patch (or task_for_pid(0) patch) removes this restriction, allowing any executable running as root to call task_for_pid for pid 0 (hence the name) and then use vm_read and vm_write to modify the kernel VM region. The entitlements get-task-allow and task_for_pid-allow are required to make AMFI happy.
https://www.theiphonewiki.com/wiki/Tfp0_patch
现在我们可以整理一下思路了：
三
方案实现
有了思路，那接下来我们要如何找到kernproc的内核地址呢？
通过阅读源码，我们知道kernproc的是一个全局变量，所以判断他的地址偏移一定是固定了，而且应该位于kernelcache,并且会在bsd_init过程中被初始化。
根据上边的线索，我们可以通过逆向kernelcache镜像文件找到他的偏移。
找到偏移后，下一个问题来了，由于ASLR的存在，我们必须要获取到kernbase才能配合偏移量定位kernproc位置，进行进一步操作。
索性GeoSn0w大神已经在github上提供了这个功能的代码，其原理是通过扫描kernel heap 找到指向内核镜像的指针，再根据这个内核景象向上回溯machO的head。详细的可以通过阅读源码来了解。
boolkernel_base_init_with_unsafe_heap_scan() {uint64_t kernel_region_base = 0xfffffff000000000;uint64_t kernel_region_end = 0xfffffffbffffc000;// Try and find a pointer in the kernel heap to data in the kernel image. We'll take the// smallest such pointer.uint64_t kernel_ptr = (uint64_t)(-1);mach_vm_address_t address = 0;for (;;) {// Get the next memory region.mach_vm_size_t size = 0;uint32_t depth = 2;struct vm_region_submap_info_64 info;mach_msg_type_number_t count = VM_REGION_SUBMAP_INFO_COUNT_64;kern_return_t kr = mach_vm_region_recurse(kernel_task_port, &address, &size,&depth, (vm_region_recurse_info_t) &info, &count);if (kr != KERN_SUCCESS) {break;}// Skip any region that is not on the heap, not in a submap, not readable and// writable, or not fully mapped.int prot = VM_PROT_READ | VM_PROT_WRITE;if (info.user_tag != 12|| depth != 1|| (info.protection & prot) != prot|| info.pages_resident * 0x4000 != size) {goto next;}// Read the first word of each page in this region.for (size_t offset = 0; offset < size; offset += 0x4000) {uint64_t value = 0;bool ok = kernel_read(address + offset, &value, sizeof(value));if (ok&& kernel_region_base <= value&& value < kernel_region_end&& value < kernel_ptr) {kernel_ptr = value;}}next:address += size;}// If we didn't find any such pointer, abort.if (kernel_ptr == (uint64_t)(-1)) {return false;}printf("found kernel pointer %p\n", (void *)kernel_ptr);// Now that we have a pointer, we want to scan pages until we reach the kernel's Mach-O// header.uint64_t page = kernel_ptr & ~0x3fff;for (;;) {bool found = is_kernel_base(page);if (found) {kernel_base = page;return true;}page -= 0x4000;}return false;}
好了，万事俱备了，现在需要的是通过代码将其实现：
// ---- Main --------------------------------------------------------------------------------------//iphone8 ios 13.4 kernel#define TARGET_KERNELCACHE_VERSION_STRING "@(#)VERSION: Darwin Kernel Version 19.4.0: Mon Feb 24 22:04:29 PST 2020; root:xnu-6153.102.3~1/RELEASE_ARM64_T8015"int main() {kernel_task_init();uint64_t kb = kernel_base_init();for (size_t i = 0; i < 8; i++) {printf("%016llx\n", kernel_read64(kb + 8 * i));}uint64_t versionstraddr = kb + 0x2FB64;char versionstr[256];if(kernel_read(versionstraddr, (void *)&versionstr, sizeof(versionstr))){printf("%s\n", versionstr);if(strcmp(TARGET_KERNELCACHE_VERSION_STRING,versionstr) == 0){printf("kernel cache hit\n");//226AF60 kernprocuint64_t kernel_proc0 = kernel_read64(kb + 0x226AF60);struct proc * proc0 = (void *)malloc(sizeof(struct proc));if(!kernel_read(kernel_proc0, (void *)proc0, sizeof(struct proc))){printf("proc0 read failed\n");return -1;}printf("uniqueid offset 0x%llx comm offset 0x%llx \n",(int64_t)&(proc0->p_uniqueid) - (int64_t)proc0, (int64_t)&(proc0->p_comm)- (int64_t)proc0);struct proc * proc1 = (struct proc *)malloc(sizeof(struct proc));uint64_t preptr = (uint64_t)(proc0->p_list.le_prev);while(preptr){if(!kernel_read(preptr, (void *)proc1, sizeof(struct proc))){printf("procnext read failed\n");return -1;}else{if(proc1->p_list.le_prev == 0){printf("proc1->p_list.le_prev == 0\n");break;}int64_t lflagoffset = (int64_t)&(proc1->p_lflag) - (int64_t)proc1;int lflagvalue = proc1->p_lflag;printf("(%llu)%s proc = 0x%llx lflag = 0x%x lflag offset = 0x%llx",proc1->p_uniqueid,proc1->p_comm,//(char *)((int64_t)proc1 + 0x258),preptr,lflagvalue,lflagoffset);if(ISSET(lflagvalue, P_LNOATTACH)){printf(" !!!P_LNOATTACH set");CLR(lflagvalue, P_LNOATTACH);KERNEL_WRITE32(preptr + lflagoffset, lflagvalue);}printf("\n");preptr = (uint64_t)(proc1->p_list.le_prev);}}printf("end\n");free(proc0);free(proc1);}else{printf("kernel cache version mismatch\n");}}else{printf("failed to read kernel version string\n");}return 0;}
完整代码可到github上下载：
https://github.com/xiaohang99/iOSFuckDenyAttach
看雪ID：xiaohang
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