内容简介:虽然现在技术文章很少人看,大家都喜欢聊安全八卦,但技术文章输出是一种很好的学习方式。更重要的是,专业的文章是给专业的人看的,并非为了取悦所有人。对于应用程序的代码插桩,有现成的Pin和DynamoRIO插桩框架,在Fuzzing中可以用来实现代码覆盖率的反馈驱动,这已经被应用到winafl,效果很好。除了挖洞,在逆向工程领域应用也很广泛。上面都是针对应用层的,内核层的,上面的Pin和DynamoRIO就派不上用场了,对于这种系统内核级的指令插桩,有时就会采用虚拟化技术为实现,比如通过Qemu或Bochs虚
虽然现在技术文章很少人看,大家都喜欢聊安全八卦,但技术文章输出是一种很好的学习方式。更重要的是,专业的文章是给专业的人看的,并非为了取悦所有人。
对于应用程序的代码插桩,有现成的Pin和DynamoRIO插桩框架,在Fuzzing中可以用来实现代码覆盖率的反馈驱动,这已经被应用到winafl,效果很好。除了挖洞,在逆向工程领域应用也很广泛。
上面都是针对应用层的,内核层的,上面的Pin和DynamoRIO就派不上用场了,对于这种系统内核级的指令插桩,有时就会采用虚拟化技术为实现,比如通过Qemu或Bochs虚拟机。
ProjectZero的j00ru大神就用bochs的插桩API为实现针对内核double fetches的监测,项目称为bochspwn,后来又采用污点追踪方式检测未初始化漏洞导致的内核信息泄露,叫bochspwn-reloaded。
Bochs Instrument API 文档参考: http://bochs.sourceforge.net/cgi-bin/lxr/source/instrument/instrumentation.txt,在编译bochs时指定插桩代码目录:
./configure [...] --enable-instrumentation="instrument/myinstrument"
下面是bochspwn中用到的API:
// Bochs初始化CPU对象时的回调函数 void bx_instr_initialize(unsigned cpu); // Bochs析构CPU对象时的回调函数 void bx_instr_exit(unsigned cpu); // Bochs访问线性内存时的回调函数 void bx_instr_lin_access(unsigned cpu, bx_address lin, bx_address phy,unsigned len, unsigned memtype, unsigned rw); // Bochs执行指令前的回调函数 void bx_instr_before_execution(unsigned cpu, bxInstruction_c *i);
bx_instr_initialize用来加载配置信息,针对不同的系统环境设置不同的数据结构偏移地址,用来提供需要的进程/线程等重要信息:
[general] trace_log_path = memlog.bin modules_list_path = modules.bin os = windows bitness = 32 version = win10_32 min_read_size = 1 max_read_size = 16 min_write_size = 1 max_write_size = 16 callstack_length = 48 write_as_text = 0 symbolize = 0 symbol_path = <symbols path> [win7_32] kprcb = 0x120 current_thread = 0x04 tcb = 0x0 process = 0x150 client_id = 0x22c process_id = 0 thread_id = 4 create_time = 0x200 image_filename = 0x16c kdversionblock = 0x34 psloadedmodulelist = 0x18 loadorder_flink = 0x0 basedllname = 0x2c baseaddress = 0x18 sizeofimage = 0x20 us_len = 0x0 us_buffer = 0x4 teb_cid = 0x20 irql = 0x24 previous_mode = 0x13a exception_list = 0x0 next_exception = 0x0 try_level = 0xc ......
Bochspwn的核心功能实现就在于 bx_instr_lin_access
与 bx_instr_before_execution
两个函数。先看下 bx_instr_before_execution
的实现逻辑:
- 忽略实模式real mode
-
忽略无关的系统调用中断指令,仅允许
int 0x2e
与int 0x80
- 获取当前进程/线程ID相关的信息,当发现漏洞时方便重现
void bx_instr_before_execution(unsigned cpu, bxInstruction_c *i) { static client_id thread; BX_CPU_C *pcpu = BX_CPU(cpu); unsigned opcode; // We're not interested in instructions executed in real mode. if (!pcpu->protected_mode() && !pcpu->long64_mode()) { return; } // If the system needs an additional invokement from here, call it now. if (globals::has_instr_before_execution_handler) { invoke_system_handler(BX_OS_EVENT_INSTR_BEFORE_EXECUTION, pcpu, i); } // Any system-call invoking instruction is interesting - this // is mostly due to 64-bit Linux which allows various ways // to be used for system-call invocation. // Note: We're not checking for int1, int3 nor into instructions. opcode = i->getIaOpcode(); if (opcode != BX_IA_SYSCALL && opcode != BX_IA_SYSENTER && opcode != BX_IA_INT_Ib) { return; } // The only two allowed interrupts are int 0x2e and int 0x80, which are legacy // ways to invoke system calls on Windows and linux, respectively. if (opcode == BX_IA_INT_Ib && i->Ib() != 0x2e && i->Ib() != 0x80) { return; } // Obtain information about the current process/thread IDs. if (!invoke_system_handler(BX_OS_EVENT_FILL_CID, pcpu, &thread)) { return; } // Process information about a new syscall depending on the current mode. if (!events::event_new_syscall(pcpu, &thread)) { return; } }
再看下 bx_instr_lin_access
实现逻辑:
- 忽略仅读写指令
- 检测CPU类型(32位或64位)
- 判断当前指令地址pc是否为内核地址,判断访问的线性内存地址是否为用户层地址
- 检测读取的内存长度是否处于0~16字节之间,长度大小范围在config.txt中配置,仅处理此范围内的指令操作
- 通过上述条件之后,就代表可能存在内核漏洞,然后反汇编指令,然后填充日志记录信息
void bx_instr_lin_access(unsigned cpu, bx_address lin, bx_address phy, unsigned len, unsigned memtype, unsigned rw) { BX_CPU_C *pcpu = BX_CPU(cpu); // Not going to use physical memory address. (void)phy; // Read-write instructions are currently not interesting. if (rw == BX_RW) return; // Is the CPU in protected or long mode? unsigned mode = 0; // Note: DO NOT change order of these ifs. long64_mode must be called // before protected_mode, since it will also return "true" on protected_mode // query (well, long mode is technically protected mode). if (pcpu->long64_mode()) { #if BX_SUPPORT_X86_64 mode = 64; #else return; #endif // BX_SUPPORT_X86_64 } else if (pcpu->protected_mode()) { // This is either protected 32-bit mode or 32-bit compat. long mode. mode = 32; } else { // Nothing interesting. // TODO(gynvael): Well actually there is the smm_mode(), which // might be a little interesting, even if it's just the bochs BIOS // SMM code. return; } // Is pc in kernel memory area? // Is lin in user memory area? bx_address pc = pcpu->prev_rip; if (!invoke_system_handler(BX_OS_EVENT_CHECK_KERNEL_ADDR, &pc, NULL) || !invoke_system_handler(BX_OS_EVENT_CHECK_USER_ADDR, &lin, NULL)) { return; /* pc not in ring-0 or lin not in ring-3 */ } // Check if the access meets specified operand length criteria. if (rw == BX_READ) { if (len < globals::config.min_read_size || len > globals::config.max_read_size) { return; } } else { if (len < globals::config.min_write_size || len > globals::config.max_write_size) { return; } } // Save basic information about the access. log_data_st::mem_access_type access_type; switch (rw) { case BX_READ: access_type = log_data_st::MEM_READ; break; case BX_WRITE: access_type = log_data_st::MEM_WRITE; break; case BX_EXECUTE: access_type = log_data_st::MEM_EXEC; break; case BX_RW: access_type = log_data_st::MEM_RW; break; default: abort(); } // Disassemble current instruction. static Bit8u ibuf[32] = {0}; static char pc_disasm[64]; if (read_lin_mem(pcpu, pc, sizeof(ibuf), ibuf)) { disassembler bx_disassemble; bx_disassemble.disasm(mode == 32, mode == 64, 0, pc, ibuf, pc_disasm); } // With basic information filled in, process the access further. process_mem_access(pcpu, lin, len, pc, access_type, pc_disasm); }
信息记录方式都是通过invoke_system_handler函数去处理自定义系统事件,目前主要支持4种操作系统(windows\linux\freebsd\openbsd),macOS还没搞过,原作者是说想继续实现macOS,这个值得尝试开发下:
const struct tag_kSystemEventHandlers { const char *system; s_event_handler_func handlers[BX_OS_EVENT_MAX]; } kSystemEventHandlers[] = { {"windows", {(s_event_handler_func)windows::init, (s_event_handler_func)windows::check_kernel_addr, (s_event_handler_func)windows::check_user_addr, (s_event_handler_func)windows::fill_cid, // 获取线程环境块TEB,读取进程/线程ID (s_event_handler_func)windows::fill_info, // 基于config.txt中配置的进线程结构offset去读取进线程信息,包括进程文件名、创建时间、栈回溯等信息 (s_event_handler_func)NULL} }, {"linux", {(s_event_handler_func)linux::init, (s_event_handler_func)linux::check_kernel_addr, (s_event_handler_func)linux::check_user_addr, (s_event_handler_func)linux::fill_cid, (s_event_handler_func)linux::fill_info, (s_event_handler_func)NULL} }, {"freebsd", {(s_event_handler_func)freebsd::init, (s_event_handler_func)freebsd::check_kernel_addr, (s_event_handler_func)freebsd::check_user_addr, (s_event_handler_func)freebsd::fill_cid, (s_event_handler_func)freebsd::fill_info, (s_event_handler_func)freebsd::instr_before_execution} }, {"openbsd", {(s_event_handler_func)openbsd::init, (s_event_handler_func)openbsd::check_kernel_addr, (s_event_handler_func)openbsd::check_user_addr, (s_event_handler_func)openbsd::fill_cid, (s_event_handler_func)openbsd::fill_info, (s_event_handler_func)openbsd::instr_before_execution} }, {NULL, {NULL, NULL, NULL, NULL, NULL}} };
最后就是输出记录的信息,比如作者发现的CVE-2018-0894漏洞信息:
------------------------------ found uninit-copy of address fffff8a000a63010 [pid/tid: 000001a0/000001a4] { wininit.exe} COPY of fffff8a000a63010 ---> 1afab8 (64 bytes), pc = fffff80002698600 [ mov r11, rcx ] Allocation origin: 0xfffff80002a11101 (ntoskrnl.exe!IopQueryNameInternal+00000071) --- Shadow memory: 00000000: 00 00 00 00 ff ff ff ff 00 00 00 00 00 00 00 00 ................ 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ --- Actual memory: 00000000: 2e 00 30 00 aa aa aa aa 20 30 a6 00 a0 f8 ff ff ..0..... 0...... 00000010: 5c 00 44 00 65 00 76 00 69 00 63 00 65 00 5c 00 \.D.e.v.i.c.e.\. 00000020: 48 00 61 00 72 00 64 00 64 00 69 00 73 00 6b 00 H.a.r.d.d.i.s.k. 00000030: 56 00 6f 00 6c 00 75 00 6d 00 65 00 32 00 00 00 V.o.l.u.m.e.2... --- Stack trace: #0 0xfffff80002698600 (ntoskrnl.exe!memmove+00000000) #1 0xfffff80002a11319 (ntoskrnl.exe!IopQueryNameInternal+00000289) #2 0xfffff800028d4426 (ntoskrnl.exe!IopQueryName+00000026) #3 0xfffff800028e8fa8 (ntoskrnl.exe!ObpQueryNameString+000000b0) #4 0xfffff8000291313b (ntoskrnl.exe!NtQueryVirtualMemory+000005fb) #5 0xfffff800026b9283 (ntoskrnl.exe!KiSystemServiceCopyEnd+00000013)
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