Linux常用保护机制

Linux程序常见用的一些保护机制

一、NX(Windows中的DEP)

NX:No-eXecute、DEP:Data Execute Prevention

  • 也就是数据不可执行,防止因为程序运行出现溢出而使得攻击者的shellcode可能会在数据区尝试执行的情况。
  • gcc默认开启,选项有:
gcc -o test test.c      // 默认情况下,开启NX保护
gcc -z execstack -o test test.c  // 禁用NX保护
gcc -z noexecstack -o test test.c  // 开启NX保护

二、PIE(ASLR)

PIE:Position-Independent Excutable、ASLR:Address Space Layout Randomization

-fpic

	Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)

	Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

	When this flag is set, the macros `__pic__` and `__PIC__` are defined to 1.

-fPIC

	If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table.This option makes a difference on AArch64, m68k, PowerPC and SPARC.

	Position-independent code requires special support, and therefore works only on certain machines.

	When this flag is set, the macros `__pic__` and `__PIC__` are defined to 2.

-fpie
-fPIE

	These options are similar to -fpic and -fPIC, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the  -pie  GCC option.

	-fpie and -fPIE both define the macros `__pie__` and `__PIE__`. The macros have the value 1 for `-fpie` and 2 for `-fPIE`.
  • 区别在于fpic/fPIC用于共享库的编译,fpie/fPIE则是pie文件编译的选项。文档中说 pic(位置无关代码)生成的共享库只能链接于可执行文件,之后根据自己编译简单C程序,pie正常运行,即如网上许多文章说的 pie 选项生成的位置无关代码可假定于本程序,但是我也没看出fpie/fPIE有啥区别,只是宏定义只为1和2的区别,貌似...
    编译命令(默认不开启PIE):
gcc -fpie -pie -o test test.c    // 开启PIE
gcc -fPIE -pie -o test test.c    // 开启PIE
gcc -fpic -o test test.c         // 开启PIC
gcc -fPIC -o test test.c         // 开启PIC
gcc -no-pie -o test test.c       // 关闭PIE
  • 而ASLR(地址空间随机化),当初设计时只负责栈、库、堆等段的地址随机化。ASLR的值存于/proc/sys/kernel/randomize_va_space中,如下:

0 - 表示关闭进程地址空间随机化。
1 - 表示将mmap的基址,stack和vdso页面随机化。
2 - 表示在1的基础上增加栈(heap)的随机化。(默认)

更改其值方式:echo 0 > /proc/sys/kernel/randomize_va_space

vDSO:virtual dynamic shared object;
mmap:即内存的映射。
PIE 则是负责可执行程序的基址随机。
以下摘自Wiki:

Position-independent executable (PIE) implements a random base address for the main executable binary and has been in place since 2003. It provides the same address randomness to the main executable as being used for the shared libraries.

PIE为ASLR的一部分,ASLR为系统功能,PIE则为编译选项。
注: 在heap分配时,有mmap()brk()两种方式,由malloc()分配内存时调用,分配较小时brk,否则mmap,128k区别。参考文章:https://blog.csdn.net/gfgdsg/article/details/42709943

三、Canary(栈保护)

  Canary对于栈的保护,在函数每一次执行时,在栈上随机产生一个Canary值。之后当函数执行结束返回时检测Canary值,若不一致系统则报出异常。

  • Wiki
  • Canaries or canary words are known values that are placed between a buffer and control data on the stack to monitor buffer overflows. When the buffer overflows, the first data to be corrupted will usually be the canary, and a failed verification of the canary data will therefore alert of an overflow, which can then be handled, for example, by invalidating the corrupted data. A canary value should not be confused with a sentinel value.

  如上所述,Canary值置于缓冲区和控制数据之间,当缓冲区溢出,该值被覆写,从而可以检测以判断是否运行出错或是受到攻击。缓解缓冲区溢出攻击。

  • 编译选项:
gcc -o test test.c                       //默认关闭
gcc -fno-stack-protector -o test test.c  //禁用栈保护
gcc -fstack-protector -o test test.c     //启用堆栈保护,不过只为局部变量中含有 char 数组的函数插入保护代码
gcc -fstack-protector-all -o test test.c //启用堆栈保护,为所有函数插入保护代码

四、RELRO(RELocation Read Only)

在Linux中有两种RELRO模式:”Partial RELRO“”Full RELRO“。Linux中Partical RELRO默认开启。

Partial RELRO:

  • 编译命令:
    gcc -o test test.c // 默认部分开启
    gcc -Wl,-z,relro -o test test.c // 开启部分RELRO
    gcc -z lazy -o test test.c // 部分开启
  • 该ELF文件的各个部分被重新排序。内数据段(internal data sections)(如.got,.dtors等)置于程序数据段(program's data sections)(如.data和.bss)之前;
  • 无 plt 指向的GOT是只读的;
  • GOT表可写(应该是与上面有所区别的)。

Full RELRO:

  • 编译命令:
    gcc -Wl,-z,relro,-z,now -o test test.c // 开启Full RELRO
    gcc -z now -o test test.c // 全部开启
  • 支持Partial模式的所有功能;
  • 整个GOT表映射为只读的。

gcc -z norelro -o a a.c // RELRO关闭,即No RELRO

Note:

  • .dtors:当定义有.dtors的共享库被加载时调用;
  • 在bss或数据溢出错误的情况下,Partial和Full RELRO保护ELF内数据段不被覆盖。 但只有Full RELRO可以缓解GOT表覆写攻击,但是相比较而言开销较大,因为程序在启动之前需要解析所有的符号。

参考文章:

posted @ 2019-03-02 22:44  Bl0od  阅读(2103)  评论(0编辑  收藏  举报