mmap() 熵的 ASLR 位 [英] ASLR bits of Entropy of mmap()

问题描述

我正在研究 x86 系统上 mmap() 的 ASLR 随机化.我在很多地方都读到过用 mmap() 加载的地址有 16 位随机化.

I am studying ASLR randomization of mmap(), on x86 system. I have read in a lot of places that there are 16bits of randomization on the address loaded with mmap().

但在我发现的源代码中:

But in the source code i have found:

static unsigned long mmap_rnd(void)
02  {
03          unsigned long rnd = 0;
04   
05         /*
06          *  8 bits of randomness in 32bit mmaps, 20 address space bits
07          * 28 bits of randomness in 64bit mmaps, 40 address space bits
08          */
09          if (current->flags & PF_RANDOMIZE) {
10                  if (mmap_is_ia32())
11                          rnd = (long)get_random_int() % (1<<8);
12                  else 
13                          rnd = (long)(get_random_int() % (1<<28));
14          }
15          return rnd << PAGE_SHIFT;
16  }

所以，那将只是 8 位的随机性.

So, that would be only 8bits of randomness.

但实际上，运行一些测试，我得到以下地址(stack-heap-mmap)bf937000,09a60000,b774b000

But in fact, running some test, i get the following address (stack-heap-mmap) bf937000,09a60000,b774b000

bfa86000,090ef000,b76e2000

bfa86000,090ef000,b76e2000

如果可以是 b77XX000 和 b76XX000 就超过 16 位了！！！

Its more than 16 bits if it can be b77XX000 and b76XX000!!!!

对此有帮助吗?

推荐答案

PAGE_SHIFT 正在将随机性转移到不同的位位置.您的 mmap 地址之间的区别确实是:

PAGE_SHIFT is shifting that randomness to a different bit position. The difference between your mmap addresses is indeed:

 b774b000
-b76e2000
---------
    69000

我不知道 PAGE_SHIFT 的值是多少，但如果它是 12，那么你有 0x69 差异，完全适合 8 位.

I don't know what the value of PAGE_SHIFT is, but if it's 12 for example, then you have 0x69 difference which perfectly fits in 8-bits.

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