ELF Msingi wa Taarifa

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Vichwa vya Programu

Vinavyoelezea kwa mzigo jinsi ya kupakia ELF kwenye kumbukumbu:

bash
readelf -lW lnstat

Elf file type is DYN (Position-Independent Executable file)
Entry point 0x1c00
There are 9 program headers, starting at offset 64

Program Headers:
Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
PHDR           0x000040 0x0000000000000040 0x0000000000000040 0x0001f8 0x0001f8 R   0x8
INTERP         0x000238 0x0000000000000238 0x0000000000000238 0x00001b 0x00001b R   0x1
[Requesting program interpreter: /lib/ld-linux-aarch64.so.1]
LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x003f7c 0x003f7c R E 0x10000
LOAD           0x00fc48 0x000000000001fc48 0x000000000001fc48 0x000528 0x001190 RW  0x10000
DYNAMIC        0x00fc58 0x000000000001fc58 0x000000000001fc58 0x000200 0x000200 RW  0x8
NOTE           0x000254 0x0000000000000254 0x0000000000000254 0x0000e0 0x0000e0 R   0x4
GNU_EH_FRAME   0x003610 0x0000000000003610 0x0000000000003610 0x0001b4 0x0001b4 R   0x4
GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10
GNU_RELRO      0x00fc48 0x000000000001fc48 0x000000000001fc48 0x0003b8 0x0003b8 R   0x1

Section to Segment mapping:
Segment Sections...
00
01     .interp
02     .interp .note.gnu.build-id .note.ABI-tag .note.package .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
03     .init_array .fini_array .dynamic .got .data .bss
04     .dynamic
05     .note.gnu.build-id .note.ABI-tag .note.package
06     .eh_frame_hdr
07
08     .init_array .fini_array .dynamic .got

The previous program has 9 program headers, then, the segment mapping indicates in which program header (from 00 to 08) each section is located.

PHDR - Program HeaDeR

Inajumuisha meza za vichwa vya programu na metadata yenyewe.

INTERP

Inaonyesha njia ya loader inayopaswa kutumika kupakia binary kwenye kumbukumbu.

Tip: Statically linked or static-PIE binaries won’t have an INTERP entry. In those cases there is no dynamic loader involved, which disables techniques that rely on it (e.g., ret2dlresolve).

LOAD

Vichwa hivi vinatumika kuonyesha jinsi ya kupakia binary kwenye kumbukumbu.
Kila LOAD header inaonyesha eneo la kumbukumbu (ukubwa, ruhusa na usawa) na inaonyesha bytes za ELF binary za kunakili huko.

Kwa mfano, ya pili ina ukubwa wa 0x1190, inapaswa kuwa kwenye 0x1fc48 ikiwa na ruhusa za kusoma na kuandika na itajazwa na 0x528 kutoka kwenye ofset 0xfc48 (haitajazi nafasi yote iliyohifadhiwa). Kumbukumbu hii itakuwa na sehemu .init_array .fini_array .dynamic .got .data .bss.

DYNAMIC

Hii header inasaidia kuunganisha programu na utegemezi wao wa maktaba na kutekeleza uhamasishaji. Angalia sehemu .dynamic.

NOTE

Hii inahifadhi taarifa za metadata za muuzaji kuhusu binary.

  • Kwenye x86-64, readelf -n itaonyesha bendera GNU_PROPERTY_X86_FEATURE_1_* ndani ya .note.gnu.property. Ikiwa unaona IBT na/au SHSTK, binary ilijengwa na CET (Ufuatiliaji wa Tawi la Kando na/au Stack ya Kivuli). Hii inaathiri ROP/JOP kwa sababu malengo ya tawi la kando lazima yaanze na amri ya ENDBR64 na marejeo yanakaguliwa dhidi ya stack ya kivuli. Angalia ukurasa wa CET kwa maelezo na vidokezo vya kupita.

CET & Shadow Stack

GNU_EH_FRAME

Inafafanua eneo la meza za kugeuza stack, zinazotumiwa na debuggers na kazi za usimamizi wa makosa ya C++.

GNU_STACK

Inajumuisha usanidi wa ulinzi wa kuzuia utekelezaji wa stack. Ikiwa imewezeshwa, binary haitakuwa na uwezo wa kutekeleza msimbo kutoka kwenye stack.

  • Angalia kwa readelf -l ./bin | grep GNU_STACK. Ili kulazimisha kubadilisha wakati wa majaribio unaweza kutumia execstack -s|-c ./bin.

GNU_RELRO

Inaonyesha usanidi wa RELRO (Relocation Read-Only) wa binary. Ulinzi huu utaashiria kama isiyo na kusoma sehemu fulani za kumbukumbu (kama GOT au meza za init na fini) baada ya programu kupakiwa na kabla ya kuanza kutekeleza.

Katika mfano wa awali inakopi bytes 0x3b8 hadi 0x1fc48 kama zisizo na kusoma zikihusisha sehemu .init_array .fini_array .dynamic .got .data .bss.

Kumbuka kwamba RELRO inaweza kuwa ya sehemu au kamili, toleo la sehemu halilindi sehemu .plt.got, ambayo inatumika kwa lazy binding na inahitaji nafasi hii ya kumbukumbu kuwa na ruhusa za kuandika ili kuandika anwani za maktaba wakati wa kwanza mahali pake inatafutwa.

Kwa mbinu za unyakuzi na vidokezo vya kupita vya kisasa, angalia ukurasa maalum:

Relro

TLS

Inafafanua meza ya entries za TLS, ambayo inahifadhi taarifa kuhusu mabadiliko ya ndani ya nyuzi.

Section Headers

Vichwa vya sehemu vinatoa mtazamo wa kina zaidi wa ELF binary.

objdump lnstat -h

lnstat:     file format elf64-littleaarch64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
0 .interp       0000001b  0000000000000238  0000000000000238  00000238  2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .note.gnu.build-id 00000024  0000000000000254  0000000000000254  00000254  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .note.ABI-tag 00000020  0000000000000278  0000000000000278  00000278  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .note.package 0000009c  0000000000000298  0000000000000298  00000298  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .gnu.hash     0000001c  0000000000000338  0000000000000338  00000338  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .dynsym       00000498  0000000000000358  0000000000000358  00000358  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .dynstr       000001fe  00000000000007f0  00000000000007f0  000007f0  2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .gnu.version  00000062  00000000000009ee  00000000000009ee  000009ee  2**1
CONTENTS, ALLOC, LOAD, READONLY, DATA
8 .gnu.version_r 00000050  0000000000000a50  0000000000000a50  00000a50  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
9 .rela.dyn     00000228  0000000000000aa0  0000000000000aa0  00000aa0  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 .rela.plt     000003c0  0000000000000cc8  0000000000000cc8  00000cc8  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 .init         00000018  0000000000001088  0000000000001088  00001088  2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
12 .plt          000002a0  00000000000010a0  00000000000010a0  000010a0  2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
13 .text         00001c34  0000000000001340  0000000000001340  00001340  2**6
CONTENTS, ALLOC, LOAD, READONLY, CODE
14 .fini         00000014  0000000000002f74  0000000000002f74  00002f74  2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .rodata       00000686  0000000000002f88  0000000000002f88  00002f88  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
16 .eh_frame_hdr 000001b4  0000000000003610  0000000000003610  00003610  2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
17 .eh_frame     000007b4  00000000000037c8  00000000000037c8  000037c8  2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
18 .init_array   00000008  000000000001fc48  000000000001fc48  0000fc48  2**3
CONTENTS, ALLOC, LOAD, DATA
19 .fini_array   00000008  000000000001fc50  000000000001fc50  0000fc50  2**3
CONTENTS, ALLOC, LOAD, DATA
20 .dynamic      00000200  000000000001fc58  000000000001fc58  0000fc58  2**3
CONTENTS, ALLOC, LOAD, DATA
21 .got          000001a8  000000000001fe58  000000000001fe58  0000fe58  2**3
CONTENTS, ALLOC, LOAD, DATA
22 .data         00000170  0000000000020000  0000000000020000  00010000  2**3
CONTENTS, ALLOC, LOAD, DATA
23 .bss          00000c68  0000000000020170  0000000000020170  00010170  2**3
ALLOC
24 .gnu_debugaltlink 00000049  0000000000000000  0000000000000000  00010170  2**0
CONTENTS, READONLY
25 .gnu_debuglink 00000034  0000000000000000  0000000000000000  000101bc  2**2
CONTENTS, READONLY

It also indicates the location, offset, permissions but also the aina ya data it section has.

Meta Sections

  • String table: Inayo kila nyuzi zinazohitajika na faili ya ELF (lakini si zile zinazotumiwa na programu). Kwa mfano inajumuisha majina ya sehemu kama .text au .data. Na ikiwa .text iko kwenye offset 45 katika jedwali la nyuzi itatumia nambari 45 katika uwanja wa jina.
  • Ili kupata mahali ambapo jedwali la nyuzi liko, ELF ina kipanga njia kwa jedwali la nyuzi.
  • Symbol table: Inayo taarifa kuhusu alama kama jina (offset katika jedwali la nyuzi), anwani, ukubwa na metadata zaidi kuhusu alama.

Main Sections

  • .text: Maagizo ya programu ya kuendesha.
  • .data: Vigezo vya kimataifa vyenye thamani iliyofafanuliwa katika programu.
  • .bss: Vigezo vya kimataifa vilivyotelekezwa (au kuanzishwa kuwa sifuri). Vigezo hapa vinakaguliwa kiotomatiki kuwa sifuri hivyo kuzuia sifuri zisizohitajika kuongezwa kwenye binary.
  • .rodata: Vigezo vya kimataifa vya kudumu (sehemu ya kusoma tu).
  • .tdata na .tbss: Kama .data na .bss wakati vigezo vya ndani ya thread vinapotumika (__thread_local katika C++ au __thread katika C).
  • .dynamic: Angalia hapa chini.

Symbols

Symbols ni eneo lenye jina katika programu ambalo linaweza kuwa kazi, kitu cha data cha kimataifa, vigezo vya ndani ya thread...

readelf -s lnstat

Symbol table '.dynsym' contains 49 entries:
Num:    Value          Size Type    Bind   Vis      Ndx Name
0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
1: 0000000000001088     0 SECTION LOCAL  DEFAULT   12 .init
2: 0000000000020000     0 SECTION LOCAL  DEFAULT   23 .data
3: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND strtok@GLIBC_2.17 (2)
4: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND s[...]@GLIBC_2.17 (2)
5: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND strlen@GLIBC_2.17 (2)
6: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND fputs@GLIBC_2.17 (2)
7: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND exit@GLIBC_2.17 (2)
8: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND _[...]@GLIBC_2.34 (3)
9: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND perror@GLIBC_2.17 (2)
10: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterT[...]
11: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND _[...]@GLIBC_2.17 (2)
12: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND putc@GLIBC_2.17 (2)
[...]

Kila kipengele cha alama kina:

  • Jina
  • Sifa za uhusiano (dhaifu, za ndani au za kimataifa): Alama ya ndani inaweza kufikiwa tu na programu yenyewe wakati alama za kimataifa zinashirikiwa nje ya programu. Kitu dhaifu ni mfano wa kazi ambayo inaweza kubadilishwa na nyingine.
  • Aina: NOTYPE (aina haijabainishwa), OBJECT (kibadilisha data za kimataifa), FUNC (kazi), SECTION (sehemu), FILE (faili ya msimbo wa chanzo kwa ajili ya wadhibiti), TLS (kibadilisha cha nyuzi za ndani), GNU_IFUNC (kazi isiyo ya moja kwa moja kwa ajili ya uhamasishaji)
  • Sehemu index ambapo iko
  • Thamani (anwani katika kumbukumbu)
  • Ukubwa

GNU Symbol Versioning (dynsym/dynstr/gnu.version)

Modern glibc inatumia toleo la alama. Utapata vipengele katika .gnu.version na .gnu.version_r na majina ya alama kama strlen@GLIBC_2.17. Linker ya dynamic inaweza kuhitaji toleo maalum wakati wa kutatua alama. Wakati wa kuunda uhamasishaji wa mikono (mfano: ret2dlresolve) lazima utoe index sahihi ya toleo, vinginevyo kutatua kutashindwa.

Sehemu ya Dynamic

readelf -d lnstat

Dynamic section at offset 0xfc58 contains 28 entries:
Tag        Type                         Name/Value
0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
0x0000000000000001 (NEEDED)             Shared library: [ld-linux-aarch64.so.1]
0x000000000000000c (INIT)               0x1088
0x000000000000000d (FINI)               0x2f74
0x0000000000000019 (INIT_ARRAY)         0x1fc48
0x000000000000001b (INIT_ARRAYSZ)       8 (bytes)
0x000000000000001a (FINI_ARRAY)         0x1fc50
0x000000000000001c (FINI_ARRAYSZ)       8 (bytes)
0x000000006ffffef5 (GNU_HASH)           0x338
0x0000000000000005 (STRTAB)             0x7f0
0x0000000000000006 (SYMTAB)             0x358
0x000000000000000a (STRSZ)              510 (bytes)
0x000000000000000b (SYMENT)             24 (bytes)
0x0000000000000015 (DEBUG)              0x0
0x0000000000000003 (PLTGOT)             0x1fe58
0x0000000000000002 (PLTRELSZ)           960 (bytes)
0x0000000000000014 (PLTREL)             RELA
0x0000000000000017 (JMPREL)             0xcc8
0x0000000000000007 (RELA)               0xaa0
0x0000000000000008 (RELASZ)             552 (bytes)
0x0000000000000009 (RELAENT)            24 (bytes)
0x000000000000001e (FLAGS)              BIND_NOW
0x000000006ffffffb (FLAGS_1)            Flags: NOW PIE
0x000000006ffffffe (VERNEED)            0xa50
0x000000006fffffff (VERNEEDNUM)         2
0x000000006ffffff0 (VERSYM)             0x9ee
0x000000006ffffff9 (RELACOUNT)          15
0x0000000000000000 (NULL)               0x0

The NEEDED directory indicates that the program inahitaji kupakia maktaba iliyoelezwa ili kuendelea. The NEEDED directory completes once the shared maktaba imefanya kazi kikamilifu na iko tayari kwa matumizi.

Dynamic loader search order (RPATH/RUNPATH, $ORIGIN)

The entries DT_RPATH (deprecated) and/or DT_RUNPATH influence where the dynamic loader searches for dependencies. Rough order:

  • LD_LIBRARY_PATH (ignored for setuid/sgid or otherwise "secure-execution" programs)
  • DT_RPATH (only if DT_RUNPATH absent)
  • DT_RUNPATH
  • ld.so.cache
  • default directories like /lib64, /usr/lib64, etc.

$ORIGIN can be used inside RPATH/RUNPATH to refer to the directory of the main object. From an attacker perspective this matters when you control the filesystem layout or environment. For hardened binaries (AT_SECURE) most environment variables are ignored by the loader.

  • Inspect with: readelf -d ./bin | egrep -i 'r(path|unpath)'
  • Quick test: LD_DEBUG=libs ./bin 2>&1 | grep -i find (shows search path decisions)

Priv-esc tip: Prefer abusing writable RUNPATHs or misconfigured $ORIGIN-relative paths owned by you. LD_PRELOAD/LD_AUDIT are ignored in secure-execution (setuid) contexts.

Relocations

The loader also must relocate dependencies after having loaded them. These relocations are indicated in the relocation table in formats REL or RELA and the number of relocations is given in the dynamic sections RELSZ or RELASZ.

readelf -r lnstat

Relocation section '.rela.dyn' at offset 0xaa0 contains 23 entries:
Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000001fc48  000000000403 R_AARCH64_RELATIV                    1d10
00000001fc50  000000000403 R_AARCH64_RELATIV                    1cc0
00000001fff0  000000000403 R_AARCH64_RELATIV                    1340
000000020008  000000000403 R_AARCH64_RELATIV                    20008
000000020010  000000000403 R_AARCH64_RELATIV                    3330
000000020030  000000000403 R_AARCH64_RELATIV                    3338
000000020050  000000000403 R_AARCH64_RELATIV                    3340
000000020070  000000000403 R_AARCH64_RELATIV                    3348
000000020090  000000000403 R_AARCH64_RELATIV                    3350
0000000200b0  000000000403 R_AARCH64_RELATIV                    3358
0000000200d0  000000000403 R_AARCH64_RELATIV                    3360
0000000200f0  000000000403 R_AARCH64_RELATIV                    3370
000000020110  000000000403 R_AARCH64_RELATIV                    3378
000000020130  000000000403 R_AARCH64_RELATIV                    3380
000000020150  000000000403 R_AARCH64_RELATIV                    3388
00000001ffb8  000a00000401 R_AARCH64_GLOB_DA 0000000000000000 _ITM_deregisterTM[...] + 0
00000001ffc0  000b00000401 R_AARCH64_GLOB_DA 0000000000000000 __cxa_finalize@GLIBC_2.17 + 0
00000001ffc8  000f00000401 R_AARCH64_GLOB_DA 0000000000000000 stderr@GLIBC_2.17 + 0
00000001ffd0  001000000401 R_AARCH64_GLOB_DA 0000000000000000 optarg@GLIBC_2.17 + 0
00000001ffd8  001400000401 R_AARCH64_GLOB_DA 0000000000000000 stdout@GLIBC_2.17 + 0
00000001ffe0  001e00000401 R_AARCH64_GLOB_DA 0000000000000000 __gmon_start__ + 0
00000001ffe8  001f00000401 R_AARCH64_GLOB_DA 0000000000000000 __stack_chk_guard@GLIBC_2.17 + 0
00000001fff8  002e00000401 R_AARCH64_GLOB_DA 0000000000000000 _ITM_registerTMCl[...] + 0

Relocation section '.rela.plt' at offset 0xcc8 contains 40 entries:
Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000001fe70  000300000402 R_AARCH64_JUMP_SL 0000000000000000 strtok@GLIBC_2.17 + 0
00000001fe78  000400000402 R_AARCH64_JUMP_SL 0000000000000000 strtoul@GLIBC_2.17 + 0
00000001fe80  000500000402 R_AARCH64_JUMP_SL 0000000000000000 strlen@GLIBC_2.17 + 0
00000001fe88  000600000402 R_AARCH64_JUMP_SL 0000000000000000 fputs@GLIBC_2.17 + 0
00000001fe90  000700000402 R_AARCH64_JUMP_SL 0000000000000000 exit@GLIBC_2.17 + 0
00000001fe98  000800000402 R_AARCH64_JUMP_SL 0000000000000000 __libc_start_main@GLIBC_2.34 + 0
00000001fea0  000900000402 R_AARCH64_JUMP_SL 0000000000000000 perror@GLIBC_2.17 + 0
00000001fea8  000b00000402 R_AARCH64_JUMP_SL 0000000000000000 __cxa_finalize@GLIBC_2.17 + 0
00000001feb0  000c00000402 R_AARCH64_JUMP_SL 0000000000000000 putc@GLIBC_2.17 + 0
00000001fec0  000e00000402 R_AARCH64_JUMP_SL 0000000000000000 fputc@GLIBC_2.17 + 0
00000001fec8  001100000402 R_AARCH64_JUMP_SL 0000000000000000 snprintf@GLIBC_2.17 + 0
00000001fed0  001200000402 R_AARCH64_JUMP_SL 0000000000000000 __snprintf_chk@GLIBC_2.17 + 0
00000001fed8  001300000402 R_AARCH64_JUMP_SL 0000000000000000 malloc@GLIBC_2.17 + 0
00000001fee0  001500000402 R_AARCH64_JUMP_SL 0000000000000000 gettimeofday@GLIBC_2.17 + 0
00000001fee8  001600000402 R_AARCH64_JUMP_SL 0000000000000000 sleep@GLIBC_2.17 + 0
00000001fef0  001700000402 R_AARCH64_JUMP_SL 0000000000000000 __vfprintf_chk@GLIBC_2.17 + 0
00000001fef8  001800000402 R_AARCH64_JUMP_SL 0000000000000000 calloc@GLIBC_2.17 + 0
00000001ff00  001900000402 R_AARCH64_JUMP_SL 0000000000000000 rewind@GLIBC_2.17 + 0
00000001ff08  001a00000402 R_AARCH64_JUMP_SL 0000000000000000 strdup@GLIBC_2.17 + 0
00000001ff10  001b00000402 R_AARCH64_JUMP_SL 0000000000000000 closedir@GLIBC_2.17 + 0
00000001ff18  001c00000402 R_AARCH64_JUMP_SL 0000000000000000 __stack_chk_fail@GLIBC_2.17 + 0
00000001ff20  001d00000402 R_AARCH64_JUMP_SL 0000000000000000 strrchr@GLIBC_2.17 + 0
00000001ff28  001e00000402 R_AARCH64_JUMP_SL 0000000000000000 __gmon_start__ + 0
00000001ff30  002000000402 R_AARCH64_JUMP_SL 0000000000000000 abort@GLIBC_2.17 + 0
00000001ff38  002100000402 R_AARCH64_JUMP_SL 0000000000000000 feof@GLIBC_2.17 + 0
00000001ff40  002200000402 R_AARCH64_JUMP_SL 0000000000000000 getopt_long@GLIBC_2.17 + 0
00000001ff48  002300000402 R_AARCH64_JUMP_SL 0000000000000000 __fprintf_chk@GLIBC_2.17 + 0
00000001ff50  002400000402 R_AARCH64_JUMP_SL 0000000000000000 strcmp@GLIBC_2.17 + 0
00000001ff58  002500000402 R_AARCH64_JUMP_SL 0000000000000000 free@GLIBC_2.17 + 0
00000001ff60  002600000402 R_AARCH64_JUMP_SL 0000000000000000 readdir64@GLIBC_2.17 + 0
00000001ff68  002700000402 R_AARCH64_JUMP_SL 0000000000000000 strndup@GLIBC_2.17 + 0
00000001ff70  002800000402 R_AARCH64_JUMP_SL 0000000000000000 strchr@GLIBC_2.17 + 0
00000001ff78  002900000402 R_AARCH64_JUMP_SL 0000000000000000 fwrite@GLIBC_2.17 + 0
00000001ff80  002a00000402 R_AARCH64_JUMP_SL 0000000000000000 fflush@GLIBC_2.17 + 0
00000001ff88  002b00000402 R_AARCH64_JUMP_SL 0000000000000000 fopen64@GLIBC_2.17 + 0
00000001ff90  002c00000402 R_AARCH64_JUMP_SL 0000000000000000 __isoc99_sscanf@GLIBC_2.17 + 0
00000001ff98  002d00000402 R_AARCH64_JUMP_SL 0000000000000000 strncpy@GLIBC_2.17 + 0
00000001ffa0  002f00000402 R_AARCH64_JUMP_SL 0000000000000000 __assert_fail@GLIBC_2.17 + 0
00000001ffa8  003000000402 R_AARCH64_JUMP_SL 0000000000000000 fgets@GLIBC_2.17 + 0

Static Relocations

Ikiwa programu imepakuliwa mahali tofauti na anwani inayopendelea (kawaida 0x400000) kwa sababu anwani hiyo tayari inatumika au kwa sababu ya ASLR au sababu nyingine yoyote, uhamasishaji wa statiki unarekebisha viashiria ambavyo vilikuwa na thamani zikitarajia binary kupakuliwa katika anwani inayopendelea.

Kwa mfano, sehemu yoyote ya aina R_AARCH64_RELATIV inapaswa kuwa na anwani iliyorekebishwa kwa bias ya uhamasishaji pamoja na thamani ya kuongeza.

Dynamic Relocations and GOT

Uhamasishaji unaweza pia kurejelea alama ya nje (kama kazi kutoka kwa utegemezi). Kama kazi malloc kutoka libC. Kisha, mpakuwaji anapopakua libC katika anwani akichunguza mahali ambapo kazi malloc imepakuliwa, itaandika anwani hii katika jedwali la GOT (Global Offset Table) (iliyotajwa katika jedwali la uhamasishaji) ambapo anwani ya malloc inapaswa kufafanuliwa.

Procedure Linkage Table

Sehemu ya PLT inaruhusu kufanya uhusiano wa polepole, ambayo inamaanisha kwamba ufumbuzi wa mahali pa kazi utafanywa mara ya kwanza inapotumika.

Hivyo wakati programu inaita malloc, kwa kweli inaita mahali husika pa malloc katika PLT (malloc@plt). Mara ya kwanza inapotumika inapata anwani ya malloc na kuihifadhi ili wakati malloc inaitwa tena, anwani hiyo inatumika badala ya msimbo wa PLT.

Modern linking behaviors that impact exploitation

  • -z now (Full RELRO) inazima uhusiano wa polepole; entries za PLT bado zipo lakini GOT/PLT imepangwa kuwa ya kusoma tu, hivyo mbinu kama GOT overwrite na ret2dlresolve hazitafanya kazi dhidi ya binary kuu (maktaba zinaweza bado kuwa sehemu RELRO). Tazama:

Relro

  • -fno-plt inafanya kompyuta kuita kazi za nje kupitia GOT entry moja kwa moja badala ya kupitia PLT stub. Utapata mfululizo wa wito kama mov reg, [got]; call reg badala ya call func@plt. Hii inapunguza matumizi mabaya ya utekelezaji wa dhana na kubadilisha kidogo uwindaji wa ROP gadget kuzunguka PLT stubs.

  • PIE vs static-PIE: PIE (ET_DYN na INTERP) inahitaji mpakuwaji wa dynamic na inasaidia mashine ya kawaida ya PLT/GOT. Static-PIE (ET_DYN bila INTERP) ina uhamasishaji unaotumika na mpakuwaji wa kernel na hakuna ld.so; tarajia hakuna ufumbuzi wa PLT wakati wa kutekeleza.

Ikiwa GOT/PLT si chaguo, hamasisha kwenye viashiria vingine vya msimbo vinavyoweza kuandikwa au tumia ROP/SROP ya kawaida ndani ya libc.

WWW2Exec - GOT/PLT

Program Initialization

Baada ya programu kupakuliwa ni wakati wa kuikimbia. Hata hivyo, msimbo wa kwanza unaotekelezwa sio kila wakati kazi ya main. Hii ni kwa sababu kwa mfano katika C++ ikiwa kigezo cha kimataifa ni kitu cha darasa, kitu hiki lazima kiwe kimeanzishwa kabla ya main kuendesha, kama katika:

cpp
#include <stdio.h>
// g++ autoinit.cpp -o autoinit
class AutoInit {
public:
AutoInit() {
printf("Hello AutoInit!\n");
}
~AutoInit() {
printf("Goodbye AutoInit!\n");
}
};

AutoInit autoInit;

int main() {
printf("Main\n");
return 0;
}

Kumbuka kwamba hizi variables za kimataifa ziko katika .data au .bss lakini katika orodha __CTOR_LIST__ na __DTOR_LIST__ vitu vya kuanzisha na kuharibu vinahifadhiwa ili kufuatilia.

Kutoka kwa msimbo wa C inawezekana kupata matokeo sawa kwa kutumia nyongeza za GNU:

c
__attributte__((constructor)) //Add a constructor to execute before
__attributte__((destructor)) //Add to the destructor list

From a compiler perspective, to execute these actions before and after the main function is executed, it's possible to create a init function and a fini function which would be referenced in the dynamic section as INIT and FIN. and are placed in the init and fini sections of the ELF.

The other option, as mentioned, is to reference the lists __CTOR_LIST__ and __DTOR_LIST__ in the INIT_ARRAY and FINI_ARRAY entries in the dynamic section and the length of these are indicated by INIT_ARRAYSZ and FINI_ARRAYSZ. Each entry is a function pointer that will be called without arguments.

Moreover, it's also possible to have a PREINIT_ARRAY with pointers that will be executed before the INIT_ARRAY pointers.

Ujumbe wa Ukatili

  • Chini ya Partial RELRO hizi arrays zinaishi katika kurasa ambazo bado zinaweza kuandikwa kabla ld.so haijabadilisha PT_GNU_RELRO kuwa isiyoandikwa. Ikiwa unapata kuandika bila mpangilio mapema vya kutosha au unaweza kulenga arrays zinazoweza kuandikwa za maktaba, unaweza kuingilia mchakato wa udhibiti kwa kuandika tena kipengee na kazi unayochagua. Chini ya Full RELRO zinakuwa zisizoandikwa wakati wa utendaji.

  • Kwa matumizi ya lazy binding ya linker ya dynamic kutatua alama zisizo za kawaida wakati wa utendaji, angalia ukurasa maalum:

Ret2dlresolve

Agizo la Uanzishaji

  1. Programu inapoingizwa kwenye kumbukumbu, mabadiliko ya kimataifa ya statiki yanaanzishwa katika .data na yasiyoanzishwa yanawekwa sifuri katika .bss.
  2. Mahitaji yote ya programu au maktaba yana anzishwa na kuunganishwa kwa dynamic kunatekelezwa.
  3. PREINIT_ARRAY kazi zinafanywa.
  4. INIT_ARRAY kazi zinafanywa.
  5. Ikiwa kuna kipengee cha INIT kinaitwa.
  6. Ikiwa ni maktaba, dlopen inamalizika hapa, ikiwa ni programu, ni wakati wa kuita nukta halisi ya kuingia (main function).

Hifadhi ya Mitaa ya Thread (TLS)

Zimefafanuliwa kwa kutumia neno muhimu __thread_local katika C++ au nyongeza ya GNU __thread.

Kila thread itahifadhi eneo la kipekee kwa ajili ya variable hii hivyo ni thread pekee inayoweza kufikia variable yake.

Wakati hii inatumika sehemu .tdata na .tbss zinatumika katika ELF. Ambazo ni kama .data (iliyowekwa) na .bss (siyo iliyowekwa) lakini kwa TLS.

Kila variable itakuwa na kipengee katika kichwa cha TLS kinachoelezea ukubwa na offset ya TLS, ambayo ni offset itakayotumia katika eneo la data la ndani la thread.

The __TLS_MODULE_BASE is a symbol used to refer to the base address of the thread local storage and points to the area in memory that contains all the thread-local data of a module.

Vektori ya Msaidizi (auxv) na vDSO

Kernel ya Linux inapita vektori ya msaidizi kwa michakato inayoelezea anwani na bendera muhimu kwa wakati wa utendaji:

  • AT_RANDOM: inaelekeza kwa bytes 16 za nasibu zinazotumiwa na glibc kwa stack canary na mbegu nyingine za PRNG.
  • AT_SYSINFO_EHDR: anwani ya msingi ya ramani ya vDSO (inasaidia kupata __kernel_* syscalls na gadgets).
  • AT_EXECFN, AT_BASE, AT_PAGESZ, n.k.

Kama mshambuliaji, ikiwa unaweza kusoma kumbukumbu au faili chini ya /proc, mara nyingi unaweza kuvuja hizi bila kuvuja taarifa katika mchakato wa lengo:

bash
# Show the auxv of a running process
cat /proc/$(pidof target)/auxv | xxd

# From your own process (helper snippet)
#include <sys/auxv.h>
#include <stdio.h>
int main(){
printf("AT_RANDOM=%p\n", (void*)getauxval(AT_RANDOM));
printf("AT_SYSINFO_EHDR=%p\n", (void*)getauxval(AT_SYSINFO_EHDR));
}

Kuvuja AT_RANDOM kunakupa thamani ya canary ikiwa unaweza kurejelea kiashiria hicho; AT_SYSINFO_EHDR inakupa msingi wa vDSO ili kutafuta gadgets au kuita syscalls haraka moja kwa moja.

References

  • ld.so(8) – Dynamic Loader search order, RPATH/RUNPATH, secure-execution rules (AT_SECURE): https://man7.org/linux/man-pages/man8/ld.so.8.html
  • getauxval(3) – Auxiliary vector and AT_* constants: https://man7.org/linux/man-pages/man3/getauxval.3.html

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