ELF Basic Information

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Program Headers

Zinaelezea kwa loader jinsi ya kupakia ELF kwenye kumbukumbu:

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 - Kichwa cha Programu

Contains the program header tables and metadata itself.

INTERP

Indicates the path of the loader to use to load the binary into memory.

Kidokezo: 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

These headers are used to indicate how to load a binary into memory.
Each LOAD header indicates a region of memory (size, permissions and alignment) and indicates the bytes of the ELF binary to copy in there.

For example, the second one has a size of 0x1190, should be located at 0x1fc48 with permissions read and write and will be filled with 0x528 from the offset 0xfc48 (it doesn’t fill all the reserved space). This memory will contain the sections .init_array .fini_array .dynamic .got .data .bss.

DYNAMIC

This header helps to link programs to their library dependencies and apply relocations. Check the .dynamic section.

NOTE

This stores vendor metadata information about the binary.

  • On x86-64, readelf -n will show GNU_PROPERTY_X86_FEATURE_1_* flags inside .note.gnu.property. If you see IBT and/or SHSTK, the binary was built with CET (Indirect Branch Tracking and/or Shadow Stack). This impacts ROP/JOP because indirect branch targets must start with an ENDBR64 instruction and returns are checked against a shadow stack. See the CET page for details and bypass notes.

CET & Shadow Stack

GNU_EH_FRAME

Defines the location of the stack unwind tables, used by debuggers and C++ exception handling-runtime functions.

GNU_STACK

Contains the configuration of the stack execution prevention defense. If enabled, the binary won’t be able to execute code from the stack.

  • Check with readelf -l ./bin | grep GNU_STACK. To forcibly toggle it during tests you can use execstack -s|-c ./bin.

GNU_RELRO

Indicates the RELRO (Relocation Read-Only) configuration of the binary. This protection will mark as read-only certain sections of the memory (like the GOT or the init and fini tables) after the program has loaded and before it begins running.

In the previous example it’s copying 0x3b8 bytes to 0x1fc48 as read-only affecting the sections .init_array .fini_array .dynamic .got .data .bss.

Note that RELRO can be partial or full, the partial version do not protect the section .plt.got, which is used for lazy binding and needs this memory space to have write permissions to write the address of the libraries the first time their location is searched.

For exploitation techniques and up-to-date bypass notes, check the dedicated page:

Relro

TLS

Defines a table of TLS entries, which stores info about thread-local variables.

Vichwa vya Sehemu

Section headers gives a more detailed view of the 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

Pia inaonyesha eneo, offset, ruhusa lakini pia aina ya data ambayo sehemu hiyo ina.

Sehemu za Meta

  • String table: Inajumuisha strings zote zinazohitajika na ELF file (lakini si zile zinazotumika moja kwa moja na programu). Kwa mfano inajumuisha majina ya sections kama .text au .data. Na ikiwa .text iko kwenye offset 45 katika strings table itatumia nambari 45 katika shamba la name.
  • Ili kupata mahali string table iko, ELF ina pointer kwa string table.
  • Symbol table: Inajumuisha taarifa kuhusu symbols kama jina (offset katika strings table), address, size na metadata nyingine kuhusu symbol.

Sehemu Kuu

  • .text: Maelekezo ya programu yanayotekelezwa.
  • .data: Global variables zilizo na thamani iliyowekwa katika programu.
  • .bss: Global variables zisizotangazwa (au zilizowekwa kuanzia kwa zero). Variables hapa huanzishwa moja kwa moja kwa zero hivyo kuzuia zero zisizohitajika kuongezwa kwenye binary.
  • .rodata: Global variables za kudumu (sehemu ya read-only).
  • .tdata na .tbss: Kama .data na .bss wakati thread-local variables zinapotumika (__thread_local in C++ au __thread in C).
  • .dynamic: Tazama hapa chini.

Symbols

Symbols ni eneo lenye jina katika programu ambalo linaweza kuwa function, global data object, thread-local variables…

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 rekodi ya alama ina:

  • Jina
  • Binding attributes (weak, local or global): Alama ya local inaweza kufikiwa tu na programu yenyewe wakati alama ya global inashirikiwa nje ya programu. Weak object kwa mfano ni function ambayo inaweza kubadilishwa na nyingine.
  • Type: NOTYPE (hakuna aina iliyoainishwa), OBJECT (variable ya data ya kimataifa), FUNC (funsi), SECTION (sehemu), FILE (faili ya chanzo kwa debuggers), TLS (thread-local variable), GNU_IFUNC (indirect function for relocation)
  • Section: index ya sehemu ambako imewekwa
  • Value (anwani katika kumbukumbu)
  • Size

GNU IFUNC (funsi zisizo za moja kwa moja)

  • GCC inaweza kutoa alama za STT_GNU_IFUNC kwa kutumia extension __attribute__((ifunc("resolver"))). The dynamic loader inaita resolver wakati wa upakiaji ili kuchagua utekelezaji halisi (kwa kawaida CPU dispatch).
  • Ukaguzi wa haraka: readelf -sW ./bin | rg -i "IFUNC"

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

glibc ya kisasa inatumia matoleo ya alama. Utaona rekodi katika .gnu.version na .gnu.version_r na majina ya alama kama strlen@GLIBC_2.17. The dynamic linker inaweza kuhitaji toleo maalum wakati wa kutatua alama. Unapofanya relocations kwa mkono (kwa mfano ret2dlresolve) lazima utoe index sahihi ya toleo, vinginevyo utatuzi utashindwa.

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

Kabrasha NEEDED inaonyesha kwamba programu inahitaji kupakia library iliyotajwa ili kuendelea. Kabrasha NEEDED inakamilika mara library ya pamoja inapotumika kikamilifu na kuwa tayari kwa matumizi.

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

Vingizo DT_RPATH (deprecated) na/au DT_RUNPATH vinaathiri mahali ambapo dynamic loader inatafuta dependencies. Mpangilio wa jumla:

  • 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 inaweza kutumika ndani ya RPATH/RUNPATH kurejelea kabrasha la main object. Kutoka kwa mtazamo wa mshambuliaji hili ni muhimu unapodhibiti muundo wa filesystem au environment. Kwa binaries zilizohifadhiwa (AT_SECURE) vigezo vingi vya mazingira vinapuuzwa na 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: Pendelea kutumia vibaya RUNPATHs vinavyoweza kuandikwa au njia zilizo misconfigured na kuhusiana na $ORIGIN zinazomilikiwa na wewe. LD_PRELOAD/LD_AUDIT are ignored in secure-execution (setuid) contexts.

Relocations

Loader pia lazima ifanye relocations kwa dependencies baada ya kuzipakia. Relocations hizi zinaonyeshwa kwenye jedwali la relocation kwa format REL au RELA na idadi ya relocations imepewa katika dynamic sections RELSZ au 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

Relocations za ‘relative’ zilizopakiwa (RELR)

  • Linkers za kisasa zinaweza kutoa relocations za relative zilizofupishwa kwa -z pack-relative-relocs. Hii inaongeza DT_RELR, DT_RELRSZ, na DT_RELRENT entries kwenye sehemu ya dynamic kwa PIEs/shared libraries (inapuuzwa kwa non-PIE executables).
  • Recon: readelf -d ./bin | egrep -i "DT_RELR|RELRSZ|RELRENT"

Relocations za statiki

Ikiwa program imepakwa mahali tofauti na anwani inayopendekezwa (kawaida 0x400000) kwa sababu anwani hiyo tayari inatumika au kwa sababu ya ASLR au sababu nyingine yoyote, static relocation inarekebisha pointers ambazo zilikuwa na thamani zinazotarajia binary ipakwe kwenye anwani iliyopendekezwa.

Kwa mfano, kila section ya aina R_AARCH64_RELATIV inapaswa kuwa imebadilisha anwani kwenye relocation bias pamoja na thamani ya addend.

Dynamic Relocations and GOT

Relocation pia inaweza kurejea external symbol (kama function kutoka dependency). Kwa mfano function malloc kutoka libC. Kisha, loader inapopakua libC kwa anwani fulani na ikikagua wapi function malloc imepakwa, itaandika anwani hiyo kwenye GOT (Global Offset Table) (kielezeshwa kwenye relocation table) pale anwani ya malloc inapaswa kuorodheshwa.

Procedure Linkage Table

Sehemu ya PLT inaruhusu lazy binding, ambayo ina maana kwamba utatatua mahali pa function mara ya kwanza inapofikiwa.

Hivyo wakati programu inaita malloc, kwa vitendo inaita mahali husika pa malloc ndani ya PLT (malloc@plt). Mara ya kwanza inapoitwa inatatua anwani ya malloc na kuihifadhi ili wakati mwingine malloc itakapoitwa, anwani hiyo itumike badala ya msimbo wa PLT.

Modern linking behaviors that impact exploitation

  • -z now (Full RELRO) inazuia lazy binding; PLT entries bado zipo lakini GOT/PLT imewekwa kuwa read-only, hivyo techniques kama GOT overwrite na ret2dlresolve hazitafanya kazi dhidi ya main binary (libraries zinaweza bado kuwa partially RELRO). See:

Relro

  • -fno-plt inafanya compiler kuita external functions kupitia GOT entry directly badala ya kupitia PLT stub. Utaona mfululizo wa call kama mov reg, [got]; call reg badala ya call func@plt. Hii inapunguza matumizi mabaya ya speculative-execution na hubadilisha kidogo mbinu za kutafuta ROP gadget karibu na PLT stubs.

  • PIE vs static-PIE: PIE (ET_DYN with INTERP) inahitaji dynamic loader na inaunga mkono PLT/GOT machinery ya kawaida. Static-PIE (ET_DYN without INTERP) ina relocations zinazotumika na kernel loader na haina ld.so; tarajia hakuna PLT resolution wakati wa runtime.

Ikiwa GOT/PLT sio chaguo, badilisha mkondo kwenda code-pointers nyingine zinazoweza kuandikwa au tumia classic ROP/SROP kuelekea libc.

WWW2Exec - GOT/PLT

Program Initialization

Baada programu imepakwa, ni wakati wa kuikimbisha. Hata hivyo, msimbo wa kwanza unaotekelezwa isn’t always the main function. Hii ni kwa sababu kwa mfano katika C++ ikiwa global variable is an object of a class, kitu hiki lazima initialized before main runs, kama katika:

#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 global variables ziko katika .data au .bss, lakini katika orodha za __CTOR_LIST__ na __DTOR_LIST__ vitu vinavyopaswa kuanzishwa na kuharibiwa vinahifadhiwa ili kufuatilia.

Kutoka kwenye msimbo wa C inawezekana kupata matokeo yale yale kwa kutumia GNU extensions :

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

Kutoka kwa mtazamo wa compiler, ili kutekeleza vitendo hivi kabla na baada ya main kutekelezwa, inawezekana kuunda function ya init na function ya fini ambazo zitatumika katika dynamic section kama INIT na FINI, na zimewekwa katika sehemu za init na fini za ELF.

Chaguo jingine, kama ilivyotajwa, ni kurejelea orodha __CTOR_LIST__ na __DTOR_LIST__ katika ingizo za INIT_ARRAY na FINI_ARRAY kwenye dynamic section na urefu wao unaonyeshwa na INIT_ARRAYSZ na FINI_ARRAYSZ. Kila kipengee ni function pointer itakayoitwa bila argumenti.

Zaidi ya hayo, pia inawezekana kuwa na PREINIT_ARRAY yenye pointers ambazo zitatekelezwa kabla ya INIT_ARRAY pointers.

Exploitation note

  • Under Partial RELRO hizi arrays zinaishi katika pages ambazo bado ni writable kabla ld.so huhamisha PT_GNU_RELRO kuwa read-only. Ikiwa unapata arbitrary write mapema vya kutosha au unaweza kulenga writable arrays za library, unaweza kuiba control flow kwa kuandika juu kipengee kwa function unayotaka. Under Full RELRO zina kuwa read-only wakati wa runtime.

  • For lazy binding abuse of the dynamic linker to resolve arbitrary symbols at runtime, see the dedicated page:

Ret2dlresolve

Mpangilio wa Uanzishaji

  1. Programu inapakiwa kwenye memory, static global variables zinaanzishwa katika .data na zile zisizoanzishwa zinawekwa kwa sifuri katika .bss.
  2. Dependencies zote za programu au libraries zinaanzishwa na dynamic linking inaendeshwa.
  3. PREINIT_ARRAY functions zinatekelezwa.
  4. INIT_ARRAY functions zinatekelezwa.
  5. Ikiwa kuna ingizo la INIT linaitwa.
  6. Ikiwa ni library, dlopen inamalizika hapa; ikiwa ni programu, ni wakati wa kuita real entry point (main function).

Thread-Local Storage (TLS)

Zimetangazwa kwa kutumia keyword __thread_local katika C++ au extension ya GNU __thread.

Kila thread itahifadhi eneo la kipekee kwa variable hii hivyo ni thread pekee itakayoweza kufikia variable yake.

Wakati hii inatumiwa, sections .tdata na .tbss zitatumika katika ELF. Ziko kama .data (initialized) na .bss (not initialized) lakini kwa TLS.

Kila variable itakuwa na rekodi katika TLS header inayoainisha size na TLS offset, ambayo ni offset itakayotumika katika eneo la data la thread.

Symbol __TLS_MODULE_BASE hutumika kurejelea base address ya thread local storage na inaonyesha eneo la memory lenye data zote za thread-local za module.

Auxiliary Vector (auxv) and vDSO

Linux kernel hupitisha auxiliary vector kwa processes ambayo ina anwani muhimu na flags kwa runtime:

  • AT_RANDOM: inaonyesha 16 random bytes zinazotumiwa na glibc kwa stack canary na vyanzo vingine vya PRNG.
  • AT_SYSINFO_EHDR: base address ya vDSO mapping (inayosaidia kupata __kernel_* syscalls na gadgets).
  • AT_EXECFN, AT_BASE, AT_PAGESZ, etc.

Kama mshambuliaji, ikiwa unaweza kusoma memory au files chini ya /proc, mara nyingi unaweza leak hizi bila infoleak katika process la lengo:

# 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));
}

Leaking AT_RANDOM inakupa thamani ya canary ikiwa unaweza dereference pointer hiyo; AT_SYSINFO_EHDR inakupa msingi wa vDSO wa kuchimba gadgets au kupiga fast syscalls moja kwa moja.

Marejeo

  • GCC Common Function Attributes (ifunc / STT_GNU_IFUNC): https://gcc.gnu.org/onlinedocs/gcc-14.3.0/gcc/Common-Function-Attributes.html
  • GNU ld -z pack-relative-relocs / DT_RELR nyaraka: https://sourceware.org/binutils/docs/ld.html
  • ld.so(8) – mpangilio wa utafutaji wa Dynamic Loader, RPATH/RUNPATH, sheria za utekelezaji salama (AT_SECURE): https://man7.org/linux/man-pages/man8/ld.so.8.html
  • getauxval(3) – vektori ya ziada na konstanti za AT_*: https://man7.org/linux/man-pages/man3/getauxval.3.html

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