HackTheBox CTF Writeups Cryptography

ret2dlresolve

Resolving our own libc functions

Broad Overview

During a ret2dlresolve, the attacker tricks the binary into resolving a function of its choice (such as system) into the PLT. This then means the attacker can use the PLT function as if it was originally part of the binary, bypassing ASLR (if present) and requiring no libc leaks.

Detailed Overview

Dynamically-linked ELF objects import libc functions when they are first called using the PLT and GOT. During the relocation of a runtime symbol, RIP will jump to the PLT and attempt to resolve the symbol. During this process a "resolver" is called.

For all these screenshots, I broke at read@plt. I'm using GDB with the pwndbg plugin as it shows it a bit better.

The PLT jumps to wherever the GOT points. Originally, before the GOT is updated, it points back to the instruction after the jmp in the PLT to resolve it.

In order to resolve the functions, there are 3 structures that need to exist within the binary. Faking these 3 structures could enable us to trick the linker into resolving a function of our choice, and we can also pass parameters in (such as /bin/sh) once resolved.

Structures

There are 3 structures we need to fake.

$readelf -d source

Dynamic section at offset 0x2f14 contains 24 entries:
  Tag        Type                         Name/Value
 0x00000005 (STRTAB)                     0x804825c
 0x00000006 (SYMTAB)                     0x804820c
 0x00000017 (JMPREL)                     0x80482d8
 [...]

JMPREL

The JMPREL segment (.rel.plt) stores the Relocation Table, which maps each entry to a symbol.

$readelf -r source

Relocation section '.rel.dyn' at offset 0x2d0 contains 1 entry:
 Offset     Info    Type            Sym.Value  Sym. Name
0804bffc  00000206 R_386_GLOB_DAT    00000000   __gmon_start__

Relocation section '.rel.plt' at offset 0x2d8 contains 2 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
0804c00c  00000107 R_386_JUMP_SLOT   00000000   gets@GLIBC_2.0
0804c010  00000307 R_386_JUMP_SLOT   00000000   __libc_start_main@GLIBC_2.0

These entries are of type Elf32_Rel:

typedef uint32_t Elf32_Addr;
typedef uint32_t Elf32_Word;
typedef struct
{
  Elf32_Addr    r_offset;               /* Address */
  Elf32_Word    r_info;                 /* Relocation type and symbol index */
} Elf32_Rel;
/* How to extract and insert information held in the r_info field.  */
#define ELF32_R_SYM(val)                ((val) >> 8)
#define ELF32_R_TYPE(val)               ((val) & 0xff)

The column name coresponds to our symbol name. The offset is the GOT entry for our symbol. info stores additional metadata.

Note the due to this the R_SYM of gets is 1 as 0x107 >> 8 = 1.

STRTAB

Much simpler - just a table of strings for the names.

SYMTAB

Symbol information is stores here in an Elf32_Sym struct:

typedef struct 
{ 
   Elf32_Word st_name ; /* Symbol name (string tbl index) */
   Elf32_Addr st_value ; /* Symbol value */ 
   Elf32_Word st_size ; /* Symbol size */ 
   unsigned char st_info ; /* Symbol type and binding */ 
   unsigned char st_other ; /* Symbol visibility under glibc>=2.2 */ 
   Elf32_Section st_shndx ; /* Section index */ 
} Elf32_Sym ;

The most important value here is st_name as this gives the offset in STRTAB of the symbol name. The other fields are not relevant to the exploit itself.

Linking the Structures

We now know we can get the STRTAB offset of the symbol's string using the R_SYM value we got from the JMPREL, combined with SYMTAB:

Here we're reading SYMTAB + R_SYM * size (16), and it appears that the offset (the SYMTAB st_name variable) is 0x10.

And if we read that offset on STRTAB, we get the symbol's name!

More on the PLT and GOT

Let's hop back to the GOT and PLT for a slightly more in-depth look.

If the GOT entry is unpopulated, we push the reloc_offset value and jump to the beginning of the .plt section. A few instructions later, the dl-resolve() function is called, with reloc_offset being one of the arguments. It then uses this reloc_offset to calculate the relocation and symtab entries.

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