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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
[email protected]. 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.
1
$readelf -d source
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​
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Dynamic section at offset 0x2f14 contains 24 entries:
4
Tag Type Name/Value
5
0x00000005 (STRTAB) 0x804825c
6
0x00000006 (SYMTAB) 0x804820c
7
0x00000017 (JMPREL) 0x80482d8
8
[...]
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JMPREL
The
JMPREL segment (.rel.plt) stores the Relocation Table, which maps each entry to a symbol.1
$readelf -r source
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​
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Relocation section '.rel.dyn' at offset 0x2d0 contains 1 entry:
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Offset Info Type Sym.Value Sym. Name
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0804bffc 00000206 R_386_GLOB_DAT 00000000 __gmon_start__
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​
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Relocation section '.rel.plt' at offset 0x2d8 contains 2 entries:
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Offset Info Type Sym.Value Sym. Name
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0804c00c 00000107 R_386_JUMP_SLOT 00000000 [email protected]_2.0
10
0804c010 00000307 R_386_JUMP_SLOT 00000000 [email protected]_2.0
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These entries are of type
Elf32_Rel:1
typedef uint32_t Elf32_Addr;
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typedef uint32_t Elf32_Word;
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typedef struct
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{
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Elf32_Addr r_offset; /* Address */
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Elf32_Word r_info; /* Relocation type and symbol index */
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} Elf32_Rel;
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/* How to extract and insert information held in the r_info field. */
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#define ELF32_R_SYM(val) ((val) >> 8)
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#define ELF32_R_TYPE(val) ((val) & 0xff)
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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.
0x0804825c is the location of STRTAB we got earlier
SYMTAB
Symbol information is stores here in an
Elf32_Sym struct:1
typedef struct
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{
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Elf32_Word st_name ; /* Symbol name (string tbl index) */
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Elf32_Addr st_value ; /* Symbol value */
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Elf32_Word st_size ; /* Symbol size */
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unsigned char st_info ; /* Symbol type and binding */
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unsigned char st_other ; /* Symbol visibility under glibc>=2.2 */
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Elf32_Section st_shndx ; /* Section index */
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} Elf32_Sym ;
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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.Resources
Last modified 1yr ago
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