Generic exploitation of invalid memory writes

Jonathan Brossard

CEO – Toucan System

[email protected]



Memory permissions

Taxonomy of invalid memory access

Determining the type of error

The case of invalid writes


Maybe you did some fuzzing and found some bugs ?

Welcome in 2010 : exploitation is more difficult than finding trivial bugs.

The goal of this talk is to detail (and automate !) the steps to successful exploitation of invalid memory writes.

Memory permissions

This is the key to modern exploitation...

Memory permissions

Under x86 : permissions are set at MMU (+ Kernel) level using three flags :

R : read permission

W : write permission

X : execution permission

5 years back, everything was in fact +X !!

Memory permissions

Where to get my shellcode executed in memory ??

Alternatively (when not using shellcode) : where to return in memory to execute something usefull (think ROP).

Memory permissions

What is or not executable depends on :

-your cpu (NX flag) + BIOS Config.

-your kernel (PaX, PAE...) and more globally your toolchain.

-the dynamic loader (shared libs).


Using readelf and /proc/pid/maps to check what permissions should be.

Using paxtest to verify what is actually executable or not.

Taxonomy of invalid

memory access

What's the impact of this bug ?

int main(int argc, char **argv){ printf(argv[1]) ; return 0;


Exemple 1

Truth is : we can't know from C source : we have to check at instruction level to see if the c library was actually built with a write primitive...

Segfault at...

mov DWORD PTR [eax], edx

eax 0x4000e030

ecx 0xbffff848

edx 0x0

ebx 0x40199ff4

=> Invalid write.

(more complex) exemple:

fld QWORD PTR [eax+0x8]

=> Read ? Write ? Both ??


Triggering the bug in xpdf

Determining the error type of complex bugs using valgrind


At instruction level, an invalid memory access can be of three kinds :

-invalid read

-invalid write

-invalid exec

Exploiting invalid exec

Trivial if jump location is user controled.

eg : call [eax]

Exploitation strategy :

Have eax pointing to our shellcode

Exploiting invalid exec (2/2)

Fairly rare in userland

Used to be a major problem in kernel land (invalid Null ptr dereference in exec mode from kernel land). First page not mappable without privs since kernel 2.6.23

Exploiting invalid reads

Cannot hijack control flow directly :(

Exploitation strategy:

-Either use no memory corruption (eg : information leakage)

-Or trigger a bug latter in the code (either invalid exec or invalid write)

-… Or it is just plain non exploitable :(

Exploiting invalid writes

This is the meat !

Exploitation strategy :

-Either overwrite important data (eg : tast_struct->uid in kernel land)

-Or try to hijack the control flow

The case of invalid writes : different levels of control

X86 allows accessing either :

-8 bytes

-4 bytes (most instructions)

-2 bytes

-1 byte

The case of invalid writes : different levels of control

-If both destination and value are fully user controled : overwrite .dtors, liked list in at_exit, function pointers...

(ideal case such as missing format strings, easy !)

-If only destination is user controled : attempt a pointer truncation on a given function pointer.

Problem is...

Out of 3Gb of user land, how to find a suitable function pointer to overwrite or truncate ?

Exploitation strategy

-Dump the content of every section of the binary

-Parse the +W ones

-Check for pointers to +X locations

=> exhaustive, reduced list of potential candidates.

Then chose one based on actual binary constraints (will truncation point to a user controled location ?)


Triggering an invalid write in Opera

Using livedump to chose potential candidates

Memory alignement

Most Intel instructions only allow reads/writes on aligned boundaries.

Typically : 4b aligned memory

(Common) Worst case


If only 0x00000000 can be written, in

4b aligned memory locations...

=> Then pointer truncation won't be possible on 4b aligned sections :-(

Naive exploitation strategy

-Trigger the bug

-set a signal handler for signal 5 which re enables single stepping

-set the « trap » cpu flag

=> Single step until exit and monitor unaligned reads/writes.

=> Slow, painfull, per thread :(

Elite exploitation strategy

with livedump

-Trigger the bug

-set the « align » cpu flag

-setup a signal handler for SIGBUS

=> every read/write to unaligned memory will be trapped and give potential truncation candidates


Using livedump to detect unaligned memory access

Thank you for coming

Questions ?