Block Oriented Programming Compiler (BOPC)
What is BOPC
NEW: The talk from CCS'18 presentation is available here.
BOPC (stands for BOP Compiler) is a tool for automatically synthesizing arbitrary, Turing-complete, Data-Only payloads. BOPC finds execution traces in the binary that execute the desired payload while adhering to the binary's Control Flow Graph (CFG). This implies that the existing control flow hijacking defenses are not sufficient to detect this style of execution, as execution does never violates the Control Flow Integrity (CFI).
Essentially, we can say that Block Oriented Programming is code reuse under CFI.
BOPC works with basic blocks (hence the name "block-oriented"). What it does is to find a set of functional blocks (i.e., blocks that perform useful computations). This step is somewhat similar with finding Return Oriented Programming (ROP) gadgets. Having the functional blocks, BOPC looks for dispatcher blocks to that are used to stitch functional blocks together. Compared to ROP (that we can move from one gadget to the next without any limitation), here we can't do that as it would violate the CFI. Instead, BOPC finds a proper sequence for dispatcher blocks that naturally lead the execution from one functional block to the next one. Unfortunately the problem of building Data-Only payloads is NP-hard. However it turns out that in practice BOPC finds solution in a reasonable amount of time.
For more details on how BOPC works, please refer to our paper, and our slides from CCS'18.
To operate, BOPC requires 3 inputs:
- A target binary that has an Arbitrary Memory Write (AWP) vulnerability (hard requirement)
- The desired payload, expressed in a high level language called SPL (stands for SPloit Language)
- The so-called "entry point", which is the first instruction in the binary that the payload execution should start. There can be more than one entry points and determining it is part of the vulnerability discovery process.
The output of BOPC is a set of "what-where" memory writes that indicate how the memory should be initialized (i.e., what values to write at which memory addresses). When the execution reaches the entry point and the memory is initialized according to the output of BOPC, the target binary execute the desired payload instead of continuing the original execution.
Disclaimer: This is a research project coded by a single guy. It's not a product, so do not expect it to work perfectly under all scenarios. It works nicely for the provided test cases, but beyond that we cannot guarantee that will work as expected.
Installation
Just run setup.sh :)
How to use BOPC
BOPC started as a hacky project, so several changes made to adapt it into an scientific
context. That is, the implementation in the paper is slightly
different from the actual implementation, as we omitted several implementation details
from the paper. The actual implementation overview is shown below:
Command line arguments explained
A good place to start are the command line arguments:
usage: BOPC.py [-h] [-b BINARY] [-a {save,load,saveonly}] [--emit-IR] [-d]
[-dd] [-ddd] [-dddd] [-V] [-s SOURCE] [-e ENTRY]
[-O {none,ooo,rewrite,full}] [-f {raw,idc,gdb}] [--find-all]
[--mapping-id ID] [--mapping MAP [MAP ...]] [--enum-mappings]
[--abstract-blk BLKADDR] [-c OPTIONS [OPTIONS ...]]
optional arguments:
-h, --help show this help message and exit
General Arguments:
-b BINARY, --binary BINARY
Binary file of the target application
-a {save,load,saveonly}, --abstractions {save,load,saveonly}
Work with abstraction file
--emit-IR Dump SPL IR to a file and exit
-d Set debugging level to minimum
-dd Set debugging level to basic (recommended)
-ddd Set debugging level to verbose (DEBUG ONLY)
-dddd Set debugging level to print-everything (DEBUG ONLY)
-V, --version show program's version number and exit
Search Options:
-s SOURCE, --source SOURCE
Source file with SPL payload
-e ENTRY, --entry ENTRY
The entry point in the binary that payload starts
-O {none,ooo,rewrite,full}, --optimizer {none,ooo,rewrite,full}
Use the SPL optimizer (Default: none)
-f {raw,idc,gdb}, --format {raw,idc,gdb}
The format of the solution (Default: raw)
--find-all Find all the solutions
Application Capability:
-c OPTIONS [OPTIONS ...], --capability OPTIONS [OPTIONS ...]
Measure application's capability. Options (can be many)
all Search for all Statements
regset Search for Register Assignments
regmod Search for Register Modifications
memrd Search for Memory Reads
memwr Search for Memory Writes
call Search for Function/System Calls
cond Search for Conditional Jumps
load Load capabilities from file
save Save capabilities to file
noedge Dump statements and exit (don't calculate edges)
Debugging Options:
--mapping-id ID Run the Trace Searching algorithm on a given mapping ID
--mapping MAP [MAP ...]
Run the Trace Searching algorithm on a given register mapping
--enum-mappings Enumerate all possible mappings and exit
--abstract-blk BLKADDR
Abstract a specific basic block and exit
Ok, there are a lot of options here (divided into 4 categories) as BOPC can do several things.
Let's start with the General Arguments. To avoid working directly with assembly, BOPC, "abstracts" each basic block into a set of "actions". For more details, please check absblk.py. Abstraction process symbolically executes each basic block in the binary and carefully monitors its actions. The abstraction process can take from a few minutes (for small binaries) to several hours (for the larger ones). Waiting that much every time that you want to run BOPC does not sound a good idea, so BOPC uses an old trick: caching.
The abstraction process depends on the binary and not on the SPL payload nor the entry point,
so we only need to calculate them once per binary. Therefore, we have to calculate the
abstractions only one time, then save them into a file and each time loading them.
The save and saveonly options save the abstractions into a file. The only difference is that
saveonly halts execution after it saves the abstractions, while save continues to search
for a solution. As you can guess, the load option loads the abstractions from a file.
The --emit-IR option is used to "dump" the IR representation of the SPL payload (this is
another intermediate result that you should not worry about it).
BOPC provides 5 verbosity levels: no option, -d, -dd, -ddd and -dddd. I recommend you
to use either the -dd or the -ddd to get a detailed progress status.
Let's get into the Search Options options. The most important arguments here are the
--source (which is a file that contains the SPL payload) and the --entry which is an
address inside the binary that indicates the entry point. Trace searching starts from the
entry point, so this is quite important.
The optimizer (-O option) is double edge knife. On the one hand, it optimizes the SPL
payload to make it more flexible. This means that it increases the likelihood to find a
solution. On the other hand, the search space (along with the execution time) is increased.
The decision is up to the user, hence the use of optimizer is optional. The 2 possible
optimizations are the out of order execution (ooo option) and the statement rewriting
(rewrite option).
The out-of-order optimization reorders payload statements. Consider for example the following SPL payload:
__r0 = 13;
__r1 = 37;
To find a solution here, BOPC must find a functional block for the first statement (__r0 = 13)
then a functional block for the second statement (__r1 = 37) and a set of dispatcher blocks
to connect these two statements. However these functional blocks may be far apart so a dispatcher
may not exist. However there's no difference if you execute the __r0 = 13 statement first
or second as it does not have any dependencies with the other statement. Thus if we rewrite
the payload as follows:
__r1 = 37;
__r0 = 13;
It may be possible to find another set dispatcher blocks, hopefully much smaller
(path A -> B may be much longer than path B -> A) and find a solution.
Internally, this is a two-step process. First the optimizer groups independent
statements together (for more details take a look here) and
generated and augmented SPL IR. Then, the trace search module, permutes statements
within each group, each time resulting in a different SPL payload. However all these
payloads are equivalent. As you can guess there are can be an exponential number of
permutations, so this can take forever. To alleviate that, you can adjust
N_OUT_OF_ORDER_ATTEMPTS configuration parameter and tell BOPC to stop after trying
N iterations, instead of trying all of them.
The statement rewriting is an under development optimization that rewrites
some statements that do not exist in the binary. For instance if the SPL payload
spawns a shell through 'execve()' but the target binary does not invoke
execve() at all, then BOPC fails as there are no functional blocks for that statement.
However, if the target binary invokes execv(), it may be possible to find a solution
by replacing execve() with execv(). The optimizer contains a list of possible replacements,
and adjust payload accordingly.
As we already explained, the output of BOPC is a set of "what-where" memory writes. There
are several ways to express the output. For instance they can be raw lines containing the
address, the value and the size of the data that should be written in memory. Or they can
be a gdb/IDA script that can run directly on the debugger and modify the memory accordingly.
The last option is the best one as it you only need to run the BOPC output into the debugger.
Currently only the gdb format is implemented.
The Application Capability options used to measure Application's capabilities, that gives us upper bounds on what payloads the target binary is capable of executing.
Finally the Debugging Options assist the audit/debugging/development process. They are used
to bypass parts of the BOP work-flow. Do not use them unless you're doing changes in the code.
Recall that BOPC finds a mapping between virtual and host registers along with a mapping
between SPL variables and underlying memory addresses. If that mapping does not lead to
a solution it goes back and tries another one. If you want to focus on a specific mapping
(e.g., let's say that code crashes at mapping 458), you don't have to wait for BOPC to try
the first 457 mappings first. By supplying the --mapping-id=458 option you can skip
all mappings and focus on that one. In case that you don't know the mapping number but you
know the actual mapping you can instead you the --mapping option: --mapping=__r0=rax __r1=rbx`
Finally, BOPC has a lot of configuration options. You see all of them in config.py and adjust them according to our needs. The default values are a nice trade off between accuracy and performance that I found during then evaluation.
Example
Let's see now how to actually use BOPC. The first thing to do is to get the basic block abstractions. This step is optional, but I expect that you are going to run BOPC several times, so it's a good idea to get the abstractions first:
./source/BOPC.py -dd --binary $BINARY --abstractions saveonly
This calculates the abstractions and saves them into a file named $BINARY.abs. Don't forget
to enable debugging to see the status on the screen.
Writing an SPL payload is pretty much like writing C:
void payload()
{
string prog = "/bin/sh\0";
int argv = {&prog, 0x0};
__r0 = &prog;
__r1 = &argv;
__r2 = 0;
execve(__r0, __r1, __r2);
}Please take a look at the available payloads to see all features of SPL. Don't expect to write crazy program with SPL; Yes, in theory you can write any program. In practice the more complicated is the SPL payload, the more the complexity increases and the harder it gets to find a solution.
Running BOPC is as simple as the following:
./source/BOPC.py -dd --binary $BINARY --source $PAYLOAD --abstractions load \
--entry $ENTRY --format gdb
If everything goes well an *.gdb file will be created that contains the set of memory writes
to execute the desired payload.
Pruning search space
A common problem is that there can be thousands of mappings (it's exponential based on the number of registers and variables that are used). Each mapping can take up to a minute to test (assuming out of order execution and other optimizations), so BOPC may run for days.
However, if you know approximately where a solution could be, you can ask BOPC to quickly find (and verify) it, without trying all mappings. Let's assume that you want to execute the following SPL payload:
void payload()
{
string msg = "This is my random message! :)\0";
__r0 = 0;
__r1 = &msg;
__r2 = 32;
write( __r0, __r1, __r2 );
}Because we have a system call, we know the register mapping:
__r0 <-> rdi, __r1 <-> rsi, __r2 <-> rdx.
Let's assume that we're on proftpd binary which contains the following "all-in-one"
functional block:
.text:000000000041D0B5 loc_41D0B5:
.text:000000000041D0B5 mov edi, cs:scoreboard_fd ; fd
.text:000000000041D0BB mov edx, 20h ; n
.text:000000000041D0C0 mov esi, offset header ; buf
.text:000000000041D0C5 call _writeThe abstractions for this basic block, will be the following (recall that to get the
abstractions for a single basic block, you need to pass the --abstract-blk 0x41D0B5
in the command line).
[22:02:07,822] [+] Abstractions for basic block 0x41d0b5:
[22:02:07,823] [+] regwr :
[22:02:07,823] [+] rsp = {'writable': True, 'const': 576460752303359992L, 'type': 'concrete'}
[22:02:07,823] [+] rdi = {'sym': {}, 'memrd': None, 'type': 'deref', 'addr': <BV64 0x66e9e0>, 'deps': []}
[22:02:07,823] [+] rsi = {'writable': True, 'const': 6787008L, 'type': 'concrete'}
[22:02:07,823] [+] rdx = {'writable': False, 'const': 32L, 'type': 'concrete'}
[22:02:07,823] [+] memrd : set([(<SAO <BV64 0x66e9e0>>, 32)])
[22:02:07,823] [+] memwr : set([(<SAO <BV64 0x7ffffffffff07f8>>, <SAO <BV64 0x41d0ca>>)])
[22:02:07,823] [+] conwr : set([(576460752303359992L, 64)])
[22:02:07,823] [+] splmemwr : []
[22:02:07,823] [+] call : {}
[22:02:07,823] [+] cond : {}
[22:02:07,823] [+] symvars : {}
[22:02:07,823] [*]
Here, __r0 <-> rdi is loaded indirectly and the value of __r1 <-> rsi (which holds the msg
variable) is 6787008 or 0x678fc0 in hex. Then we enumerate all possible mappings with the
--enum-mappings option. Here, there are 287 possible mappinges, but there are instances that
we have thousands of mappings:
If we look at the output we can quickly search for the appropriate mapping, which in our case is mapping #89:
[.... TRUNCATED FOR BREVITY ....]
[21:59:28,471] [*] Trying mapping #88:
[21:59:28,471] [*] Registers: __r0 <-> rdi | __r1 <-> rsi | __r2 <-> rdx
[21:59:28,471] [*] Variables: msg <-> *<BV64 0x7ffffffffff1440>
[21:59:28,614] [*] Trying mapping #89:
[21:59:28,614] [*] Registers: __r0 <-> rdi | __r1 <-> rsi | __r2 <-> rdx
[21:59:28,614] [*] Variables: msg <-> 0x678fc0L
[21:59:28,762] [*] Trying mapping #90:
[21:59:28,762] [*] Registers: __r0 <-> rdi | __r1 <-> rsi | __r2 <-> rdx
[21:59:28,762] [*] Variables: msg <-> *<BV64 r12_56287_64 + 0x28>
[.... TRUNCATED FOR BREVITY ....]
[22:00:04,709] [*] Trying mapping #287:
[22:00:04,709] [*] Registers: __r0 <-> rdi | __r1 <-> rsi | __r2 <-> rdx
[22:00:04,709] [*] Variables: msg <-> *<BV64 __add__(((0#32 .. rbx_294059_64[31:0]) << 0x5), r12_294068_64, 0x10)>
[22:00:04,979] [+] Trace searching algorithm finished with exit code 0
Now that we know the actual mapping, we can tell BOPC to focus on this one. All we have to
do is to pass the --mapping-id 89 option.
We run this and after 1 minute and 51 seconds later, we get the solution:
#
# This file has been created by BOPC at: 29/03/2018 22:04
#
# Solution #1
# Mapping #89
# Registers: __r0 <-> rdi | __r1 <-> rsi | __r2 <-> rdx
# Variables: msg <-> 0x678fc0L
#
# Simulated Trace: [(0, '41d0b5', '41d0b5'), (4, '41d0b5', '41d0b5'), (6, '41d0b5', '41d0b5'), (8, '41d0b5', '41d0b5'), (10, '41d0b5', '41d0b5')]
#
break *0x403740
break *0x41d0b5
# Entry point
set $pc = 0x41d0b5
# Allocation size is always bigger (it may not needed at all)
set $pool = malloc(20480)
# In case that rbp is not initialized
set $rbp = $rsp + 0x800
# Stack and frame pointers aliases
set $stack = $rsp
set $frame = $rbp
set {char[30]} (0x678fc0) = {0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x6d, 0x79, 0x20, 0x72, 0x61, 0x6e, 0x64, 0x6f, 0x6d, 0x20, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x21, 0x20, 0x3a, 0x29, 0x00}
set {char[8]} (0x66e9e0) = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
Let's take a closer look here. The Simulated Trace comment shows the path that BOPC followed.
This is a list of ($pc, $src, $dst) tuples. $pc is the program counter of the SPL statement.
$src is the address of the functional block for the current SPL statement and $dst is the
address of the next functional block.
Before it runs, script adjusts $rip to point to the entry point, and makes sure that
stack pointers ($rsp, $rbp) are valid. It also allocates a "variable pool" (for
more details please look at simulate.py) which in our case is not
used.
Then we have the two actual memory writes at 0x678fc0 and at 0x66e9e0. If you load
the binary in gdb and run this script you will see your payload being executed:
(gdb) break main
Breakpoint 5 at 0x4041a0
(gdb) run
Starting program: /home/ispo/BOPC/evaluation/proftpd
Breakpoint 1, 0x00000000004041a0 in main ()
(gdb) continue
Continuing.
Breakpoint 3, 0x000000000041d0b5 in pr_open_scoreboard ()
(gdb) continue
Continuing.
Breakpoint 2, 0x0000000000403740 in write@plt ()
(gdb) continue
Continuing.
This is my random message! :)
Program received signal SIGSEGV, Segmentation fault.
0x00007fffffffde60 in ?? ()
Note that BOPC stops after executing the desired payload (hence the crash). If you
want to avoid this situation you can use the returnto SPL statement to naturally
transfer execution to a safe location.
Measuring application capabilities
NOTE: This is a new concept, which is not mentioned in the paper.
Beyond finding Data-Only payloads, BOPC provides some basic capability measurements.
Although it is not related to the Block Oriented Programming, it can provide upper
bounds and strong "indications" on what types of payloads can be executed and what
are not. This is very useful as we can quickly find types of payloads that cannot
be executed in the target binary.
To get the all application capabilities run the following code:
./source/BOPC.py -dd --binary $BINARY --abstractions load --capability all save
If you want to simply dump all functional gadgets for a specific statement, you can do it as follows:
./source/BOPC.py -dd --binary $BINARY --abstractions load --capability $STMT noedge
Where $STMT can be one ore more from {all, regset, regmod, memrd, memwr, call, cond}.
The noedge option is to boost things up (essentially it does not calculate edges in the
capability graph; Each node in the capability graph represents a functional block from
the binary while and edge represents the context-sensitive shortest path distance
between two functional blocks).
Final Notes (please read them carefully!)
-
When the symbolic execution engine deals with filesystem (i.e., it has to
opena file), we have to provide it a valid file. Filename is defined inSYMBOLIC_FILENAMEin coreutils.py. -
If you want to visualize things, just uncomment the code in search.py. I'm too lazy to add CLI flags to trigger it :P
-
In case that addresses used by concolic execution do not work, adjust them from simulate.py
-
Make sure that
$rspis consistent indump()in simulate.py -
For any questions/concerns regarding the code, you can contact ispo
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