Hell Oh Entropy!

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FUDCon Notes on my Security Exploits Session

Posted: Nov 10, 2011, 10:17

I was asked¬† by a couple of people for notes on the security exploits session that I conducted at FUDCon. I had posted the code samples on the talk page, but that is probably a little terse, so here’s a little write-up to support the code samples. To repeat what I had said multiple times earlier; I am not a security researcher, not even a security freak. This topic was suggested to me by Amit Shah, and I developed an interest in it due to my original interest, which is operating systems tools. The preparation of this talk got me interested in security, but only through the perspective of operating systems tools and programs, so I am still relatively indifferent to the subject of web-based security.

I started preparing for the session fairly late; i.e. 2 days before FUDCon. I am a little familiar with glibc code and with the way the compiler, linker, loader, etc. work on Linux, so that helped me understand a lot of the concepts behind exploits fairly easily. But concepts != working code and getting exploit code to work was the real challenge, especially when I had just about 3 evenings+nights for it. I had started with an idea of showing stack smashing and privilege escalation examples, but given the time constraint, audience level (college students) and also the constraint of my knowledge, I decided to restrict it to stack based attacks. All of the examples have a buffer which is being written to without checking for bounds of that buffer, typically with an strcpy.

The shellcode sample:

The shellcode sample as well as the final vulnerability demo (smash.c and vulnerable.c) were derived from the article Stack Smashing for Fun and Profit. That is a great article that explains in much more detail than I went into in my session, as to how the shellcode exploit can be developed.The core idea of this is:

The exploit is fairly straightforward, except that the instructions no longer work as is on Linux. These instructions require that the process image is set up in a manner that the page mapped to implement the program stack should have execute permissions. By default on recent Linux distributions (I tried this on F-15, but I am certain this should be true for at least F-13, if not earlier), the linker writes out binaries in a manner that the stack, when set up for a process, only has read and write permissions.

I spent a lot of time trying to figure out where this was set and finally found the -z option of the linker. So to write out a binary that sets up an executable stack, I had to call the linker with -z execstack. This finally enabled me to get the shellcode working.

The actual exploit

Once the shellcode was done, I could get the final vulnerability working and I immediately set about trying it. The exploit is based on the above shellcode example, except for one difference. The shellcode example is just that, an example. It is not an actual exploit; it is just a roundabout way to get a shell. The exploit I was about to do was a real crack. The idea now is to accept a string as input, which is then fed in to make a regular and buggy program provide you with a shell.

To imagine how this would work, think of the program that gives you a login prompt. In the context of this exploit, you should be able to input a crafted string into this login prompt and have it give you a shell without actually knowing the password! This is what the actual exploit ought to look like.

Again, writing the exploit was the easy part; getting it to run was quite another thing altogether. The exploit works as follows:

In all of this, there is one assumption that caused the program sample to not work; the assumption that memory maps are at predictable addresses. Recent kernels (quite some time ago actually) have a new security mechanism called Address Space Randomization which ensures that memory pages are loaded at random offsets. This meant that our educated guess would no longer work. So to be able to actually do this demo, I would have to disable address space randomization. I do that with:

echo 0 > /proc/sys/kernel/randomize_va_space

Even with this, my example would not execute by itself and would end with a SIGILL. I suspect this has something to do with the fact that my systm is x86_64 while the samples are all 32-bit. Our overflow string does not seem to agree with the instruction set on my system. In any case, it seems to run just fine inside a debugger. So if you run smash to get a shell, run gdb vulnerable and then run it with $EGG, you get the shell! At least I had a demo now.

Jump to libc

While I was trying out the shellcode example, I continued thinking about various other ways in which I could get a shell. One of the methods I thought of was to overwrite the return address with the address of the system() glibc call and pass the string via stack. I later found out via Huzaifa that this is in fact a documented way to exploit unchecked buffers on stack. Huzaifa also said that I may be missing out on something there and gave me some tips on finding the right resources for this. I still could not get this working, but at least I found out why the exploit did not work.

This exploit seemed attractive to me because it does not require an executable stack. The instructions I want to execute are already there in memory. So I only have to overwrite the return address and continue writing “/bin/sh” on the stack. I first tried with x86_64 in this case, because I was going by my own idea at that time. I soon figured out that the system() function on x86_64 did not take function arguments from stack. It took the argument from the %rdi register. My devious plan had been foiled! I did not give up however and looked at the system() implementation on i686. This retained the old behaviour of popping arguments from the stack, so my exploit was still possible here.

Not. My code was correct, but every time I run the program, the address of system() had just 3 bytes set. So it would always look something like: 0x00aabbcc. This was bad news because this meant that I cannot continue writing the shell string into the stack (strcpy stops copying when it encounters a 0x0). This means that I can call system() (like I was able to on x86_64 too), but I cannot pass it an argument. After trying enough number of times, I concluded that this must be a security feature. This was backed up by the tip Huzaifa had shared with me to (ironically) get the exploit to work. This was perhaps the first documentation of a return to libc exploit by Solar Designer. In his explanation, Solar designer mentioned that a way to fix this would be to ensure a 0x00 in the address, which is precisely what is happening here.

This obviously does not deny the fact that such an exploit can be carried out if you want to call functions that do not have arguments. Think for example, of a function that executes a shell ;)

Conclusion

The last modify example was a simple little trick I wrote on the last day to demonstrate how buffer overflows work and how they can be used to alter program flow. That again is not an exploit at all. At most, it can be called… a buffer overflow ;)

I had even more fun preparing for this session than actually presenting it because it taught me a lot more than I could ever have done by just reading literature. I hope those who attended my session at FUDCon enjoyed the session too.

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FUDCon Pune 2011 Day 3: Hack and eat

Posted: Nov 07, 2011, 03:44

The last day of FUDCon. I had not slept much the last couple of nights, so I slept in a little late. Due to this I reached the venue late too and found that a lot of the speakers had reached late and that seemed to have got the volunteers (understandably) a little annoyed. All of the action was to happen just in the auditorium and the seminar halls this time and I stuck mostly to the auditorium for most of the day. I had planned to work on the libgqpid library, but I had not decided what it is that I was going to do. I started off by writing my day 1 post. That was quickly followed by lunch, where we hogged on pizzas.

I told Kushal by that time that I will work on autotoolizing libgqpid since he was busy working on his book. I started working on that after lunch, quickly finding out that there are a few things that wouldn’t work very easily there.

libgqpid is a wrapper around the apache c++ client library and the library check for the qpid client library would have to be in c++. autotools does not have any macros for c++ library checks, so I had to write a check that looked like this:

AC_LANG_PUSH([C++])

AC_MSG_CHECKING([if qpid c++ client libraries are present]) AC_LINK_IFELSE( [AC_LANG_PROGRAM([#include <qpid/messaging/Connection.h>], [qpid::messaging::Connection con])], [QPID_LIBS=“$LIBS”], [AC_MSG_ERROR([qpid c++ client development libraries not found])])

AC_LANG_POP

After this one little hurdle it was pretty much smooth sailing and the result was a pull request to Kushal for the patch. By early evening, everyone was done and ready to go. The volunteers, especially the girls were very excited about getting their pictures clicked and were calling all of the major organizers (Rahul, Amit, Satya, etc.) to get their pictures clicked with them. It was pretty entertaining to watch.

This was followed by a cake, which was mostly eaten and the rest of it smeared on Rahul and Jared’s faces. To end the event, we had a feedback session with the volunteers and they gave us a few good tips on how we could have done some things better.

By the time we were back at Magarpatta City, everyone looked tired. Rahul had organized a parting dinner at the Cocoon for all the speakers and organizers, which was a lot of fun. I had a great time chatting with Heherson, Izhar, Arun S A G, Srishti Sethi, Anurag and Eugene Teo. After dinner it was time to head back home.

We did a lot of things right in this event and kudos to Rahul, Amit Shah and all of the core team for getting together a really great event. I hope that at least some of all of those college students make the transition from being users to being contributors, especially contributing code to Fedora and upstream projects.

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FUDCon Pune 2011 Day 2: Me, followed by lunch, followed by me, followed by me...

Posted: Nov 07, 2011, 03:12

The title pretty much summarizes what most of my day looked like on day 2 of FUDCon. Well, not exacly, but it comes quite close. I had three sessions lined up in a single day and I was worried that I might lose my voice by the end of it. Ankur Sinha had all of 4 talks in the single day, so I was definitely better off that him.

The day started with Harish Pillay’s keynote on the community architecture team. The turnout on day 2 was less than that on day 1, which was a little surprising. Most of them trickled later in the day, so it meant that a large number of the attendees in Harish’s talk were Red Hatters and the CoEP volunteers. We probably started a little too early for a Saturday.

Immediately following that was my session on qpid messaging. The attendance in the session was modest (about 8-10 people), but the best part was that they were very involved in the session and that made the session worthwhile. Mrugesh Karnik also joined the session mid-way and asked some really good questions that actually helped my session. We ended up doing a queue design for a fictitious stock trading system and I was able to show how the design could scale very easily with a qpid messaging broker in place. Unfortunately, most of the attendees did not have laptops, so I could not engage them in a hands-on session. In fact, that was my story of the day to a large extent. I had intended all my sessions to be hands-on, but most of it never really materialized because most of the audience did not have laptops.

After the qpid session, I spent some time chatting with Sankarshan, Mrugesh, Anurag and Nisha over lunch. After that I decided to double-check my exploit code samples because it was the one session that I had never done before and it was something that is not my area of expertise. The only aspect of the exploits that I was really comfortable with was how they worked and how I could explain that using the usual tools like gdb, objdump, etc.

I was sitting in the speakers lounge cleaning up my examples when Aditya Patawari came in and asked me about my session. That reminded me that I had to actually go into the session :D We quickly left for the classroom and found pjp finishing up his python session, which had a packed audience. Once he was done, a lot of people left, which led me to think that even this talk was going to have a modest audience. However, people trickled in as I was about to begin and by the time I did begin, the room was full.

The exploits session was probably one of the best sessions I have done so far, mainly because I personally enjoyed it. The audience also consisted of people who were interested (exploits are sexy, as someone said later) and I got a lot of questions during and after the session. The talk also seemed to give some people from the audience the impression that I am a security expert, which is flattering but incorrect.

Then came the awesome part where Pai and Yogesh Babar followed up my session with impromptu sessions, which the audience lapped up eagerly as well. Pai talked about extensibility of postgresql by making it call routines in perl (typical dinosaur stuff ;) ) and Yogesh did a talk on kdump. I learnt later that Rahul Sundaram did something similar in one of the seminar halls by asking the audience to “ask him anything about Fedora and Open Source”. Pretty cool stuff.

After Pai and Yogesh were done, it was again time for me to get on to the platform for another session, this time on autotools. This was something I had done multiple times with the same examples, so it was pretty uneventful.

Day 2 was probably awaited by a lot of people for another reason – the FUDPub! We went to Park Estique near Vimaan Nagar for dinner. There was loud music and bling bling lights and food and drink. I enjoyed the food and drink; the lights gave my a headache and the loud music was, well, too loud. In any case it was fun chatting with people and having the really good food.

Like the first day, I did not get to attend any other sessions, this time for a different reason. I’ll probably submit less sessions in the next conference so that I actually get to attend other sessions and meet and talk to more people. I did meet a lot of interesting people on day 2, so all of that hectic schedule was completely worth it.

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FUDCon Pune 2011 Day 1: The lounge monitor edition

Posted: Nov 06, 2011, 07:55

So we’re on the final day of FUDCon Pune now and I realized that I hadn’t written about my experience at the event at all. The last two days were very hectic and I got only about 6-7 hours of sleep on both days altogether.

I arrived at the venue at 7:30 AM to help out with registrations. Satya was in charge of that and we did not have any hiccups there. People trickled in fairly slowly and hence it was easier than we had thought it would be. We had a good crowd in; going by the fact that the auditorium was about half full, thee may have been over 250-300 people in there. The auditorium capacity is 900.

After a quick welcome note from the CoEP management, we kicked off the event with Jared Smith’s keynote on Fedora. Jared came up with the familiar analogy of streams to explain the way Fedora and upstream communities interact. He talked about how community members could transition from users of Fedora to contributors in Fedora. It was a great talk in general. I had one personal bone to pick in that however, which is the kind of options conveyed to students as avenues to contribute to Fedora.

I have been interacting with students on and off for about 2 years or so now and also been noticing the avenues that students seem to prefer to contribute to Fedora. One of the things I have always been concerned about in this regard is the fact that most students seem to prefer being ambassadors and volunteers and generally doing as less code as possible. In that context, stressing about non-technical ways to contribute was not something I agree with. Of course, it is not something that is wrong with Jared’s message in the talk; it is the fact that the message may be received incorrectly that I am concerned about. This is also why I am generally more inclined towards doing technical sessions instead of broad talks especially if students are the audience.

Anyway, so that was pretty much the only talk I attended, since I was the designated hall monitor for the speakers lounge. My job was to make sure that things run smoothly there and help speakers if they needed anything. While sitting there, I decided to work on the code for my exploits demo. I had wasted about 4 hours last night trying to get the shellcode exploit to work. I had found out in the end that the gold linker builds binaries without execute permissions for the stack and to et an executable stack, I would have to provide ‘-z execstack’ as an extra argument to the linker.

Huzaifa and Eugene Teo came in while I was working on this and Huzaifa introduced me to Eugene. This gave me a chance to pick their brains about the stuff I was about to do in the exploits workshop. Huzaifa pointed me to a few resources and also gave tips about various things I could add to my session to make it more complete. I gave up working on this after the crowd in the lounge increased and returned to chatting with people, some who I was meeting after a long time and others who I had met for the very first time.

I occasionally walked over to the sessions happening nearby to see how things were going and was pleasantly surprised that these sessions had good attendance too. I was concerned about the fact that we may find it difficult to direct attendees to the classrooms since they were a good 5-7 minute walk from the main auditorium and the seminar halls. On the contrary Arun Sag’s introductory session on Linux and Linux commands was hugely successful with the classroom being packed full.

So the first day of FUDCon was a good day of meeting friends for me. The second day was very hectic too and I will write a separate post about it since now it is time for me to work on Kushal’s libgqpid.

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FUDCon Pune 2011: Less than 48 hours to go!

Posted: Nov 02, 2011, 18:19

Firstly,

and I’ve got a lot of things to do once I’m there!

FUDCon Pune 2011 has a wonderful line-up of talks and sessions going for it this weekend. I have added three workshops of my own too:

I finally got involved in preparations for FUDCon this week to try and help wherever needed. Rahul suggested I help out with the schedule arrangement, since I had some ideas on the layout of some sessions. We slugged it out for about 4 hours yesterday and another 3 hours today at the Red Hat office, trying to make sure we schedule talks in a manner that attendees can follow tracks (virtualization, security, web apps, embedded, etc.) and at the same time, have enough flexibility to add talks and sessions without having to shuffle things around all the time. Saleem has become a pro at copy-pasting across cells in spreadsheets as a result.

The end result is pretty impressive and we’re expecting even more submissions to keep people busy. There were concerns on whether there were too many sessions running in parallel, but I don’t think that matters. The event attendance is expected to be quite large, so there will e enough audience to keep all speakers busy. Besides, I don’t know a single conference where one gets to attend each and every talk in the conference. The real fun of the conference is to meet friends, collaborators and fellow geeks and not just sitting in rooms and listening to people talk.

Oh, and we have hackfests throughout the day on Sunday. The one I am particularly psyched about is Kushal Das’ libgqpid. It was an idea we had brainstormed about earlier and he wrote a lot of the code in it. Hopefully we can get a release out on Sunday with the core qpid client features ready. For the uninitiated (which is pretty much everyone I guess, since Kushal has not published the code yet), libgqpid is a glib based C wrapper around the qpid C++ client.

Looking forward to having a great time at FUDCon!

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