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Project Zero Exploits 'Unexploitable' Glibc Bug
NotInHere (3654617) writes with news that Google's Project Zero has been busy at work. A month ago they reported an off-by-one error in glibc that would overwrite a word on the heap with NUL and were met with skepticism at its ability to be used in an attack. Google's 'Project Zero' devised an exploit of the out-of-bounds NUL write in glibc to gain root access using the setuid binary pkexec in order to convince skeptical glibc developers. 44 days after being reported, the bug has been fixed.
They even managed to defeat address space randomization on 32-bit platforms by tweaking ulimits. 64-bit systems should remain safe if they are using address space randomization.
Actually, I find the arrogance of calling an obvious bug "unexploitable" disturbing.
Most ARM is 32 bit...
Wow, sent an e-mail as suggested when clicking on "use classic" banner, and got a fast response that addressed my msg
CAN YOU HEAR ME NOW?? HELLO?
Never say never.
Unexploitable? Srsly? GAC.
An acquaintance recently posted "Six Stages of Debugging" on his g+ page. (1. That can't happen, 2. That doesn't happen on my machine, 3. That shouldn't happen, 4. Why does that happen? 5. Oh, I see, and 6. How did that ever work). Doesn't an software dev who has been working for more than about three years go straight to No. 4?
The things they don't teach you in a CS degree.
Embedded stuff would typically use uClibc. Android uses Bionic libc.
Most ARM might be 32 bit but most ARM doesn't use Glibc.
The first part is arrogance. The second part is pragmatic humility.
I read through the thread and at no point was the bug considered "Unexploitable". Even skepticism is too strong of a word to use. The only doubt that was raised was asking "How likely is that?"
Reminds me of this overflow bug which was fixed in sudo 1.6.3p6. It writes a single NUL byte past the end of a buffer, calls syslog(), and the restores the original overwritten byte. Seems unexploitable, right?
Wrong. Here's the detailed writeup of the exploit. It requires some jiggering with the parameters to get the exploit to work on a particular system, but you don't need a local root exploit to work every time, you just need it to work once and you own the system.
One that a slight slip anywhere in millions of lines of code could produce random memory corruptions with unpredictable consequences. Who would have believed that anybody would even dream of using a language with constructs such as ptr++. And we are surprised to find bugs...
The word you're looking for is 'skeptical', and then they went and fixed it when they were proven wrong. This is actually the opposite of arrogant.
They should have fixed the bug as soon as they realized it was there, and not waited until someone proved it was an especially bad bug.
"First they came for the slanderers and i said nothing."
Meanwhile, slopping programming in any language results in unintended side effects.
Yes, but the lack of bounds checking in C is kind of crazy. The compiler is now going out of its way to delete error-checking code simply because it runs into "undefined behavior," but no matter how obvious a bounds violation is, the compiler won't even mention it. Go ahead and try it. Create an array, index it with an immediate value of negative one, and compile. It won't complain at all. ...but god-forbid you accidentally write code that depends upon signed overflow to function correctly, because that's something the compiler needs to notice and do something about, namely, it needs to remove your overflow detection code because obviously you've memorized the C standards in their entirety and you're infallible, and there's no chance whatsoever that anyone ever thought that "undefined behavior" might mean "it'll just do whatever the platform the code was compiled for happens to do" rather than "it can do anything at all, no matter how little sense it makes."
Due to just how well GCC optimizes code, bounds checking wouldn't be a huge detriment to program execution speed. In some cases the compiler could verify at compile time that bounds violations will not occur. At other times, it could find more logical ways to check, like if there's a "for (int i = 0; i < some_variable; i++)" used to index an array, the compiler would know that simply checking "some_variable" against the bounds of the array before executing the loop is sufficient. I've looked at the code GCC generates, and optimizations like these are well within its abilities. The end result is that bounds checking wouldn't hinder execution speeds as much as everyone thinks. A compare and a conditional jump isn't a whole lot of code to begin with, and with the compiler determining that a lot of those tests aren't even necessary, it simply wouldn't be a big deal.
No.
Off by ones are much easier to fix than to prove safe. The amounts of bugs called "unexploitable" until an exploit was provided is staggering. No mildly security aware person will avoid fixing a buffer overflow because it is unexploitable.
Shachar
While you have a point, you shouldn't forget the Raspberry Pi. It is probably the most popular internet facing non-mobile ARM platform today. Literally millions of these run glibc and at least hundreds of thousands are in some way or form directly connected to the internet. While I don't believe that this bug can be exploited without first gaining RCE on the raspberry pi, once an attacker gets access to the rpi, this bug should be able to get them to escalate to root privileges.
There are quite a few people that put a full debian (or other) distribution on their NAS server. I own a zyxel NSA 325 and it is possible to install a full debian release on this and some other NAS boxes. These might be a limited amount of systems overall, but it's significant enough to deserve mentioning because they too often are internet facing.
I was promised a flying car. Where is my flying car?
You can already ask some compilers to do what you are asking - it's just often not on in shipped builds.
At compilation time warnings can be generated for out of bounds accesses that can be determined statically. Clang has -fsanitize=bounds, GCC has -Warray-bounds.
As an Anonymous Coward pointed out, it can be hard to detect runtime allocations overruns at compilation time. For these something like Clang's AddressSanitizer (GCC has added it too will help but at a cost of both time (slow down factor of 2) and space which is why you're unlikely to find it enabled on your precompiled SSH server binary. It's true there are cheaper checks (such as GCC's FORTIFY_SOURCE) that are less thorough/specialized that are often enabled by distros.
It's an oldschool attitude to not touch things, from back in the day where software was so flaky that chances were someone had already 'exploited' the bug to do something non-malicious.
Ah, that actually makes sense, good analysis.
. It's pretty obvious from the description what the bug is, so saying you aren't going to fix it is, as you say, pure laziness.
This sort of thing worries me about glibc, and the attitude that 'bugs are no big deal' is a dangerous one that is infecting software developers all over.
"First they came for the slanderers and i said nothing."
64-bit systems should remain safe if they are using address space randomization.
Nah. It just takes more crashes before the exploit achieves penetration.
(Address space randomization is a terrible idea. It's a desperation measure and an excuse for not fixing problems. In exchange for making penetration slightly harder, you give up repeatable crash bug behavior.)
The OpenBSD philosophy says that the difference between a bug and a vulnerability is the intelligence of the attacker. There are lots of categories of bugs (null pointer dereferences, integer overflows) that were thought to be unexploitable, right up until someone exploited them. It's the same as with cryptosystems: the fact that you can't break your encryption algorithm doesn't mean that it's secure.
I am TheRaven on Soylent News
The first part is also pragmatic. Releasing a security fix is a lot of work, not just for the developers, but also for everybody else. So you only do that if you have reasonable suspicion that the bug is a security risk. They were good reasons to believe that it is not the case here, although in the end they did not apply in every situation.
If you treat every bug as a security issue, you end up with the Google situation where only one version, the latest, is ever supported. And for libc that is not an acceptable option.
It is one thing to say we will not fix it right now because of the costs and the unlikely of seeing this in the wild. It is quite another to call it unexploitable. The former is pragmatism. The later is hubris.