New cloud attack takes full control of virtual machines with little effort
arstechnica.com
arstechnica.com
Memory errors are particularly disturbing because they are often highly dependent on data and access patterns, and can be extremely difficult to pinpoint without special testing tools. I've personally experienced a situation where a system which otherwise appears to work perfectly well would always corrupt one specific bit of a file when extracting one particular archive.
As a testing tool, MemTest86+ has always worked well for me, and the newer versions can detect rowhammer, although there is this interesting discussion about whether it is actually a problem (to which I say a resounding YES!!!) or if there's some sort of cover-up by the memory industry:
http://www.passmark.com/forum/memtest86/5903-rowhammer-probl...
http://www.passmark.com/forum/memtest86/5475-memtest86-v6-2-...
Run it on your hardware and if it fails, I think you should definitely complain and get it fixed.
Meanwhile, small teams in academia are building CPU's that knock out those and other issues. Worth bringing up given the fix you want isnt doable for most HW dedigners. RAM vendors might eventually use it as a differentiator but that's not guaranteed.
Server grade hardware is certainly available to cloud/VPS providers, but it turns out people are unwilling to pay $2 for a VM if there's one going elsewhere for $1.50.
The customers expect the RAM they bought to work correctly. They might have even read papers on ASIC verification where the hardware companies brag about all these techniques they use to prevent recalls like one Intel had. The issue is that the companies stopped doing or reduced verification on specific components to reduce costs. What they bring in on the chips is way more than it takes to do that. So, the reason must be greed driving the profits up a little bit.
This one is the companies' fault. I'd have assigned blame differently if we were talking security of regular, consumer products or even operating systems. Verification of repeating pieces of hardware circuits is an industry-standard practice, though. Except for RAM providers apparently.
One of the main use-cases for these technologies is trusted computing in a cloud environment - the customer can assert that the hardware is securing the program state from the eyes of the computer owner!.
However, the cloud is actually made from cheap commodity boxes without server-grade anything! ;)
Encrypting RAM pages would prevent the hypervisor from deduping pages between virtual machines, and this would be very negative for cloud providers who want to up the occupancy on each box as much as possible...
In a few years, or perhaps longer, perhaps proper DDR4 and other immune memory will be mainstream in clouds. But until then, it seems we'll have a cloud fitted out with increasingly aging cheap machines with no rowhammer immunity.
Google, for example, are famous for making big data centres out of cheap commodity boxes, and I double Amazon are any different. I certainly know the rackspace blades I've played with didn't make my grade of either! :)
You know, I refuse to buy anything that does not support ECC for my home desktops (and don't even pay much for it). Only my laptop got a pass from this because there was literally no option available with it.
Good to know cloud providers are not as careful... But honestly, shouldn't be a surprise.
1. RAM that works at this price.
2. RAM that allows more crashes or corruption of your files for slightly-lower price.
The Right Thing suddenly looks more obvious except to cheap skates. Now I just need one with ChipKill built-in. That's the next level of ECC. I haven't heard whether Intel or AMD got something similar.
Intel's encrypted RAM is authenticated quite nicely, but it's not (yet?) designed for general purpose use -- it's for SGX only right now. Using it for everything would (if I understand correctly) add considerable space overhead and possibly considerable latency.
I see no reason to prevent someone from implementing this sort of error correction for GPG and other important cryptography.
Rowhammer is too powerful a technique to expect secure software to run on machines affected by it. This is an attack based on using rowhammer to change bits in other VM's memory. The only sane response to that, from the perspective of writing secure software, is despair. You can't deal with attackers in possession of that primitive.
In theory they could attack a new bit every few minutes, but that requires a system that allows the victim page to be remapped multiple times. KSM does not; any other memory-merging system could work the same way to mitigate things.
Even if they could keep remapping, it's a very slow attack that way. Reloading the checksum every ten minutes would keep you safe.
I too am of this opinion and am surprised this view isn't widely shared. With DDR4, we should be asking for a refund and/or starting a class-action suit, yet we're putting up with software 'mitigations' instead.
This isn't like the 2008 Phenom TLB bug [1] where the CPU was locking up so AMD released a workaround that kept it from freezing at the expense of a 14% performance penalty. This is like the floating point division bug [2] where the device no longer meets basic operational and accuracy guarantees. RAM cells bleeding into each other ought to be considered a fatal flaw, not some intellectual curiosity.
[1] http://techreport.com/review/13741/phenom-tlb-patch-benchmar...
But my original intention was pointing out that the failure mode of the Phenom was such that it wasn't exploitable for anything other than potentially denial-of-service; it was just inconvenient, and only affected a subsystem of the CPU which worked fine without it using a firmware workaround.
Though you don't expect your CPU to halt and lock up, I believe it's far more insidious when you feed a device inputs and get the wrong output without any obvious indication that something went wrong, like in the case of rowhammer-vulnerable memory and FDIV.
Rowhammer is a much more complex errata and I don't feel qualified to comment on, especially the safety of the published mitigations, but it is in a class of bugs where the outcomes are not generally predictable due to more variables involved.
My reason for replying initially though, is that I don't think that the line for what types of hardware defects are open to software workarounds is so cut and dry, and I don't think many people outside of kernel/OS dev realize how many errata are on the chips they use everyday with workarounds they don't notice.
I extensively test all the hardware I buy (CPU: LINPACK, RAM: MemTest86+) and if it fails any of those tests, it gets returned as "not fit for purpose". I've done this successfully a few times. A lot of other enthusiasists/power users do the same too, especially if they're overclocking, and searches on other forums show plenty of users testing and finding (mostly other, not rowhammer) errors in newly-bought RAM even when not overclocking. But as noted in the threads I linked to, manufacturers may be trying to cover this up and downplay its severity. Even in the original paper on rowhammer, the authors didn't disclose which manufacturers and which modules were affected, although I think this should really be treated like the FDIV bug: name and shame. I blame political correctness...
I assume just compiling it and execute some tests that are part of it?
It's funny you mention this, since the problem only affects newer DDR3 and DDR4 modules and older RAM (EDO modules are apparently still in limited production and being sold) does tend to be significantly more expensive. Unfortunately the rest of the hardware needs to be compatible.
This also means all the older hardware that gets scrapped in massive quantities daily is likely to contain RAM immune to this problem, which is somewhat ironic... maybe it's just a (sad) continuation of the "newer is more volatile" trend that can be traced back to thousand-year-old stone tablets which remain readable today.
About the main point, why isn't ECC fixing this for everybody? I'll surely get cheaper, more volatile RAM, and use some of it on redundancy so it works better than the more expensive, less volatile kind.
The market has chosen to adopt the cost benefits of smaller transistors and higher capacity for the same $. It's a mix of physics and market forces, not malfunctioning hardware.
So you're not obtaining original private keys, you're altering original public keys so that you can more quickly factor a private key that will be accepted.
If this is an SSH public key, then you can obtain SSH access. If it's a PGP key trusted by the package manager, then you can craft signatures on packages that would be accepted as valid, assuming you can also get the target machine to download said package.
I think SSH is probably the most interesting attack vector assuming you can get network access to the host once you've jumped through the myriad hoops to perform this attack.
It's a serious issue that should be addressed (probably via forced from-disk reads or at minimum integrity checks), but I think the authors are perhaps a little too eager on the practical implications of corrupting in-memory public keys.
That the attackers illustrated it by changing public keys so they could push updates or ssh into a box doesn't mean that's all the ways they could have compromised. You can't say "I don't use SSH so I'm safe!" or anything like that.
DDR memory is laid out in an array of rows and columns, which are assigned in large blocks to various applications and operating system resources. To protect the integrity and security of the entire system, each large chunk of memory is contained in a "sandbox" that can be accessed only by a given app or OS process. Bit flipping works when a hacker-developed app or process accesses two carefully selected rows of memory hundreds of thousands of times in a tiny fraction of a second. By hammering the two "aggressor" memory regions, the exploit can reverse one or more bits in a third "victim" location. In other words, selected zeros in the victim region will turn into ones or vice versa.
The other bit is the newer idea (well, old idea, newer actual implementation); memory deduplication by your hypervisor leads to a very minor timing fingerprint when you write to a page of memory that had previously been deduplicated IE the same physical page was shared amongst multiple VM's because it was identical.. until you wrote to it and the OS/hardware had to then Copy-on-Write it out to your own dedicated copy; that has a higher latency than a memory page that is already available directly for writing to you.
Why not turn it off entirely?
Seems the savings would be somewhat offset by having your whole business destroyed because its easy to crack.
It's just that in the face of defective hardware, it's not safe. But this is not surprising, because nothing is safe, so it isn't particularly a criticism of page sharing. This specific attack may have used it, but Rowhammer is a powerful tool. This is not the only way it can be used; it is merely an exemplar.
Another benefit of the salting mechanism is that it allows the administrator to define groups of VMs that are trusted in which sharing will be performed.
disclaimer: I work at VMware and wrote the salting code.
But that's just this attack - the fact that they have that much control over memory means there are FAR FAR FAR more possible attacks.
If you can control memory to that level then you are limited only by your imagination.
The only mitigation I can think of at the moment is ECC memory. And shame on Intel for only supporting that on Xeon.
Allowing multiple domU VMs on the same dom0 (or the equivalent in other hypervisor platforms) to re-use memory and balloon/contract memory on the fly is what enables this.
I'm considering deploying a custom unikernel for protecting the private key data for my app[1], until I have enough money for a Hardware Security Module.
[1] http://security.stackexchange.com/questions/135457/penetrati...
Unless you're very careful about which ranges of physical memory are mapped to each VM.
But the vendor's cloud will not disable sharing pages of physical memory because ____.
This is a great counterpoint to the salesman trying to sell you on "cloud" anything.
Why is it less expensive to use the "cloud"?
One reason is because you do not get your own physical server, including your own RAM.
When the "cloud" buzz began to gain momentum years ago I raised the issue of not knowing who your "neighbors" were on these physical servers that customers are sharing with other customers in datacenters.
As usual, these concerns will just fade into the background... again.
https://forums.aws.amazon.com/thread.jspa?messageID=739485&t...
The 'best' solution is better ram: some vendors are more vulnerable than others.
That AWS don't boast that they are not susceptible to this suggests that perhaps at least some of their setup is?
"In our experience, ECC memory is necessary for server infrastructure, and all the hardware underlying Amazon EC2 uses ECC memory."
While ECC does apparently not completely mitigate Rowhammer, it helps.
This "Flip Feng Shui" wouldn't work in SmartOS simply because the hypervisor does not implement memory deduplication.
Good luck with VMware though.
For those that cannot be on a dedicated server, what changes could be made to the shared VM memory setup to reduce this attack surface?
For the attacks to work, the cloud hosting the VMs must have deduplication enabled so that physical pages are shared between customers.
its not (afaik) heartbleed time. It's bad but the effort required is high and afaik the attacker will replace your key with their key - making it clear you are compromised.
Also, as others have pointed out, this is a hardware issue and the clear solution is to swap out the vulnerable RAM. Yeah, paying more is an "easy" way to have peace of mind (if that's even an option for you as a "cloud hoster"), but that's just backwards IMHO: a security vulnerability on the host's side should not translate into an upsell.
The attack relies on glitching the in-memory key.
Seems to me that a public key should be identified by a cryptographic hash of it, rather than the public keys itself. Then the attacker would need to replace the entire hash, rather than just a few bits, because the hash changes completely just by flipping a single bit in the input.
Even with say a 64 bit checksum then there's only a 1 in 2^64 chance of the randomly modified key/checksum pair matching. But you could use a cryptographic hash as your checksum if you wanted.
I only suggest this not because I think it would be a complete defence against all Rowhammer attacks - it wouldn't - but because the general fragility of the RSA construction means that doing it with any potentially corrupted input gives me the willies. There are other sources of bitflips other than Rowhammer and it just strikes me as a generally good idea not to leak the results of RSA operations performed on potentially bitflipped inputs.