Xen Project Spectre/Meltdown FAQ
blog.xenproject.org
blog.xenproject.org
> Meltdown and Spectre are, by themselves, only information leaks. There is no suggestion that speculative execution can be used to modify memory or cause a system to do anything it might not have done already.
Aren't they a bit too casual here? The answer is not wrong in the sense that Meltdown and Spectre themselves can't directly be used for privilege escalation.
On the other hand the question they posed was: "Is there any risk of privilege escalation?" and I wouldn't be so quick about that. If I can read arbitrary kernel memory isn't there a chance (at least under some circumstances) that I can find something (clear text or hashed root password maybe) that enables trivial privilege escalation?
BUT you might be able use the privileged information you might gain to launch another "active" attack using another method. This will have to be specific to what data you found and if there is an attack surface for it.
E.g. a ssh connection over the internet where the attacker gains access to the keys necessary to connect as a user or join an active session.
But that is just not enough. As their advisory states this might allow reading memory of other guests, and who knows what that guest is doing and what next attack that might lead to. Anything you can imagine, really. The attacker might find keys that allow access to systems not even running on that platform. Maybe user passwords. Maybe private mails or documents. Any data the other guest ever processes is at risk.
It's not a guaranteed win and very limited possibilities to automate. But the potential harm could be unbelievable.
>Can an untrusted guest attack the hypervisor using Meltdown or Spectre?
>Can a guest user-space program attack a guest kernel using Meltdown or Spectre?
There are two angles if you maintain Xen yourself. However the vast majority of people aren't Xen customers, they are customers of Amazon or other cloud provider. In which case the main concern is:
Can a fellow customer guest running on an AWS instance attack my guest account?
It seems like the answer is "No", but it looks like the answer might be "Only if dom0 is patched", and might even be "Yes". Since it's not in AWS interests to publicize that the answer is Yes, and since AWS is a large user of Xen, I find the it unsettling that this question is unanswered. It makes me thing it is unanswered for a reason. And if the response is "Oh, we didn't think about it from that perspective", then that would be even more disturbing.
This is incorrect: the BTB can be poisoned to speculatively jump anywhere in the text segment of the supervisor.
For the 3 bugs, the biggest one only affect Intel CPUs, for bug 2 and 3:
AMD bug only affects THE SAME PROCESS, unlike Intel, which allows exploits to cross processes:
https://googleprojectzero.blogspot.com/2018/01/reading-privi...
"As shown, AMD was only vulnerable to "the ability to read data inside mis-speculated execution within the same process, without crossing any privilege boundaries."
To me, the 'biggest' problem here is Spectre which affects all modern high-performance processors, regardless of make or model due to it being a failure of a fundamental technique used to create them.
The Meltdown bug is not more serious than Spectre, if anything, its the reverse as Meltdown has a realistic mitigation.
I see no attempt by Intel to minimise this as it quite clearly is an industry-wide problem, don't start looking for scapegoats where there is none.
There isn't always someone at fault.
The "worse" part of Spectre is mainly about the fact that it affects everyone (but not to the same degree). But on a per chip-maker basis, Intel is still getting the shortest end of the stick here.
Spectre style issues had JS pulling browser process memory using timing--the patches being "put every page in its own process" (Chrome) and "don't let people get accurate timings" (Firefox). They are way worse in the grand scheme of things, because even if they aren't as easy to exploit, they will continue to show up, probably for the foreseeable future (next 5-10 years), long after Meltdown is patched and old news.
Of course every website needs it's own process. It should really be in it's own VM too.
Does all this create an opportunity for another chip line (architecture) to capture some of the server market?
I'm thinking specifically of SPARC. How far behind is SPARC now? A decade? Is that gap surmountable?
Given that 1) both AMD and Intel have issues, and 2) the commentariat have written the variety of ARMs are a mess too, and 3) maybe the people who care (those with server farms) will mitigate risks by adding a completely different architecture to the mix (prefer hetero vs mono culture).
Just curious.
Also, it is unclear whether it allows speculation beyond privilege boundaries and it is also unclear whether ordering semantics for memory are different enough to mitigate somewhat.
I wish I still had access to modern sparc hardware to find out.
While it’s best to assume everything is vulnerable unless proven otherwise I’d really like to see a thorough evaluation.
In the Meltdown paper, the authors report that they couldn't yet make the proof-of-concept exploit work on AMD or ARM, despite having confirmed the general vulnerability on those processors. So based on current knowledge, AMD and ARM are vulnerable to Meltdown with quite a high probability.
The current PoC created by the researchers doesn't work on other manufacturer's CPU's, but the vulnerability and the Meltdown class of attacks should work on other manufacturer's CPUs. Or at least we don't know any reason it shouldn't. The authors' "toy example" works on AMD and ARM too. (See the section 6.4 in the paper).