Its hard to get your head around how big a deal this is. This vulnerability is so bad they killed x86 indirect jump instructions. It's so bad compilers --- all of them --- have to know about this bug, and use an incantation that hacks ret like an exploit developer would. It's so bad that to restore the original performance of a predictable indirect jump you might have to change the way you write high-level language code.
It's glorious.
It truly is difficult to predict all the ripple effects from this. I can't think of a single computer bug in the last 30 years that's similar in reach to this Intel Meltdown.
[EDITED following text to replace "Intel bug" with "Spectre bug" based on ars and jcranmer clarification. The Intel Meltdown can be fixed with operating system update patches for kpti instead of a complete recompile.]
Journalists like to overuse the bombastic metaphor "shaken the very foundations" but this Spectre bug actually seems very fitting of it. Off the top of my head:
- browsers like Chrome & Firefox have to compile with new defensive compilation flags because it runs untrusted Javascript
- cloud providers have to recompile and patch their code to protect themselves from hostile customer vms
- operating systems like Linux/Windows/MacOS have to recompile and patch code to protect users from malware
Imagine the economics of all these mitigations. Also imagine that each of the cloud vendors AWS/Google/Azure/Rackspace had very detailed Excel spreadsheets extrapolating cpu usage for the next few years to plan for millions of $$$ of capital expenditures. Because of the severe performance implications of the bugfix (5% to 50% slowdown?), the cpu utilization assumptions in those spreadsheets are now wrong. They will have to spend more than they thought they did to meet goals of workload throughput.
There are dozens of other scenarios that we can't immediately think of.
Wrong bug. Intel meltdown is bad, but not anywhere near as bad as Spectre which affects everything! No AMD immunity here.
Not meaning to be that rude, yet this itself summarises (and the issue perhaps will shed more light on) how stupid an idea is to let everybody run untrusted code from other peoples, let alone third party stuff like "privacy-intrusion-as-a-service" startups et aliae.
From the spectre paper:
"A minor variant of this could be to instead use an out-of-bounds read to a function pointer to gain control of execution in the mis-speculated path. We did not investigate this variant further."
If I were developing processors, I'd be having emergency meetings on trying to craft exploits to figure out where our processors' weaknesses are. While being happy that Intel is getting all the bad PR for this and I'm not.
Agreed. This is an entirely new class of vulnerabilities, and we're just at the beginning.
For any other non-sanboxed application you pretty much have to trust the code anyway. Privilege escalation is always a bad thing of course, but for single user desktop machines getting user shell access as an attacker means that you can do pretty much anything you want.
As far as I can see the only surface of attack for my current machine would be a website running untrusted JS. For all other applications running on my machine if one of them is actually hostile them I'm already screwed.
Frankly I'm more annoyed at the ridiculous over-engineering of the Web than at CPU vendors. Because in 2017 you need to enable a turing complete language interpreter in your browser in order to display text and pictures on many (most?) websites.
Gopher should've won.
It ends with the performance advantages of OOO execution being effectively negated by the workarounds to address the security issues it causes.
The following parable is edifying: https://www.cs.utexas.edu/users/EWD/transcriptions/EWD05xx/E...
[Edit] Or, how far down does the rabbit hole go?
Additionally, it is quite fascinating to me to compare the complexity of modern CPRUs with, say, a compiler.
I believe the generalized fix is to restore the entire CPU state after a mispredict. You’d either need to add an extra copy of the entire processor state (tens of megabits) for every simultaneous predict you support ($$$) or keep track of how to revert all changes and revert them one at a time ($, slow).
- we didn't have hyperconverged cloud infrastructures running arbitrary entities' code next to each other
What's new and surprising is the power of these side-channel attacks--you can use these, reliably, to exfiltrate arbitrary memory, including across privilege modes in some cases (apparently, some ARM cores are affected by the latter vulnerability, in addition to Intel).
Are you sure?
The right fix is to prevent speculatively executed code from leaking information.
Here that perhaps means associating cache lines with a speculative branch somehow so that they aren't accessible until/unless the speculative branch becomes the real branch. (I have no idea exactly how that would be done or what the performance cost might be... I'd really need to know the details of how speculative execution is implemented in a particular CPU to even be able to guess.)
Ouch! This is independent of other performance hurts, like from the kernel syscall overhead that was the hot topic yesterday. This is pretty crazy.
Will be intrigued to see how processor manufacturers respond to this. If they were even slightly relaxed about it prior to disclosure I expect there's going to be some very hurried attempts to engineer some solutions pronto. This is the sort of thing where it might even be worth throwing away all of your future roadmap plans and just getting a revision of the current chips out there ASAP, whatever that may do to the rest of your roadmap.
Although actually, we already are - binary distros already don't take into account per-microarchitecture scheduling, nor any ISA extensions above a common baseline (e.g. just SSE2, no autovectorising to AVX2 etc).
This might provide enough impetus to restructure how binary distros work and get the whole distro compiled with some newer CPU flags (march={first corrected architecture}?) but in the short term i assume every package will take the hit.
Great time to learn about source-based distros!
> However, real-world workloads exhibit substantially lower performance impact.
I feel like you could have mentioned this.
I feel bad for all of the engineers currently working on performance sensitive applications in these languages. There's a whole lot of Java, .NET, and JavaScript that's about to get slower[1]. Enterprise-y, abstract class heavy (i.e.: vtable using) C++ will get slower. Rust trait objects get slower. Haskell type classes that don't optimize out get slower.
What a mess.
[1] These mitigations will need to be implemented for interpreters, and JITs will want to switch to emitting "retpoline" code for dynamic dispatch. There's no world in which I don't expect the JVM, V8, and others to switch to these by default soon.
Maybe I'm being naive, but would a simple modulo instruction work? Consider the example code from https://googleprojectzero.blogspot.com/2018/01/reading-privi...:
unsigned long untrusted_offset_from_caller = ...;
if (untrusted_offset_from_caller < arr1->length) {
unsigned char value = arr1->data[untrusted_offset_from_caller];
...
}
unsigned char value = arr1->data[untrusted_offset_from_caller % arr1->length];
You only need to wipe between syscalls that have side effects. Number crunching AVX heavy subroutines should never have to deal with safety once entered.
The obvious drawback that it effectively disables sharing code in memory, it would still allow sharing code on disk though. So it would be a middle ground between the current state in dynamic and static linking.
https://www.technovelty.org/c/position-independent-code-and-...
* https://lkml.org/lkml/2018/1/3/797 (https://news.ycombinator.com/item?id=16066968)
What do people more knowledgeable in the field think about this?
All of these attacks assume you are running something you don't trust on your CPU, whether it is another user's program, a non-root executable, or a JavaScript program from a website.
When do we stop hacking processors, kernels, and compilers and revisit our assumptions of what we can and can't do securely.
side channels have always been some of the most insidious exploits. Many are basically un-solvable (timing attacks are always going to leak some information, and compression is basically completely at odds with secure information storage), many more are easily enough overlooked that it would be easy to maliciously include them without raising any eyebrows, and the "fixes" for them almost always murder performance.
I think the only real fully-encompassing solution to this is a redesign in how we use computers. Either a massive step backwards on performance and turning off most automatic "optimizations" until they can be proven through a much more rigorous process (both in compilers, and in hardware), or a significant change in how computers are architected adding more hardware level isolation for processes and systems running on the machine (just daydreaming now, but something like a cluster of isolated micro-CPUs that run one application only).
You can do a lot by separating these speedup mechanisms across security boundaries. The biggest factor that makes this hard to mitigate is in-process security boundaries. Total isolation between processes is neither necessary nor sufficient.
https://marc.info/?l=openbsd-misc&m=118296441702631&w=2
(from 2007)
RISCV has already had to fix its memory consistency model, so it is not without problems. But it that is a spec bug, not an implementation bug. Whether there is an out of order, speculative execution RISCV core in the wild which suffers from this is as far as I know very unlikely. If there is, no doubt it's designers have had a busy time lately.
I'd presume that the slowest RISC-V designs are immune due to not speculating enough, while any high-performance implementation is vulnerable.
>> While makeshift processor-specific countermeasures are possible in some cases, sound solutions will require fixes to processor designs as well as updates to instruc- tion set architectures (ISAs) to give hardware architects and software developers a common understanding as to what computation state CPU implementations are (and are not) permitted to leak.
Early processors had speculative execution? I thought this had been added to Intel/AMD/ARM about 20 years ago?
Trusting in a compiler you hope was used to build all the executables on your system isn't trustworthy enough to be the final solution.
[1] https://www.win.tue.nl/~aeb/linux/hh/thompson/trust.html
Unless the compiler is also patched to either disallow inserted assembly, or to modify the inserted assembly (this being both hard and dangerous), someone who wants to exploit the bug will just add their own inserted assembly code that exploits the bug, and a patched compiler won't help one bit in that case.
* https://lkml.org/lkml/2018/1/4/432 * http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a...
It appears that Skylake and later can actually predict retpolines? Some hardware features called IBRS, IBPB, STIBP (not a lot of details on this are out there) are supposedly coming in a microcode update.
There's a lot of prominence being given to all kinds of damage malicious users might inflict, and ways to prevent or mitigate, but little to the malice itself. Whence does it arise? What emotions drive those users? What unmet needs?
Meanwhile, when these slowing-down patches for Sceptre and Meltdown arrive, I intend to not run them, to the possible extent. I intend to keep aside a VM with patches for critical stuff, like banking or others' data entrusted to me. But I don't want my machine to be slowed down just because someone, sometime, might invest effort in targeting these attacks at it. Given how transparent I want to be with my life, that's a risk I'm willing to take.
Sure, you might not have anything you want to hide in your life, but the drive-by javascript doesn't care about your secrets - it'll hack you anyway. Best-case scenario, you lose access to a bunch of accounts you used to use and need to create new identities from scratch. Worst-case, they clean you out financially, steal your identity, etc.
Also, any insight about performance impact here?
Most programs have indirect jumps somewhere. Higher-level languages with virtual function calls have lots of indirect jumps, because they parameterize functions: to get the "length" of the variable "foo", the function "bar" has to call one of 30 different functions, depending on the type of "foo"; the function to call is read out of a table at some offset from the base address of "foo". Or, another example is switch statements, which can compile down to jump tables.
What we want, to mitigate Spectre, is to be able to disable speculative execution for indirect jumps. The CPU doesn't provide a clean way to do that directly.
So we just stop using the indirect jump instructions. Instead, we abuse the fact that "ret" is an indirect jump.
"Call" and "ret" are how CPUs support function calls. When you "call" a function, the CPU pushes the return address --- the next instruction address after the "call" --- to the stack. When you return from a function, you pop the return address and jump to it. There's a sort of "jmp %register" hidden in "ret".
You abuse "ret" by replacing indirect jumps with a sequence of call/mov/jump, where the mov does a switcheroo on the saved return address.
The obvious next question to ask here is, "why don't CPUs predict and speculatively execute rets?" And, they do. So the retpoline mitigates this: instead of just "call/pop/jump", it does "call/...pause/jmp.../mov/jmp", where the middle sequence of instructions set off in "..." is jumped over and not executed, but captures the speculative execution that the CPU does --- the CPU expects the "ret" to return to the original "call", and does not know how to predict around the fact that we did the switcheroo on the return address.
How'd I do?
Is the overhead of the retpoline such that it's no longer a benefit to compile switches to jump tables?
Here is an example of the most common programming patterns that end up causing indirect jumps/calls:
Imagine every virtual function call in a C++ program being mispredicted and taking twice as long.
(Instead of forcing us to recompile the world, maybe Intel should just disable branch prediction in microcode.)
> (Instead of forcing us to recompile the world, maybe Intel should just disable branch prediction in microcode.)
Wouldn't the performance impact be dramatic ? In this[1] example there's a 6 times slowdown between situation with and without correct branch prediction.
[1]: https://stackoverflow.com/questions/11227809/why-is-it-faste...
Something like that could allow the CPU to speculate agressively while preventing information leak exploits.
The bug here is that the CPU is not aborting the speculation when fetches occur to addresses marked as "access denied". Instead the fetch happens and a line of normally inaccessible memory is put into cache by code that should not be able to get it read into the cache normally.
One hardware fix would be to plug that hole. Speculative reads get blocked when they encounter permission denied errors from the paging system and do not change the cache state. That blocks the Meltdown attack, but not the Spectre attack.
Also maybe the context switching would need to be made faster, because you would need to do that whenever eg javascript calls browser interfaces.
It also sets a very bad precedent: I understand people want to mitigate/fix as much as possible, but this is basically giving an implicit message to the hardware designers: "it doesn't matter if our instructions are broken, regardless of how widespread in use they already are --- they'll just fix it in the software."
What are any other options? It's hardware, that cannot be patched. Of course they will change chip designs going forward, but what else do you suggest folks do with the billions of chips that exhibit this problem?
> We built multi-tenant cloud computing on top of processors and chipsets that were designed and hyper-optimized for
> single-tenant use. We crossed our fingers that it would be OK and it would all turn out great and we would all profit.
> In 2018, reality has come back to bite us.
This is the root of all the problems.
Right now many function calls don't safely wipe registers and the new side channel caches found in Spectre. There really needs to be two kinds of function calls. Maybe a C PRAGMA?
The complier has parent function call wiping as a flag; the code has pragmas that over-ride the flag.
Wayback Machine: https://web.archive.org/web/20180104131631/https://reviews.l...
:)