Messages in this thread | | | Date | Tue, 7 Sep 2010 05:44:17 +0200 | From | Ingo Molnar <> | Subject | Re: disabling group leader perf_event |
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* Avi Kivity <avi@redhat.com> wrote:
> On 09/06/2010 06:47 PM, Ingo Molnar wrote: > > > >>The actual language doesn't really matter. > >There are 3 basic categories: > > > > 1- Most (least abstract) specific code: a block of bytecode in the form > > of a simplified, executable, kernel-checked x86 machine code block - > > this is also the fastest form. [yes, this is actually possible.] > > Do you then recompile it? [...]
No, it's machine code. It's 'safe x86 bytecode executed natively by the kernel as a function'.
It needs a verification pass (because the code can come from untrusted apps) so that we can copy, verify and trust it (so obviously it's not _arbitrary_ x86 machine code - a safe subset of x86) - maybe with a sha1 based cache for already-verified snippets (or a fast verifier).
> x86 is quite unpleasant.
Any machine code that is fast and compact is unpleasant almost by definition: it's a rather non-obvious Huffman encoding embedded in an instruction architecture.
But that's the life of kernel hackers, we deal with difficult things. (We could have made a carreer choice of selling icecream instead, but it's too late i suspect.)
> > 2- Least specific (most abstract) code: A subset/sideset of C - as it's > > the most kernel-developer-trustable/debuggable form. > > > > 3- Everything else little more than a dot on the spectrum between the > > first two points. > > > > I lean towards #2 - but #1 looks interesting too. #3 is distinctly > > uninteresting as it cannot be as fast as #1 and cannot be as > > convenient as #2. > > Curious - how do you guarantee safety of #1 or even #2? [...]
Safety of #1 (x86 bytecode passed in by untrusted user-space, verified and saved by the kernel and executed natively as an x86 function if it passes the security checks) is trivial but obviously needs quite a bit of work.
We start with trivial (and useless) special case of something like:
#define MAX_BYTECODE_SIZE 256
int x86_bytecode_verify(char *opcodes, unsigned int len) {
if (len-1 > MAX_BYTECODE_SIZE-1) return -EINVAL;
if (opcodes[0] != 0xc3) /* RET instruction */ return -EINVAL;
return 0; }
... and then we add checks for accepted/safe x86 patterns of instructions step by step - always keeping it 100% correct.
Initially it would only allow general register operations with some input and output parameters in registers, and a wrapper would save/restore those general registers - later on stack operands and globals could be added too.
That's not yet Turing complete but already quite functional: an amazing amount of logic can be expressed via generic register ops only - i think the filter engine could be implemented via that for example.
We'd eventually make it Turing complete in the operations space we care about: a fixed-size stack sandbox and a virtual memory window sandbox area, allow conditional jumps (only to instruction boundaries).
The code itself is copied into kernel-space and immutable after it has been verified.
The point is to decode only safe instructions we know, and to always have a 'safe' core of checking code we can extend safely and iteratively.
Safety of #2 (C code) is like the filter engine: it's safe right now, as it parses the ASCII expression in-kernel, compiles it into predicaments and executes those predicament (which are baby instructions really) safely.
Every extension needs to be done safely, of course - and more complex language constructs will complicate matters for sure.
Note that we have (small) bits of #1 done already in the kernel: the x86 disassembler. Any instruction pattern we dont know or dont trust we punt on.
( Also note that beyond native execution this 'x86 bytecode' approach would still allow JIT techniques, if we are so inclined: x86 bytecode, because we fully verify it and fully know its structure (and exclude nasties like self-modifying code) can be re-JIT-ed just fine.
Common sequences might even be pre-JIT-ed and cached in a hash. That way we could make sequences faster post facto, via a kernel change only, without impacting any user-space which only passes in the 'old' sequence. Lots of flexibility. )
> Can you point me to any research?
Nope, havent seen this 'safe native x86 bytecode' idea mentioned/researched anywhere yet.
> Everything I'm aware of is bytecode with explicit measures to prevent > forged pointers, but I admit I've spent no time on it. It's > interesting stuff, though.
I think some Java-like bytecode is roughly the same amount of conceptual work as an x86 bytecode verifier, with the big disadvantage that even with a JIT it's much slower [and a JIT is far from simple] - not to mention the non-technical complications of Java.
> I have a truly marvellous patch that fixes the bug which this > signature is too narrow to contain.
Make sure you write down a short but buggy version of the patch on the margin of a book. Pass on the book to your heirs and enjoy the centuries long confusion from the heavens.
Thanks,
Ingo
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