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SubjectRe: disabling group leader perf_event

* 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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