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SubjectRe: disabling group leader perf_event
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On Tue, Sep 7, 2010 at 4:44 AM, Ingo Molnar <mingo@elte.hu> wrote:
>
> * 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.

Native Client: A Sandbox for Portable, Untrusted x86 Native Code, IEEE
Symposium on Security and Privacy, May 2009
http://nativeclient.googlecode.com/svn/data/docs_tarball/nacl/googleclient/native_client/documentation/nacl_paper.pdf

The "Inner Sandbox" they talk about verifies a subset of x86 code.
For indirect control flow (computed jumps), they introduce a new
instruction that can do run-time checking of the destination address.

IIRC they have a patched gcc toolchain that can compile to this subset of x86.

Stefan
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