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SubjectRe: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
On Wed, Sep 23, 2020 at 7:39 AM Florian Weimer <> wrote:
> * Solar Designer:
> > While I share my opinion here, I don't mean that to block Madhavan's
> > work. I'd rather defer to people more knowledgeable in current userland
> > and ABI issues/limitations and plans on dealing with those, especially
> > to Florian Weimer. I haven't seen Florian say anything specific for or
> > against Madhavan's proposal, and I'd like to. (Have I missed that?)
> There was a previous discussion, where I provided feedback (not much
> different from the feedback here, given that the mechanism is mostly the
> same).
> I think it's unnecessary for the libffi use case. Precompiled code can
> be loaded from disk because the libffi trampolines are so regular. On
> most architectures, it's not even the code that's patched, but some of
> the data driving it, which happens to be located on the same page due to
> a libffi quirk.
> The libffi use case is a bit strange anyway: its trampolines are
> type-generic, and the per-call adjustment is data-driven. This means
> that once you have libffi in the process, you have a generic
> data-to-function-call mechanism available that can be abused (it's even
> fully CET compatible in recent versions). And then you need to look at
> the processes that use libffi. A lot of them contain bytecode
> interpreters, and those enable data-driven arbitrary code execution as
> well. I know that there are efforts under way to harden Python, but
> it's going to be tough to get to the point where things are still
> difficult for an attacker once they have the ability to make mprotect
> calls.
> It was pointed out to me that libffi is doing things wrong, and the
> trampolines should not be type-generic, but generated so that they match
> the function being called. That is, the marshal/unmarshal code would be
> open-coded in the trampoline, rather than using some generic mechanism
> plus run-time dispatch on data tables describing the function type.
> That is a very different design (and typically used by compilers (JIT or
> not JIT) to implement native calls). Mapping some code page with a
> repeating pattern would no longer work to defeat anti-JIT measures
> because it's closer to real JIT. I don't know if kernel support could
> make sense in this context, but it would be a completely different
> patch.

I would very much like to see a well-designed kernel facility for
helping userspace do JIT in a safer manner, but designing such a thing
is likely to be distinctly nontrivial. To throw a half-backed idea
out there, suppose a program could pre-declare a list of JIT

static bool ffi_trampoline_verifier(void *target_address, size_t
target_size, void *source_data, void *context);

struct jit_verifier {
.magic = 0xMAGIC_HERE,
.verifier = ffi_trampoline_verifier,
} my_verifier __attribute((section("something special here?)));

and then a system call something like:

instantiate_jit_code(target, source, size, &my_verifier, context);

The idea being that even an attacker that can force a call to
instantiate_jit_code() can only create code that passes verification
by one of the pre-declared verifiers in the process.

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