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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 23 Sep 2020 11:09:29 -0700
Message-ID: <CALCETrUqct4tDrjTSzJG4+=+cEaaDbZ+Mx=LAUdQjVV=CruUcw@mail.gmail.com>
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Solar Designer <solar@openwall.com>, Pavel Machek <pavel@ucw.cz>,
        "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        linux-integrity <linux-integrity@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, X86 ML <x86@kernel.org>,
        Andrew Lutomirski <luto@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Mark Rutland <mark.rutland@arm.com>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>,
        Rich Felker <dalias@libc.org>
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On Wed, Sep 23, 2020 at 7:39 AM Florian Weimer <fweimer@redhat.com> 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
verifiers:

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.