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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Mon, 8 Oct 2018 10:42:54 -0700
Message-ID: <CALCETrWMvtH4tS5jkZcEP=YuwV6XOtRqx-0TF5dCTABa_zC-Xg@mail.gmail.com>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        mhelsley@vmware.com,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        Andrew Lutomirski <luto@kernel.org>
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On Mon, Oct 8, 2018 at 10:30 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> On 8 October 2018 at 19:25, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Mon, Oct 8, 2018 at 9:40 AM Peter Zijlstra <peterz@infradead.org> wr=
ote:
> >>
> >> On Mon, Oct 08, 2018 at 09:29:56AM -0700, Andy Lutomirski wrote:
> >> >
> >> >
> >> > > On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <peterz@infradead.org> =
wrote:
> >> > >
> >> > > On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote:
> >> > >>> Can't we hijack the relocation records for these functions befor=
e they
> >> > >>> get thrown out in the (final) link pass or something?
> >> > >>
> >> > >> I could be talking out my arse here, but I thought we could do th=
is,
> >> > >> too, then changed my mind.  The relocation records give us the
> >> > >> location of the call or jump operand, but they don=E2=80=99t give=
 the address
> >> > >> of the beginning of the instruction.
> >> > >
> >> > > But that's like 1 byte before the operand, right? We could even do=
uble check
> >> > > this by reading back that byte and ensuring it is in fact 0xE8 (CA=
LL).
> >> > >
> >> > > AFAICT there is only the _1_ CALL encoding, and that is the 5 byte=
: E8 <PLT32>,
> >> > > so if we have the PLT32 location, we also have the instruction loc=
ation. Or am
> >> > > I missing something?
> >> >
> >> > There=E2=80=99s also JMP and Jcc, any of which can be used for rail =
calls, but
> >> > those are also one byte. I suppose GCC is unlikely to emit a prefixe=
d
> >> > form of any of these. So maybe we really can assume they=E2=80=99re =
all one
> >> > byte.
> >>
> >> Oh, I had not considered tail calls..
> >>
> >> > But there is a nasty potential special case: anything that takes the
> >> > function=E2=80=99s address. This includes jump tables, computed goto=
s, and
> >> > plain old function pointers. And I suspect that any of these could
> >> > have one of the rather large number of CALL/JMP/Jcc bytes before the
> >> > relocation by coincidence.
> >>
> >> We can have objtool verify the CALL/JMP/Jcc only condition. So if
> >> someone tries to take the address of a patchable function, it will err=
or
> >> out.
> >
> > I think we should just ignore the sites that take the address and
> > maybe issue a warning.  After all, GCC can create them all by itself.
> > We'll always have a plain wrapper function, and I think we should just
> > not patch code that takes its address.  So we do, roughly:
> >
> > void default_foo(void);
> >
> > GLOBAL(foo)
> >   jmp *current_foo(%rip)
> > ENDPROC(foo)
> >
> > And code that does:
> >
> > foo();
> >
> > as a call, a tail call, a conditional tail call, etc, gets discovered
> > by objtool + relocation processing or whatever and gets patched.  (And
> > foo() itself gets patched, too, as a special case.  But we patch foo
> > itself at some point during boot to turn it into a direct JMP.  Doing
> > it this way means that the whole mechanism works from very early
> > boot.)
>
> Does that mean that architectures could opt out of doing the whole
> objtool + relocation processing thing, and instead take the hit of
> going through the trampoline for all calls?
>

I don't see why not.

--Andy