Re: [RFC PATCH for 4.15 00/24] Restartable sequences and CPU op vector v11

[Andy Lutomirski]

> On Nov 14, 2017, at 1:32 PM, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> ----- On Nov 14, 2017, at 4:15 PM, Andy Lutomirski luto@xxxxxxxxxxxxxx wrote:
>
>
> One thing I kept however that diverge from your recommendation is the
> "sign" parameter to the rseq syscall. I prefer this flexible
> approach to a hardcoded signature value. We never know when we may
> need to randomize or change this in the future.
>
> Regarding abort target signature the vs x86 disassemblers, I used a
> 5-byte no-op on x86 32/64:
>
>  x86-32: nopl <sig>
>  x86-64: nopl <sig>(%rip)

I still don't see how this can possibly work well with libraries.  If
glibc or whatever issues the syscall and registers some signature,
that signature *must* match the expectation of all libraries used in
that thread or it's not going to work.  I can see two reasonable ways
to handle it:

1. The signature is just a well-known constant.  If you have an rseq
abort landing site, you end up with something like:

nopl $11223344(%rip)
landing_site:

or whatever the constant is.

2. The signature varies depending on the rseq_cs in use.  So you get:

static struct rseq_cs this_cs = {
  .signature = 0x55667788;
  ...
};

and then the abort landing site has:

nopl $11223344(%rip)
nopl $55667788(%rax)
landing_site:

The former is a bit easier to deal with.  The latter has the nice
property that you can't subvert one rseq_cs to land somewhere else,
but it's not clear to me how what actual attack this prevents, so I
think I prefer #1.  I just think that your variant is asking for
trouble down the road with incompatible userspace.

--Andy