On Fri, Oct 19, 2018 at 08:36:45AM -0700, Kees Cook wrote:

On Fri, Oct 19, 2018 at 4:24 AM, Will Deacon [off-list ref] wrote:

quoted

FWIW: I think we should be entertaining a prctl() interface to use a new
key on a per-thread basis. Obviously, this would need to be used with care
(e.g. you'd fork(); use the prctl() and then you'd better not return from
the calling function!).

Assuming we want this (Kees -- I was under the impression that everything in
Android would end up with the same key otherwise?), then the question is
do we want:

  - prctl() get/set operations for the key, or
  - prctl() set_random_key operation, or
  - both of the above?

Part of the answer to that may lie in the requirements of CRIU, where I
strongly suspect they need explicit get/set operations, although these
could be gated on CONFIG_CHECKPOINT_RESTORE=y.

Oh CRIU. Yikes. I'd like the get/set to be gated by the CONFIG, yes.
No reason to allow explicit access to the key (and selected algo) if
we don't have to.

Makes sense.

As for per-thread or not, having a "pick a new key now" prctl() sounds
good, but I'd like to have an eye toward having it just be "automatic"
on clone().

I thought about that too, but we're out of clone() flags afaict and there's
no arch hook in there. We could add yet another clone syscall, but yuck (and
I reckon viro would kill us).

Or are you saying that we could infer the behaviour from the existing set
of flags?

Will