Re: General flags to turn things off (getrandom, pid lookup, etc)

Andy Lutomirski [off-list ref] writes:

On Jul 27, 2014 5:06 PM, "Theodore Ts'o" [off-list ref] wrote:

quoted

On Fri, Jul 25, 2014 at 11:30:48AM -0700, Andy Lutomirski wrote:

quoted

There is recent interest in having a way to turn generally-available
kernel features off.  Maybe we should add a good one so we can stop
bikeshedding and avoid proliferating dumb interfaces.

I believe the seccomp infrastructure (which is already upstream)
should be able to do most of what you want, at least with respect to
features which are exposed via system calls (which was most of your
list).

Seccomp can't really restrict lookups of non-self pids.  In fact, this
feature idea started out as a response to a patch adding a kind of
nasty seccomp feature to make it sort of possible.

I agree that that seccomp can turn off GRND_RANDOM, but how is it
supposed to do it in such a way that the filtered software will fall
back to something sensible?  -ENOSYS?  -EPERM?  Something else?

I think that -ENOSYS is clearly wrong, but standardizing this would be
nice.  Admittedly, adding something fancy like this for GRND_RANDOM
may not be appropriate.

Andy you seem to be arguing here for two system calls.
get_urandom() and get_random().

Where get_urandom only blocks if there is not enough starting entropy,
and get_random(GRND_RANDOM) blocks if there is currently not enough
entropy.

That would allow -ENOSYS to be the right return value and it would
simply things for everyone.

Eric