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

[ebiederm@xmission.com (Eric W. Biederman)]

Andy Lutomirski <luto@amacapital.net> writes:

> On Jul 27, 2014 5:06 PM, "Theodore Ts'o" <tytso@mit.edu> wrote:
>>
>> On Fri, Jul 25, 2014 at 11:30:48AM -0700, Andy Lutomirski wrote:
>> >
>> > 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