Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753603AbaG3Vbd (ORCPT ); Wed, 30 Jul 2014 17:31:33 -0400 Received: from lxorguk.ukuu.org.uk ([81.2.110.251]:39371 "EHLO lxorguk.ukuu.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753347AbaG3Vba (ORCPT ); Wed, 30 Jul 2014 17:31:30 -0400 Date: Wed, 30 Jul 2014 22:29:03 +0100 From: One Thousand Gnomes To: ebiederm@xmission.com (Eric W. Biederman) Cc: Andy Lutomirski , Paolo Bonzini , linux-crypto@vger.kernel.org, Henrique de Moraes Holschuh , "linux-kernel\@vger.kernel.org" , James Morris , LSM List , Al Viro , Linux API , Julien Tinnes , "Theodore Ts'o" , Greg Kroah-Hartman , Paul Moore , David Drysdale , Kees Cook , Meredydd Luff , Christoph Hellwig Subject: Re: General flags to turn things off (getrandom, pid lookup, etc) Message-ID: <20140730222903.4c83a652@alan.etchedpixels.co.uk> In-Reply-To: <87r412g04a.fsf@x220.int.ebiederm.org> References: <20140727210617.GY6725@thunk.org> <87oawa740c.fsf@x220.int.ebiederm.org> <20140730153713.736881f0@alan.etchedpixels.co.uk> <87r412g04a.fsf@x220.int.ebiederm.org> Organization: Intel Corporation X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.23; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 30 Jul 2014 11:41:41 -0700 ebiederm@xmission.com (Eric W. Biederman) wrote: > One Thousand Gnomes writes: > > >> 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. > > > > So you replace the "no file handle" special case with the "unsupported or > > disabled syscall" special case, which is even harder to test. > > > > Interfaces have failure modes. People who can't deal with that shouldn't > > be writing code that does anything important in languages which don't > > handle it for them. > > Perhaps I misread the earlier conversation but it what I have read of > this discussion people want to disable some of get_random() modes with > seccomp. Today get_random does not have any failure codes define except > -ENOSYS. > > get_random(0) succeeding and get_random(GRND_RANDOM) returning -ENOSYS > has every chance of causing applications to legitimately assume the > get_random system call is not available in any mode. Or more likely it'll be used like this get_random(foo); /* always works */ Now the existing failure mode is is open(...) /* forget the check */ read() /* forget the check */ and triggered by evil local attacks on file handles. The "improved" behaviour is unchecked -ENOSYS returns which are likely to occur systemically when users run stuff on old kernels, in vm's with it off etc. So you've swapped the odd evil user attack on a single target for the likelyhood of mass generation of flawed keys with no error reporting. In fact you could do a better job of the whole mess in libc rather than the kernel, because in libc you'd write it like this if (open(.. ) < 0) kill(getpid(), 9); if (read(...) < expected) kill(getpid(), 9); close(fd); and a) on an older library you'd get a good failure (unable to execute the binary) b) on a newer system you'd get "do or die" behaviour and can improve its robustness as desired Alan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/