Date: Wed, 30 Jul 2014 22:29:03 +0100
From: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
To: ebiederm@xmission.com (Eric W. Biederman)
Cc: Andy Lutomirski <luto@amacapital.net>, Paolo Bonzini <pbonzini@redhat.com>,
        linux-crypto@vger.kernel.org,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        James Morris <james.l.morris@oracle.com>,
        LSM List <linux-security-module@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux API <linux-api@vger.kernel.org>, Julien Tinnes <jln@google.com>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Paul Moore <paul@paul-moore.com>, David Drysdale <drysdale@google.com>,
        Kees Cook <keescook@chromium.org>,
        Meredydd Luff <meredydd@senatehouse.org>,
        Christoph Hellwig <hch@infradead.org>
Subject: Re: General flags to turn things off (getrandom, pid lookup, etc)
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On Wed, 30 Jul 2014 11:41:41 -0700
ebiederm@xmission.com (Eric W. Biederman) wrote:

> One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk> 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
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