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

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Message-ID: <20140730222903.4c83a652@alan.etchedpixels.co.uk>
Date:	Wed, 30 Jul 2014 22:29:03 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	Andy Lutomirski <luto@...capital.net>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-crypto@...r.kernel.org,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	James Morris <james.l.morris@...cle.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux API <linux-api@...r.kernel.org>,
	Julien Tinnes <jln@...gle.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Moore <paul@...l-moore.com>,
	David Drysdale <drysdale@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: General flags to turn things off (getrandom, pid lookup, etc)

On Wed, 30 Jul 2014 11:41:41 -0700
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> One Thousand Gnomes <gnomes@...rguk.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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