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

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Date:	Wed, 30 Jul 2014 19:37:47 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
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)

One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> writes:

> 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

I have said enough about the silliness of disabling this syscall with
seccomp or related infrastructure.

The aspect I like about get_random() is that it will silence the
requests from people to enable binary sysctl support in the kernel.
Just so they can get random numbers when /dev/random and /dev/urandom
are absent in their chroots.

sysctl(2) is finally legitmately going fading away.

Eric

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