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

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Date:	Fri, 25 Jul 2014 16:15:07 -0400
From:	Dave Jones <davej@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Julien Tinnes <jln@...gle.com>,
	David Drysdale <drysdale@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Paolo Bonzini <pbonzini@...hat.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Moore <paul@...l-moore.com>,
	James Morris <james.l.morris@...cle.com>,
	Linux API <linux-api@...r.kernel.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	linux-crypto@...r.kernel.org
Subject: Re: General flags to turn things off (getrandom, pid lookup, etc)

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.
 > 
 > Things that might want to be turn-off-able include:
 >  - getrandom with GRND_RANDOM [from the getrandom threads]
 >  - Any lookup of a non-self pid [from the capsicum thread]
 >  - Any lookup of a pid outside the caller thread group [capsicum]
 >  - Various architectural things (personal wishlist), e.g.:
 >     - RDTSC and userspace HPET access
 >     - CPUID?
 >     - 32-bit GDT code segments [huge attack surface]
 >     - 64-bit GDT code segments [probably pointless]

I'm not sure there's value in disabling cpuid dev interface,
when the instruction is unprivileged.

 > I would propose a new syscall for this:
 > 
 > long restrict_userspace(int mode, int type, int value, int flags);

do the restrictions happen system-wide like in say SELinux,
or only within the calling process, like seccomp ?

	Dave

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