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

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Date:	Fri, 25 Jul 2014 11:30:48 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"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>,
	Andy Lutomirski <luto@...capital.net>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	linux-crypto@...r.kernel.org
Subject: General flags to turn things off (getrandom, pid lookup, etc)

[new thread because this sort of combines two threads]

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 would propose a new syscall for this:

long restrict_userspace(int mode, int type, int value, int flags);

mode is RESTRICT_SET, RESTRICT_GET, or RESTRICT_LOCK.

type is RESTRICT_GRND_RANDOM, RESTRICT_PID_SCOPE, RESTRICT_X86_TIMING, etc.

Value is zero if RESTRICT_GET.  Otherwise value is the desired value,
generally 0 or 1.  For RESTRICT_PID_SCOPE, value would be
RESTRICT_PID_SCOPE_ANY, RESTRICT_PID_SCOPE_THREADGROUP, or
RESTRICT_PID_SCOPE_SELF.

flags must be zero.  Someday, someone will propose a thread-sync flag.

restrict_userspace requires either no_new_privs or CAP_SYS_ADMIN in
the current user namespace.

Thoughts?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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