linux-kernel - Re: [RFC] kernel/panic: place an upper limit on number of oopses

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Message-ID: <20160113002043.GA17146@openwall.com>
Date:	Wed, 13 Jan 2016 03:20:43 +0300
From:	Solar Designer <solar@...nwall.com>
To:	Daniel Axtens <dja@...ens.net>
Cc:	Jann Horn <jann@...jh.net>, kernel-hardening@...ts.openwall.com,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Baoquan He <bhe@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [RFC] kernel/panic: place an upper limit on number of oopses

Jann Horn <jann@...jh.net> wrote:
> To prevent an attacker from turning a mostly harmless oops into an
> exploitable issue using a refcounter wraparound caused by repeated
> oopsing, limit the number of oopses.

This may also reduce the likelihood of successful exploitation of some
other vulnerabilities involving memory corruption, where an unsuccessful
attempt may inadvertently trigger an Oops.  The attacker would then need
to succeed in fewer than the maximum allowed number of Oops'es.  Jann's
currently proposed default of 0x100000 is too high to make a difference
in that respect, but people may set it differently.

On Wed, Jan 13, 2016 at 10:34:39AM +1100, Daniel Axtens wrote:
> I'm torn between making the limit configurable and not adding to the
> massive proliferation of config options.

What about reusing panic_on_oops for the configurable limit?  The
currently supported values of 0 and 1 would retain their meaning,
2 would panic after 2nd Oops, and so on.

There's overlap with grsecurity's banning of users on Oops, but I think
it makes sense to have both the trivial change proposed by Jann (perhaps
with the reuse of panic_on_oops for configuration) and grsecurity-style
banning (maybe with a low configurable limit, rather than always on
first Oops).

Alexander