linux-kernel - Re: [kernel-hardening] Re: [PATCH 1/2] sysctl: expand use of proc_dointvec_minmax

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Date:	Sat, 23 Jan 2016 19:20:17 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jann Horn <jann@...jh.net>
Cc:	kernel-hardening@...ts.openwall.com,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Richard Weinberger <richard@....at>,
	Andy Lutomirski <luto@...capital.net>,
	Robert Święcki <robert@...ecki.net>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	David Howells <dhowells@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [kernel-hardening] Re: [PATCH 1/2] sysctl: expand use of proc_dointvec_minmax_sysadmin

Jann Horn <jann@...jh.net> writes:

> On Fri, Jan 22, 2016 at 09:10:07PM -0600, Eric W. Biederman wrote:
>> Kees Cook <keescook@...omium.org> writes:
>> 
>> > Several sysctls expect a state where the highest value (in extra2) is
>> > locked once set for that boot. Yama does this, and kptr_restrict should
>> > be doing it. This extracts Yama's logic and adds it to the existing
>> > proc_dointvec_minmax_sysadmin, taking care to avoid the simple boolean
>> > states (which do not get locked). Since Yama wants to be checking a
>> > different capability, we build wrappers for both cases (CAP_SYS_ADMIN
>> > and CAP_SYS_PTRACE).
>> 
>> Sigh this sysctl appears susceptible to known attacks.
>> 
>> In my quick skim I believe this sysctl implementation that checks
>> capabilities is susceptible to attacks where the already open file
>> descriptor is set as stdout on a setuid root application.
>> 
>> Can we come up with an interface that isn't exploitable by an
>> application that will act as a setuid cat?
>
> Adding the struct file * to the parameters of all proc_handler
> functions would work, right? (Or just filp->f_cred? That would be
> less generic.)
>
> A quick grep says that's just about 160 functions that'll need to
> be changed. :/

Yep.  That is about the size of it.  file * used to be passed to the
sysctl methods but it was removed several years ago because no one was
using it.

Eric