linux-kernel - Re: [PATCH v3] fs: introduce pipe-only dump mode suid

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Message-ID: <CAGXu5jLOJrZTNKh=G75wP2fo-cPj_mM-Urc+4hDVvyJpiy+7Dg@mail.gmail.com>
Date:	Fri, 22 Jun 2012 15:07:45 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Rob Landley <rob@...dley.net>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Doug Ledford <dledford@...hat.com>,
	Marcel Holtmann <marcel@...tmann.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Joe Korty <joe.korty@...r.com>,
	David Howells <dhowells@...hat.com>,
	James Morris <james.l.morris@...cle.com>,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3] fs: introduce pipe-only dump mode suid_dumpable=3

On Fri, Jun 22, 2012 at 2:57 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Fri, 22 Jun 2012 14:51:54 -0700
> Kees Cook <keescook@...omium.org> wrote:
>
>> > And how serious is the security vulnerability, in real-world terms?
>> > Serious enough to risk this amount of bustage?
>>
>> If they're running in mode "2" and they do not have a coredump pipe
>> handler defined, local users can gain root access.
>
> But the kernel can detect this case and avoid it?  If we do that at the same
> time, we can avoid any mode=2 non-back-compatible breakage?

What? Do you mean detect if it's going to disk or to a pipe?

suid core dumps going to disk is not safe. The "mode=2" stuff was
added in an attempt to make it safe, but it has never actually be
safe. Some Linux systems with integrated crash handlers (i.e.
core_pattern with a pipe) want to catch crashes even in suid
processes, so mode=2 makes sense for them since they're handling the
core dump directly, making decisions about it, etc. However, if that
core_pattern is not a pipe, this leads to local users being able to
trick root processes into doing things to give the user root access.

mode=2 to disk _should_ break, is my point. It is not safe. Hence, my
original change to just disallow a mode=2 coredump from going to disk.
It's fine to throw it at the pipe, so leave that as-is.

-Kees

-- 
Kees Cook
Chrome OS Security
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