linux-kernel - Re: [PATCH 0/9] proc: protect /proc/<pid>/* files across execve

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Date:	Sat, 10 Mar 2012 16:01:09 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Kees Cook <keescook@...omium.org>,
	Solar Designer <solar@...nwall.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	James Morris <james.l.morris@...cle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg KH <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...e.hu>, Stephen Wilson <wilsons@...rt.ca>,
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH 0/9] proc: protect /proc/<pid>/* files across execve

On Sat, Mar 10, 2012 at 3:25 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
>
> 1) Use the target exec_id to bind files to their exec_id task:
>
> For the REG files /proc/<pid>/{environ,pagemap,mem} we set the exec_id
> of the proc_file_private to the target task, and we continue with
> permission checks at open time, later on each read/write call the
> permission checks are done + check the target exec_id if it equals the
> exec_id of the proc_file_private that was set at open time, in other words
> we bind the file to its task's exec_id, this way new exec programs can not
> operate on the passed fd.

So the exec_id approach was totally broken when it was used for
/proc/<pid>/mem, is there any reason to believe it's a good idea now?

It's entirely predictable, and you can make the exec_id match by
simply forking elsewhere and then passing the fd around using unix
domain sockets, since the exec_id is just updated by incrementing a
counter.

I would in general suggest strongly against using exec_id for anything
that involves files. It isn't designed for that, it's designed for the
whole "check the parent exec_id" thing for ptrace, where that whole
"pass things around to another process" approach doesn't work.

                Linus
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