linux-kernel - Re: [PATCH v2 resend] vfs: new O

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Message-ID: <m13a4qsiap.fsf@fess.ebiederm.org>
Date:	Sat, 07 Nov 2009 03:09:18 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	pavel@....cz, alan@...rguk.ukuu.org.uk, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk, dhowells@...hat.com, hch@...radead.org,
	adilger@....com, mtk.manpages@...il.com,
	torvalds@...ux-foundation.org, drepper@...il.com,
	jamie@...reable.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 resend] vfs: new O_NODE open flag

Miklos Szeredi <miklos@...redi.hu> writes:

> On Fri, 06 Nov 2009, ebiederm@...ssion.com (Eric W. Biederman wrote:
>> So far no one who believes this to be a security hole has found it
>> worth their while to look at nd->intent.open in proc_pid_follow_link
>> and write a patch.
>
> A rather disgusting patch that would be.  The fact is, checking
> permissions on follow_link makes little to no sense.  Consider
> truncate(2), for example.  Will we add another intent for that?  I
> really hope not

No.  I was just thinking we have the open intent that is there for
combining lookup and open. We can look test for LOOKUP_OPEN and do
exactly what we need.

> I'm more and more convinced, that the current behavior is the right
> one.

I think the 15 or so years we have had the current behavior without
problems is persuasive.

I think it is an interesting puzzle on how to get dup instead of
reopen as there are cases where that could be useful behavior as well.

The usefulness of an O_NONE flag increases significantly if you can
open the reference file later with more permissions.  Essentially
making a hardlink into a running program.  Hmm.  Weird cases do seem
to show up when the last dir entry is removed.  

I wonder if we want a rule that you can't open a file with link count
of 0.  Reasoning may get truly strange otherwise.

Eric
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