linux-kernel - Re: Upstream first policy

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Date:	Mon, 8 Mar 2010 15:37:38 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Ingo Molnar <mingo@...e.hu>,
	James Morris <jmorris@...ei.org>, linux-kernel@...r.kernel.org,
	Kyle McMartin <kyle@...artin.ca>,
	Alexander Viro <viro@....linux.org.uk>
Subject: Re: Upstream first policy

On Mon, 8 Mar 2010, Rik van Riel wrote:
> 
> On the other hand, '/etc/shadow' has the opposite constraint,
> where the system will not trust most of the applications with
> the data from that file.

Umm. No.

/etc/shadow is in no way at all different from /etc/passwd. Both of them 
have pathname-based security issues. The fact that both of them _also_ 
have content-based security issues is an independent issue that I just 
assumed everybody would take for granted.

Clearly I assumed too much.

So I was assuming that everybody realized that the normal inode-based UNIX 
security obviously means that you can only open /etc/passwd read-only as 
any normal user (and not open /etc/shadow at all: but that is in _no_ way 
different from /etc/passwd).

That's an example of non-pathname-based security, where you actually mark 
the content itself restricted some way. It's very naturally done with 
labels on the inode itself. It's what UNIX has _always_ done

Nobody has ever suggested removing that. That would be crazy.

But that thing is _independent_ from the other totally unrelated issue, 
namely the fact that "/etc/passwd" is a special name in the namespace. In 
other words, there is "content security", but then there is also 
"namespace security". 

Of course, you can make /etc unwritable, and that is indeed the 
traditional UNIX model of handling namespace security: by just 
implementing it as "content security" of the directory.

The sgid and sticky bits can be used to further try to make it more 
fine-grained (exactly becuase it is _not_ sufficient to say "you can't 
read or write this directory" on a whole-directory basis), and obviously 
SELinux has extensions of its own too.

Can you really not see the difference between security of naming thigns 
certain things (like "/etc/passwd") - pathname based issues - and the 
separate security of limiting access to any named device - actual markings 
on the inode itself?

				Linus
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