linux-ext4 - Re: Atomic non-durable file write API

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Date:	Wed, 29 Dec 2010 09:35:38 +1100
From:	Neil Brown <neilb@...e.de>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Olaf van der Spek <olafvdspek@...il.com>,
	Christian Stroetmann <stroetmann@...olinux.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, "Ted Ts'o" <tytso@....edu>,
	Nick Piggin <npiggin@...il.com>
Subject: Re: Atomic non-durable file write API

On Tue, 28 Dec 2010 17:15:57 -0500 Greg Freemyer <greg.freemyer@...il.com>
wrote:

> On Tue, Dec 28, 2010 at 5:06 PM, Olaf van der Spek <olafvdspek@...il.com> wrote:
> > On Tue, Dec 28, 2010 at 11:00 PM, Greg Freemyer <greg.freemyer@...il.com> wrote:
> >> create temp file
> >> write out new data
> >> delete old file
> >> rename temp file to primary name
> >> ===
> >>
> >> If so there is still a little window of vulnerability where the whole
> >> file can be lost.  (Or at least only the temp file is present).
> >
> > Delete isn't used, rename will overwrite the old file. So it's safe.
> > Meta-data is probably lost, file owner is certainly lost.
> >
> > Olaf
> 
> So ACLs are lost?
> 
> That seems like a potentially bigger issue than loosing the owner/group info.
> 
> And I assume if the owner changes, then the new owner has privileges
> to modify ACLs he didn't have previously.
> 
> So if I want to instigate a simple denial of service in a multi-user
> environment, I edit a few key docs that I have privileges to edit.  By
> doing so I take ownership.  As owner I  change the permissions and
> ACLs so that no one but me can access them.
> 
> Seems like a security hole to me.

Giving someone you don't trust uncontrolled write access to something you
value has always been a security issue - long before ACLs or editors or
computers.

NeilBrown
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