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Re: [PATCH] xfs: change the immutable in xfs_open_by_handle

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH] xfs: change the immutable in xfs_open_by_handle
From: Matthias Schniedermeyer <ms@xxxxxxx>
Date: Wed, 11 Sep 2013 17:50:17 +0200
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Greg Banks <gbanks@xxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52307CB6.3060506@xxxxxxx>
References: <20130910184724.726933044@xxxxxxx> <20130911135502.GA21898@xxxxxxxxxxxxx> <52307CB6.3060506@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On 11.09.2013 09:22, Mark Tinguely wrote:
> On 09/11/13 08:55, Christoph Hellwig wrote:
> >On Tue, Sep 10, 2013 at 01:47:20PM -0500, Mark Tinguely wrote:
> >>This patch allows clients like DMF to modify an immutable file
> >>without changing the immutable capability on the file, which
> >>would expose the file to change.
> >>
> >>This patch is restricted to holders of the CAP_LINUX_IMMUTABLE,
> >>so no addition security risk has been introduced.
> >
> >The immutable flag means that the file can't be modified, and
> >CAP_LINUX_IMMUTABLE allows setting/removing that restriction, but not
> >ignoring it.
> >
> >So: NAK, this is a change in semantics and long-standing behaviour.
> >
> As you said, the CAP_LINUX_IMMUTABLE allows the holder of the
> capability to turn on/off the restriction. The holder of
> CAP_LINUX_IMMUTABLE could turn off immutability, modify it and then
> turn it back on, but during that window, others could modify it too
> which may be more undesirable than changing the behavior.

It's like setting a file you own "444" only because you CAN set it to 
666 doesn't mean that the check should be short-circuted.

$ touch test
$ chmod 444 test
$ echo 'Hallo' > test
-bash: test: Permission denied



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