linux-kernel - Re: [PATCH v2] remove no longer use of pdflush interface

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Date:	Tue, 5 Jun 2012 21:01:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wanpeng Li <liwp.linux@...il.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Rob Landley <rob@...dley.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	"David S. Miller" <davem@...emloft.net>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	David Howells <dhowells@...hat.com>,
	James Morris <james.l.morris@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Michel Lespinasse <walken@...gle.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Gavin Shan <shangw@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] remove no longer use of pdflush interface

On Wed, 6 Jun 2012 11:21:27 +0800 Wanpeng Li <liwp.linux@...il.com> wrote:

> >Secondly, this code will permit unprivileged users to flood the logs,
> >by repeatedly reading /proc/sys/vm/nr_pdflush_threads.  We try to avoid
> >this, as it is a form of denial-of-service attack.
> >
> >This is a bit hard to fix.  The typical way of addressing this is to
> >use printk_once(), so the message only appears once per boot.  But that
> >doesn't work for a generic function - we'd need to add one bit of state
> >to the ctl_table to do this.  We can of course do that, but it's not
> >obvious that it's _worth_ doing that just for handling obsolete
> >entries.
> >
> >So perhaps the solution is to give up on the generic proc_obsolete()
> >idea, and just add a handler specifically for nr_pdflush_threads, whcih
> >uses printk_once().
> 
> What about modify the generic proc_obsolete just to put the warning message into
> the buffer, then transfer to userspace, in order to users can see this warning.Do
> you think this is a better idea?

err, no.  That's deliberately corrupting the output of an existing
interface.  In response to this change userspace might crash, hand over
root privileges or tune to a porn channel.  Instead we should yield
a well-formed response such as "0" (or maybe non-zero, if a zero
response might be unexpected) and inform the system operator via
conventional means.  ie: syslog.  
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