linux-kernel - Re: [PATCH] Enable writing to /proc/PID/mem.

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Date:	Thu, 3 Mar 2011 14:38:02 -0500
From:	Stephen Wilson <wilsons@...rt.ca>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Rientjes <rientjes@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Enable writing to /proc/PID/mem.

On Thu, Mar 03, 2011 at 11:22:59AM +0900, KOSAKI Motohiro wrote:
> > For a long time /proc/PID/mem has provided a read-only interface, at least since
> > 2.4.0.  However, a write capability has existed "forever" in tree via the
> > function mem_write, disabled with an #ifdef along with the comment "this is a
> > security hazard".  Charles Wright, back in 2006, gave some history on the
> > subject:
> > 
> >     http://lkml.org/lkml/2006/3/10/224
> > 
> > Later, in commit 638fa202c, Roland McGrath updated mem_write to call
> > check_mem_permission which ensures an identical security policy for
> > /proc/PID/mem as for ptrace().  IOW, the proc interface provides a simpler, more
> > efficient, but otherwise equivalent mechanism for probing a processes memory as
> > available via ptrace.
> > 
> > There is no longer a security hazard and the world can safely use read/write
> > instead of ptrace PEEK/POKE's.  Remove the #ifdef.
> > 
> > Signed-off-by: Stephen Wilson <wilsons@...rt.ca>
> 
> I haven't found any problem in this patch. But, I really believe we need
> to understand why it was marked "security hazard". Al, I guess you know it,
> right? So, can you please talk us your mention?

I did a bit more digging trying to find why mem_write was marked a security
hazard.

It goes back to 2.4.0-test10pre4.  Unfortunately, the changelog entry is
not at all informative either:

	- disable writing to /proc/xxx/mem. Sure, it works now, but it's
	  still a security risk.

For the interested, some of the history is visible here:

	http://tinyurl.com/4aj4d3v


Personally, I have doubts that there is much to be gleaned from this
"security hazard" comment or any amount of archeology.

The code in question has been maintained for over a decade without use.
However, the implementation looks sane to me from a security POV as it
mirrors the policy for ptrace.  But it would be great to have a few more
eyes audit this change.

-- 
steve

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