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

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Date:	Thu, 3 Mar 2011 19:46:26 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Stephen Wilson <wilsons@...rt.ca>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 02:38:02PM -0500, Stephen Wilson wrote:
> > 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.

Think what happens if the target execs suid-root binary in the middle of your
call.  After you've done your check.  E.g. during copy_from_user().

On the read side we actually recheck permissions after having copied into
buffer and if the check fails we don't copy that buffer into userland.
Not feasible on the write side...
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