linux-kernel - Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error bug

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Date:	Mon, 26 Mar 2007 14:24:38 +0100
From:	David Howells <dhowells@...hat.com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, bryan.wu@...log.com,
	linux-kernel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error bug 

Pekka J Enberg <penberg@...helsinki.fi> wrote:

> We don't touch private mappings at all as they're a snapshot to the inode 
> _before_ it was revoked. So private mappings don't really matter all: you 
> don't see any new data after it has been revoked nor do you flush anything 
> to the disk.

Okay, so that's not a problem.

> Well, assuming we would use revoke for things like SAK, this doesn't 
> really work out too well because all a malicious process has to is create 
> a shared mapping and they've effectively blocked the whole thing.

In NOMMU-mode, there's probably[*] nothing stopping a malicious process
running completely amok and changing stuff directly - even the kernel isn't
guaranteed to be safe - so I wouldn't worry about such a case.

[*] The FRV, for example, does have some limited protection capability - but
it is really limited and not really useful in this case.

> It's antisocial for sure but the only way to guarantee revoke() succeeds on
> a NOMMU setup.  Oh well, lets disable it for now and see if anyone even
> wants revoke() for NOMMU.

Agreed.

David
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