linux-kernel - Re: Out of tree module using LSM

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Date:	Thu, 29 Nov 2007 22:09:33 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	tvrtko.ursulin@...hos.com, Al Viro <viro@....linux.org.uk>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, Valdis.Kletnieks@...edu
Subject: Re: Out of tree module using LSM

Alan Cox <alan@...rguk.ukuu.org.uk> writes:

> If I want I can have 16 threads executing code in a shared object being
> written to by ten other programs at once and shared over a network while
> we are at it. Its probably not a good idea but I can do it if I have
> reason to.

Actually the kernel prevents writing to files which are executed currently
and you can also not execute what is currently open for writing.
That is what VM_DENYWRITE is good for.

Does not work for shared libraries though -- you cannot set VM_DENYWRITE
in user mmap aka ld.so Man page says

  MAP_DENYWRITE
              This flag is ignored.  (Long ago, it signalled that attempts to write to the underlying file should fail
              with ETXTBUSY.  But this was a source of denial-of-service attacks.)

-Andi
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