Subject: Re: [PATCH] mm/nommu.c: Fix improperly call of security API in mmap
From: Eric Paris <eparis@redhat.com>
To: David Howells <dhowells@redhat.com>
Cc: graff.yang@gmail.com, linux-kernel@vger.kernel.org,
       gyang@blackfin.uclinux.org, akpm@linux-foundation.org,
       uclinux-dist-devel@blackfin.uclinux.org,
       Graff Yang <graf.yang@analog.com>,
       linux-security-module@vger.kernel.org
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Date: Fri, 16 Oct 2009 11:55:29 -0400
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On Fri, 2009-10-16 at 16:43 +0100, David Howells wrote:
> Eric Paris <eparis@redhat.com> wrote:
> 
> > That would still call cap_file_mmap() and wouldn't solve your problem.
> 
> Hmmm...  I guess I don't see the problem occur because I always run the
> programs as root.

most likely yes, your processes are going to have CAP_SYS_RAWIO and will
happily sail through the cap_file_mmap().

> I would guess that cap_file_mmap() and selinux_file_mmap() are, perhaps, too
> strict.  The hint shouldn't be rejected unless MAP_FIXED is also set, surely,
> but should rather be revised upwards.

On the mmu side we don't check until the kernel has turned the hint into
a real address.  Which is apparently different then where the hook was
originally placed in the nommu side.

> Certainly, addr==NULL and !MAP_FIXED is a reasonable case to permit, even in
> tightly secured MMU and SELinux mode...  After all, the manual page says:
> 
> 	If addr is NULL, then the kernel chooses the address at which to create
> 	the mapping; this is the most portable method of creating  a  new  map-
> 	ping.

I agree, I think the choices are

A) agree that we just shouldn't check the address on nommu
   A1) change kconfig to not allow the setting of these
	(easy to do for the LSM check, but not easy as it stands today
	 for the cap_file_mmap() check)
   A2) change the addr_only to be a flags which indicate not to check
	(my original suggestion)
   A3) push the config_nommu down into the security code
	(your patch)
B) actually check the address, which requires moving the hook to a
location where it has been resolved.
	(graff yang's patch)

I'll leave it up to you to determine which you like the best since you
know the implications of nommu.

-Eric

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