Re: [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking

Re: [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking

[Dave Jones]

On Tue, Nov 16, 2010 at 11:59:03PM +0000, Linux Kernel wrote:
 > Gitweb:     http://git.kernel.org/linus/ec6743bb06510c7b629603ce35713d6ae9273579
 > Commit:     ec6743bb06510c7b629603ce35713d6ae9273579
 > Parent:     ca768b663131ca644689fcadc9ca092dcc96a758
 > Author:     Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
 > AuthorDate: Wed Nov 10 10:05:55 2010 +0100
 > Committer:  Martin Schwidefsky <sky@mschwide.boeblingen.de.ibm.com>
 > CommitDate: Wed Nov 10 10:05:54 2010 +0100
 > 
 >     [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking
 >     
 >     Provide the devmem_is_allowed() routine to restrict access to
 >     kernel memory from userspace.
 >     Set the CONFIG_STRICT_DEVMEM config option to switch on checking.
 >     
 >     Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
 >     Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

[snip add kconfig]

...

 > +static inline int devmem_is_allowed(unsigned long pfn)
 > +{
 > +	return 0;
 > +}

So you add the config option that promises security, but then you always
allow the access.  This seems pointless ?  Why bother having the option at all?

	Dave

Re: [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking

[Martin Schwidefsky]

On Wed, 17 Nov 2010 13:03:21 -0500
Dave Jones <davej@redhat.com> wrote:

> On Tue, Nov 16, 2010 at 11:59:03PM +0000, Linux Kernel wrote:
>  > Gitweb:     http://git.kernel.org/linus/ec6743bb06510c7b629603ce35713d6ae9273579
>  > Commit:     ec6743bb06510c7b629603ce35713d6ae9273579
>  > Parent:     ca768b663131ca644689fcadc9ca092dcc96a758
>  > Author:     Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
>  > AuthorDate: Wed Nov 10 10:05:55 2010 +0100
>  > Committer:  Martin Schwidefsky <sky@mschwide.boeblingen.de.ibm.com>
>  > CommitDate: Wed Nov 10 10:05:54 2010 +0100
>  > 
>  >     [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking
>  >     
>  >     Provide the devmem_is_allowed() routine to restrict access to
>  >     kernel memory from userspace.
>  >     Set the CONFIG_STRICT_DEVMEM config option to switch on checking.
>  >     
>  >     Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
>  >     Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> 
> [snip add kconfig]
> 
> ...
> 
>  > +static inline int devmem_is_allowed(unsigned long pfn)
>  > +{
>  > +	return 0;
>  > +}
> 
> So you add the config option that promises security, but then you always
> allow the access.  This seems pointless ?  Why bother having the option at all?

It is the other way around, access is always disallowed. So it is very secure,
access via /dev/mem is disabled if STRICT_DEVMEM=y.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.