Re: [PATCH] x86, PAT: honor CONFIG_STRICT_DEVMEM if pat is disable]

["H. Peter Anvin" <hpa@zytor.com>]

On 07/22/2011 02:11 AM, Michal Hocko wrote:
> Hi,
> I have just come across a strange behavior of /dev/[k]mem when PAT is
> configured while STRICT_DEVMEM is disabled. 
> One would expect that /dev/kmem would allow to access also the
> system RAM in that configuration but that is not obviously true as pat
> code defines range_is_allowed to protect from accessing that memory.
> 
> AFAICS this behavior was introduced in 0124cecf (x86, PAT: disable
> /dev/mem mmap RAM with PAT) which says that it disables [k]mem with PAT
> because it is safer. There is no explanation why it allows to access
> that memory if CONFIG_NONPROMISC_DEVMEM (CONFIG_STRICT_DEVMEM now).
> 
> The thing is even more complicated by the fact that the access is
> allowed when nopat kernel parameter is specified because
> range_is_allowed just does't call devmem_is_allowed in that case.
> 
> While I do agree that the feature is not safe in general we should honor
> STRICT_DEVMEM setting in some way IMO.
> 
> What do you think about the following fix? I have tried to preserve
> "disabled for PAT" by default behavior.

The reason it is disabled for PAT is that it is very hard to track maps
of that memory that are created by mapping /dev/[k]mem, since those maps
don't have a defined PAT type and really should be transparently
tracking the consensus caching type; this is a facility that *could* be
created but has no other user.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.