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

[Michal Hocko]

On Fri 22-07-11 09:15:08, H. Peter Anvin wrote:
> 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.

Thanks for the clarification!
What do you think about fixing the nopat with STRICT_DEVMEM like the
patch does?

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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