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



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