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

[Venki Pallipadi]

On Tue, Aug 2, 2011 at 1:03 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> On Mon 01-08-11 16:33:18, Venki Pallipadi wrote:
>> On Fri, Jul 22, 2011 at 2:01 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
>> >
>> > since 0124cecf (x86, PAT: disable /dev/mem mmap RAM with PAT) we are
>> > disabling access to the system RAM as if STRICT_DEVMEM was enabled by
>> > default. If we, however, disable pat by nopat kernel parameter we can
>> > access that memory without considering CONFIG_STRICT_DEVMEM.
>>
>> Looking at the code, I don't think this problem exists. Code in pat.c
>> disables /dev/mem mmap only when !CONFIG_STRICT_DEVMEM.
>
> Ahh yes, you are right. I got confused by having two separate
> range_is_allowed functions and didn't realize which one is called from
> which context.
>        mmap_mem
>          -> range_is_allowed [drivers/char/mem.c]
>               -EPERM
>          -> phys_mem_access_prot_allowed
>               -> range_is_allowed [arch/x86/mm/pat.c]
>
> So we really get stopped at the first check and do not get to the later
> one.
>
> While we are at it. {read,write}_mem checks only range_is_allowed (mem.c
> version) and it doesn't call phys_mem_access_prot_allowed so we do not
> consider PAT. {read,write}_kmem doesn't check the range at all.
> Is this correct?
> Why do we care _only_ about mmap?
>

Yes. The problem with PAT and /dev/mem mmap is with untracked mmaps in
user address space. Once mapped the mappings can stay in user space
and if there is a page attribute change for that physical address
later kernel doesn't have handle on all such user space mappings that
may exist.
Read/Write on the other hand use direct mappings and will not cause
this problem. I have generally been suggesting tools that want to
access /dev/mem and are unable to mmap it due to PAT to use read/write
instead. /dev/mem on 32 bit has some problems of its own and the
recent patchset on mailing list fixes most of the issues there.

Thanks,
Venki
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