Re: [Xen-devel] /dev/mem

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > What I'm seeing is that accesses to /dev/mem offset 0x100000 get a
> > failure in __direct_remap_pfn_range() (including, if
> > debugging is turned on, two log messages from the hypervisor). If
> > these accesses went to kernel memory instead,
> > consumers of this interface would get exactly what they get on a
> > native OS.
>
> Does anyone sane peek /dev/mem to get at kernel code/data? I doubt it.

In the old days of 2.4 (and some of 2.6???), /dev/mem would only map memory 
with the reserved bit set, because the only users watned device memory, not 
real RAM (and because implementing full functionality upset memory management 
in a way I'm not too clear on now...).  As a result peeking kernel code / 
data wasn't even possible on native Linux in the recent past...

Cheers,
Mark

> > Further I would think that such map attempts shouldn't even make it to
> > the hypervisor, thus avoiding the log messages
> > (which I started looking into only because I thought they might
> > indicate a latent bug of some sort). This would,
> > however, require knowledge in the kernel which addresses (or address
> > ranges) represent physical memory (rather than
> > potential I/O memory regions).
>
> Non-debug Xen builds don;t print those messages so won't scare users.
>
> > Finally, while following the involved code paths, I noted a check that
> > is made in __ioremap, by calling
> > is_local_lowmem(). It would seem to me that calling this function just
> > for the first address is insufficient - the range
> > specified may begin in non-lowmem but end in lowmem. Or it may begin
> > in lowmem and end in non-lowmem, in which case I
> > would think that virt_to_page() wouldn't work correctly anymore
> > (permitting, say, an erroneous dd command issued by root
> > to result in an oops or maybe worse things).
>
> It's assumed people using those interfaces know what they're doing. How
> can a 'dd' end up in __ioremap() anyway?
>
>   -- Keir
>
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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