Re: [Xen-devel] changing attributes of a page!

[Tim Deegan <tim@xxxxxxx>]

Hi, 

At 10:22 +0330 on 09 Jan (1326104566), Mohamad Rezaei wrote:
> Hi,
> 
> I am trying to change attributes of a page from Dom0.

Do you mean a page of dom0's memory? 

> The reason is
> that I want to make a kernel module completely read-only to other
> parts of kernel. I will update it from hypervisor itself. I have tried
> to do this by this code:
> 
> // I have the mfn of the page in Dom0's address space.
> void hamed_set_entry(struct p2m_domain *p2m, mfn_t mfn) {
>     unsigned long gfn = mfn_to_gfn(p2m->domain,mfn);
>     p2m_type_t p2mt;
>     p2m_access_t p2ma;
>     p2m_lock(p2m);
>     p2m->get_entry(p2m, gfn, &p2mt, &p2ma, p2m_query);
>     p2m->set_entry(p2m, gfn, mfn, 0, p2mt, p2m_access_rwx);
>     p2m_unlock(p2m);
> }

That looks plausible for a HVM guest, but dom0 is a PV guest and doesn't
have a p2m table, so you're likely to crash Xen if you try to to this
to dom0.

Do you have a serial console set up on your test machine?  It's _very_
useful for finding out why the system crashed, since Xen will usually
print a backtrace when it crashes. 

> But whenever it runs Dom0 restarts. I am not even sure this is the
> right way to do this. I am grateful for any help!

To do this to dom0 you could
 (a) get dom0 to make the memory read-only in its own pagetables; and
 (b) enforce that read-only property in the PTE validation code in mm.c

Or you could run dom0 under shadow pagetables and enforce the read-only
property in _sh_propagate().  That will have a performace hit, though. 

Cheers,

Tim.

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