Re: [Xen-devel] Re: [PATCH]Add a flag for shadow pages

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 11:57 +0000 on 04 Mar (1236167835), Keir Fraser wrote:
> I'm working on this by the way. I'll clean up everything except shadow uses
> of page_get_owner(). The only two possibly suspect uses I can see (most are
> just ASSERT/BUG_ON uses I think are okay):
>  * sh_mfn_is_a_pagetable()
>  * shadow_get_page_from_l1e()
> 
> It'd be good if Tim or Gianluca would check whether these need to be more
> careful -- could page_get_owner() return a duff non-NULL value in either of
> these functions? This could only happen if the pages they work on could
> possibly actually be shadow pages with clobbered page owner field.

shadow_get_page_from_l1e() should never be handling a pointer to a
shadow -- if it does that then we've let the guest see the shadows and
all invariants go out the window. 

sh_mfn_is_a_pagetable() looks OK too; it only gets called based on the
contents of shadow PTEs or the MFNs that guests are writing to, both of
which should be safe.

It all feels a bit fragile to me though, compared to the old layout
where we always knew the owner field would be NULL.

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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