Re: [Xen-devel] [RESEND] Question about recursive mappings

[Mathieu Ropert <mro@xxxxxxxxxx>]

Keir Fraser wrote:

> On 23 May 2006, at 10:41, Ian Pratt wrote:
>
> > Can you explain the pagtable structure a little better please. Does the
> > L3 contain an entry point at itself (recursive), or at another L3
> > (foreign)?
> >
> > I've never come across non-root pagetable linear mappings other than in
> > the crock that is PAE (3 level) where you have to use 4 entries in an L2
> > to point to all the L2s due to the limited address space.
>
> And that's something we don't currently support. A slightly more 
> generic linear pagetable logic that would permit us to support linear 
> pagetables on PAE would be a good thing.
>  -- Keir
A bit more explanation:
- All my L4 tables (kernel and user) have one of their entries pointing 
on themselves.
- As I can't use the same tables in kernel and user mode on x86_64, the 
kernel table is always the same, and sometimes needs to map a user L4 to 
modify.
- In order to do that, i set an entry of the kernel L4 to the machine 
address of the wanted L4 user table. So my L4 user recursive entry is 
used as a L3 one in kernel tables.
- When i want to access a user table in kernel mode, i do 2 mmu updates:
1/ Set a L4 kernel table entry with the machine address of the L4 user 
table i want to change. This step currently works.
2/ Set the L4 user table recursive as valid (i clear the valid bit when 
i'm done with my changes, don't want the user to be able to read his 
page tables). This is where Xen refuse to update the tables.
About the possible fix, won't something like trying a 
get_linear_pagetable() at all level works? Will it cause too much overhead?
Generally speaking, is allowing a R/O mapping of another table the 
domain own (whichever level it is) safe? At first thought, i don't see 
any way of exploiting it.
Regards,
Mathieu

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