Re: [Xen-devel] pagetable RO

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.13 at 10:59, åæ<zhangzhi2022@xxxxxxxxxxx> wrote:
>> Date: Fri, 11 Oct 2013 09:36:44 +0100
>> From: JBeulich@xxxxxxxx 
>> >>> On 11.10.13 at 09:37, åæ<zhangzhi2022@xxxxxxxxxxx> wrote:
>> >> Date: Fri, 11 Oct 2013 08:15:43 +0100
>> >> From: JBeulich@xxxxxxxx 
>> >> >>> On 11.10.13 at 08:30, åæ<zhangzhi2022@xxxxxxxxxxx> wrote:
>> >> >       Say, the sub-op: MMU_NORMAL_PT_UPDATE,
>> >> > if the page that ptr of struct mmu_update points to is 
>> >> > PGT_l2_page_table, 
>> >> > then
>> >> > the address part of val of struct mmu_update points
>> >> > to a page, which is PGT_l1_page_table.
>> >> > However, I couldn't find the protections for this PGT_l1_page_table
>> >> > page in the do_mmu_update, is it already set as read-only
>> >> > before launching MMU_UPDATE hypercall?
>> >> 
>> > 
>> >> Of course - a page can't be PGT_l1_page_table when there still is
>> >> some writable mapping around. 
>> > Thanks, but when will the page be set as read-only before the MMU_UPDATE 
>> > hypercall ? is there any hint?
>> 
>> Whenever the guest kernel wants to do that.

Did you read this at all? Because it answers ...

>> >> So for an L2 entry update it is sufficient to verify that all contained 
>> >> L1 
>> > entries point to R/O
>> >> pages (and transition them to PGT_l1_page_table if they aren't
>> >> already).
>> > 
>> > Besides, in the function do_mmu_update for an L2 entry update, the new 
>> > PGT_l1_page_table page, containing all L1 entries,  does not have the 
> process 
>> > of checking whether they are pointing to the pages of page table type. 
>> 
>> It surely does: mod_l2_entry() -> get_page_from_l2e() ->
>> get_page_and_type_from_pagenr(mfn, PGT_l1_page_table, ...).
>> 
> 
> Not exactly, get_page_and_type_from_pagenr(mfn, PGT_l1_page_table, ...) -> 
> ...->alloc_page_type(page, PGT_l1_page_table, 0) -> alloc_l1_table(page) 
> See, in the function: alloc_l1_table(page), only when the function: 
> get_page_from_l1e(pl1e[i], ...) returns 1, will the function: 
> l1e_remove_flags(pl1e[i], _PAGE_RW)  does execute. This is what I've just 
> said "R/W flag is removed from the L1 entry" . Further, the function: 
> get_page_from_l1e(pl1e[i], ...) will return 1 only when this check: ( 
> !mfn_valid(mfn) || (real_pg_owner = page_get_owner_and_reference(page)) == 
> dom_io ) succeeds. 
> From above, it is concluded that do_mmu_update does not set the L1 entries 
> in the new PGT_l1_page_table , which point to pages of page table type, as 
> read-only. 
> In a nutshell, where does xen hypervisor set the new PGT_l1_page_table as 
> read-only?

... this question: The hypervisor does not do this at all. It only
verifies that the guest kernel did.

Jan

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