Re: [RESEND] Question about recursive mappings

[Mathieu Ropert <mro@adviseo.fr>]

Done some investingations in the mm code of Xen, i think the problem is 
that linear page tables entries are checked only on level 4 entries.
In my case, my recursive entry is in a L3 table, already validated as 
another type, hence not "valid" for Xen.
Will authorizing L3 or L2 recursive mappings induce a security hole or 
vulnerability?
If not, I'll try to make a patch to address this issue (any hint welcome 
:)).

Regards,
Mathieu

Mathieu Ropert wrote:

> [Previous one didn't hit list after 3 days, trying a resend, sorry if 
> both finally show up]
>
> Hi,
>
> are recursive mappings (ie: a page table entry pointing back to itself)
> supported by Xen (on x86_64 at least)?
> I'm asking cause i'm seeing many error logs from get_page_type() telling
> something like "saw L3_page_table expected L2_page_table" or "saw
> L4_page_table expected L3_page_table" (finally leading to a failing 
> mmu_update, i guess others happens on user pagetables switches).
> Or maybe is there any workaround needed? (I think i saw something like
> setting entry to 0 first then to the recursive entry somewhere, but
> can't remember where).
> By the way, i'm using recursive mappings in kernel page directory (which
> seems ok) and i temporay make user page directory recursive when i map
> a user PGD in kernel space (mapping user PGD to a L4 entry of kernel 
> tables, then using kernel L4 slot and user PGD recursive entry to 
> access user page tables).
>
> [edit]
> Done some little research about the problem. Seems like NetBSD use the 
> same thing and works, but there is no x86_64 ports for now. I'm 
> starting to think that may be a x86_64 issue, maybe because recursive 
> mappings don't lead to conflicting types with only 2 levels.
> Xen interface states that a page can only be of one type (PGD, PT, 
> LDT, GDT  and R/W). I don't know why there is a need to distinguish 
> page table levels, but i'm afraid this restriction will conflit with 
> some MMU implementation on x86_64 like NetBSD and OpenBSD, and maybe 
> others (FreeBSD on top of my mind, don't know how much the pmap 
> implementation diverged).
> [/edit]
>
> Regards,
>
> Mathieu
>
>
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