Re: [Xen-devel] [PATCH] x86/mm: Adjust linear uses / entries when a page loses validation

[Jürgen Groß <jgross@xxxxxxxx>]

On 22.11.19 19:52, George Dunlap wrote:
> "Linear pagetables" is a technique which involves either pointing a
> pagetable at itself, or to another pagetable the same or higher level.
> Xen has limited support for linear pagetables: A page may either point
> to itself, or point to another page of the same level (i.e., L2 to L2,
> L3 to L3, and so on).
>
> XSA-240 introduced an additional restriction that limited the "depth"
> of such chains by allowing pages to either *point to* other pages of
> the same level, or *be pointed to* by other pages of the same level,
> but not both.  To implement this, we keep track of the number of
> outstanding times a page points to or is pointed to another page
> table, to prevent both from happening at the same time.
>
> Additionally, XSA-299 introduced a mode whereby if a page was known to
> have been only partially validated, _put_page_type() would be called
> with PTF_partial_set, indicating that if the page had been
> de-validated by someone else, the type count should be left alone.
>
> Unfortunately, this change did not account for the required accounting
> for linear page table uses and entries; in the case that a previously
> partially-devalidated pagetable was fully-devalidated by someone else,
> the linear_pt_counts are not updated.
>
> This could happen in one of two places:
>
> 1. In the case a partially-devalidated page was re-validated by
> someone else
>
> 2. During domain tear-down, when pages are force-invalidated while
> leaving the type count intact.
>
> The second could be ignored, since at that point the pages can no
> longer be abused; but the first requires handling.  Note however that
> this would not be a security issue: having the counts be too high is
> overly strict (i.e., will prevent a page from being used in a way
> which is perfectly safe), but shouldn't cause any other issues.
>
> Fix this by adjusting the linear counts when a page loses validation,
> regardless of whether the de-validation completed or was only partial.
>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Release-acked-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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