Re: [Xen-devel] [PATCH] x86/xpti: really hide almost all of Xen image

[Juergen Gross <jgross@xxxxxxxx>]

On 02/03/18 12:23, Jan Beulich wrote:
> Commit 422588e885 ("x86/xpti: Hide almost all of .text and all
> .data/.rodata/.bss mappings") carefully limited the Xen image cloning to
> just entry code, but then overwrote the just allocated and populated L3
> entry with the normal one again covering both Xen image and stubs.
> 
> Drop the respective code in favor of an explicit clone_mapping()
> invocation. This in turn now requires setup_cpu_root_pgt() to run after
> stub setup in all cases. Additionally, with (almost) no unintended
> mappings left, the BSP's IDT now also needs to be page aligned.
> 
> Note that the removed BUILD_BUG_ON()s don't get replaced by anything -
> there already is a suitable ASSERT() in xen.lds.S.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Tested-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

> ---
> What should we do with the TSS? Currently together with it we expose
> almost a full page of other per-CPU data. A simple (but slightly
> hackish) option would be to use one of the two unused stack slots.

Either one of the unused stack pages or directly after the GDT (we could
then drop NR_RESERVED_GDT_PAGES and reduce NR_RESERVED_GDT_ENTRIES to
a lower value, e.g. 16 or 32).

> Talking of the stack: While APs properly have the guard page mirrored
> into the cloned page tables, this is a debug-build only thing _and_
> doesn't cover the BSP. Should we perhaps add code to fully mirror the
> guard pages, including on release builds?

+1


Juergen

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