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Re: [PATCH 2/4] x86/P2M: relax guarding of MMIO entries

  • To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Tue, 31 Aug 2021 15:16:22 +0200
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Tue, 31 Aug 2021 13:16:31 +0000
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On 30.08.2021 15:02, Jan Beulich wrote:
> One of the changes comprising the fixes for XSA-378 disallows replacing
> MMIO mappings by unintended (for this purpose) code paths. At least in
> the case of PVH Dom0 hitting an RMRR covered by an E820 ACPI region,
> this is too strict. Generally short-circuit requests establishing the
> same kind of mapping that's already in place.
> Further permit "access" to differ in the "executable" attribute. While
> ideally only ROM regions would get mapped with X set, getting there is
> quite a bit of work. Therefore, as a temporary measure, permit X to
> vary. For Dom0 the more permissive of the types will be used, while for
> DomU it'll be the more restrictive one.
> While there, also add a log message to the other domain_crash()
> invocation that did prevent PVH Dom0 from coming up after the XSA-378
> changes.
> Fixes: 753cb68e6530 ("x86/p2m: guard (in particular) identity mapping 
> entries")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Btw, I had meant to have this post-commit-message remark here:

TBD: This could be generalized to all of R, W, and X. Dealing with just X
     is merely the minimum I found is immediately necessary.




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