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Re: [PATCH] x86/mm: PGC_page_table is used by shadow code only
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Tue, 29 Nov 2022 21:10:31 +0000
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHZBAKvvRJ2heCKJ0SiESCucWQuIK5WYfSAgAAEDYA=
- Thread-topic: [PATCH] x86/mm: PGC_page_table is used by shadow code only
On 29/11/2022 20:56, Andrew Cooper wrote:
> On 29/11/2022 14:55, Jan Beulich wrote:
>> By defining the constant to zero when !SHADOW_PAGING we give compilers
>> the chance to eliminate a little more dead code elsewhere in the tree.
>> Plus, as a minor benefit, the general reference count can be one bit
>> wider. (To simplify things, have PGC_page_table change places with
>> PGC_extra.)
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Ahead of making this change, can we please rename it to something less
> confusing, and fix it's comment which is wrong.
>
> PGC_shadowed_pt is the best I can think of.
>
>> ---
>> tboot.c's update_pagetable_mac() is suspicious: It effectively is a
>> no-op even prior to this change when !SHADOW_PAGING, which can't be
>> quite right. If (guest) page tables are relevant to include in the
>> verification, shouldn't this look for PGT_l<N>_page_table as well? How
>> to deal with HAP guests there is entirely unclear.
> Considering the caller, it MACs every domheap page for domains with
> CDF_s3_integrity.
>
> The tboot logical also blindly assumes that any non-idle domain has an
> Intel IOMMU context with it. This only doesn't (trivially) expose
> because struct domain_iommu is embedded in struct domain (rather than
> allocated separately), and reaching into the wrong part of the arch
> union is only mitigated by the tboot logic not being invoked on
> non-intel systems. (Also the idle domain check is useless, given that
> it's in a for_each_domain() loop).
Wow I really failed at typing here. "The tboot logic", and "doesn't
(trivially) explode".
~Andrew
>
> It does look a little like the caller is wanting to MAC all Xen data
> that describes the guest, but doing this unilaterally for all shadowed
> guests seems wrong beside the per-domain s3_integrity setting.
>
> ~Andrew
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