Re: [PATCH v17 1/2] mem_sharing: fix sharability check during fork reset

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Apr 23, 2020 at 08:30:06AM -0700, Tamas K Lengyel wrote:
> When resetting a VM fork we ought to only remove pages that were allocated for
> the fork during it's execution and the contents copied over from the parent.
> This can be determined if the page is sharable as special pages used by the
> fork for other purposes will not pass this test. Unfortunately during the fork
> reset loop we only partially check whether that's the case. A page's type may
> indicate it is sharable (pass p2m_is_sharable) but that's not a sufficient
> check by itself. All checks that are normally performed before a page is
> converted to the sharable type need to be performed to avoid removing pages
> from the p2m that may be used for other purposes. For example, currently the
> reset loop also removes the vcpu info pages from the p2m, potentially putting
> the guest into infinite page-fault loops.
> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

I've been looking however and I'm not able to spot where you copy the
shared_info data, which I think it's also quite critical for the
domain, as it contains the info about event channels (when using L2).
Also for HVM forks the shared info should be mapped at the same gfn as
the parent, or else the child trying to access it will trigger an EPT
fault that won't be able to populate such gfn, because the shared_info
on the parent is already shared between Xen and the parent.

Thanks, Roger.