[Xen-devel] tracking of Xen heap pages shared with guest

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

I assume I'm overlooking something, but can someone explain how page
tracking works in the following two cases:

a) A guest unintentionally or maliciously frees (e.g. through
decrease_reservation) a page shared from the Xen heap (e.g. the
shared info page). From what I can see, such a page would have a
reference count of 1 (from share_xen_page_with_guest(), assuming
the guest doesn't have the page mapped), and would hence be
immediately freed with the corresponding put_page(). Nevertheless
Xen itself may continue to write to such a page.

b) A domU that had a xenoprof buffer allocated gets killed. Since the
xenoprof code directly calls free_xenheap_pages() on the buffer,
any mapping dom0 may have to it would not be considered, and hence
dom0 would retain a mapping to free memory. Additionally, the
put_page() in unshare_xenoprof_page_with_guest() could revert the
singe reference to the page established through
share_xen_page_with_guest() (i.e. if dom0 never mapped or already
unmapped the buffer), which again would result in the buffer getting
freed (and thus d->xenoprof->rawbuf becoming stale).

Apparently I'm just failing to find the places where extra reference
counts are being established for such pages...

Thanks, Jan


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