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Re: [Xen-devel] [PATCH] include/public/memory.h: remove the XENMEM_rsrc_acq_caller_owned flag
- To: Paul Durrant <paul.durrant@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 19 Jul 2019 13:41:49 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- Delivery-date: Fri, 19 Jul 2019 12:42:00 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 19/07/2019 13:25, Paul Durrant wrote:
> When commit 3f8f1228 "x86/mm: add HYPERVISOR_memory_op to acquire guest
> resources" introduced the concept of directly mapping some guest resources,
> it was envisaged that the memory for some resources associated with a guest
> may not actually be assigned to that guest, specifically the IOREQ server
> resource introduces in commit 6e387461 "x86/hvm/ioreq: add a new mappable
> resource type...". Such resources were dubbed "caller owned" and resulted
> in the owned resources" and acquiring them resulted in the
> XENMEM_rsrc_acq_caller_owned flag being passed back to the caller of the
> memory op.
>
> Unfortunately the implementation led to XSA-276, which was mitigated
> by commit f6b6ae78 "x86/hvm/ioreq: fix page referencing" and then a related
> memory accounting problem was worked around by commit e862e6ce
> "x86/hvm/ioreq: use ref-counted target-assigned shared pages". This latter
> commit removed the only instance of a "caller owned" resource, but the
> flag was left in header and checked in one place in the core code.
> This patch removes that now redundant check and removes the definition of
> XENMEM_rsrc_acq_caller_owned from the public header. Also, since this was
> the only flag defined for the XENMEM_acquire_resource memory op, it removes
> the 'flags' field of struct xen_mem_acquire_resource and replaces it with
> an equivalently sized 'pad' field.
>
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
This is a modification to a public header, but in this specific case, I
think it is the correct thing to do.
XENMEM_rsrc_acq_caller_owned is totally unsafe to use. The only people
who are going to be impacted by this change are those who haven't yet
fixed XSA-276 and have an unstable system anyway, at which point, the
build breakage is arguably doing them a favour.
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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