Re: [Xen-devel] [V9 3/3] Differentiate IO/mem resources tracked by ioreq server

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 06 January 2016 08:59
> To: Zhang Yu
> Cc: Andrew Cooper; Paul Durrant; Wei Liu; Ian Jackson; Stefano Stabellini;
> Kevin Tian; zhiyuan.lv@xxxxxxxxx; Shuai Ruan; xen-devel@xxxxxxxxxxxxx; Keir
> (Xen.org)
> Subject: Re: [Xen-devel] [V9 3/3] Differentiate IO/mem resources tracked by
> ioreq server
> 
> >>> On 31.12.15 at 10:33, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > On 12/21/2015 10:45 PM, Jan Beulich wrote:
> >>>>> On 15.12.15 at 03:05, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> >>> @@ -2593,6 +2597,16 @@ struct hvm_ioreq_server
> *hvm_select_ioreq_server(struct domain *d,
> >>>           type = (p->type == IOREQ_TYPE_PIO) ?
> >>>                   HVMOP_IO_RANGE_PORT : HVMOP_IO_RANGE_MEMORY;
> >>>           addr = p->addr;
> >>> +        if ( type == HVMOP_IO_RANGE_MEMORY )
> >>> +        {
> >>> +             ram_page = get_page_from_gfn(d, p->addr >> PAGE_SHIFT,
> >>> +                                          &p2mt, P2M_UNSHARE);
> >>> +             if ( p2mt == p2m_mmio_write_dm )
> >>> +                 type = HVMOP_IO_RANGE_WP_MEM;
> >>> +
> >>> +             if ( ram_page )
> >>> +                 put_page(ram_page);
> >>> +        }
> >>
> >> You evaluate the page's current type here - what if it subsequently
> >> changes? I don't think it is appropriate to leave the hypervisor at
> >> the mercy of the device model here.
> >
> > Well. I do not quite understand your concern. :)
> > Here, the get_page_from_gfn() is used to determine if the addr is a MMIO
> > or a write-protected ram. If this p2m type is changed, it should be
> > triggered by the guest and device model, e.g. this RAM is not supposed
> > to be used as the graphic translation table. And it should be fine.
> > But I also wonder, if there's any other routine more appropriate to get
> > a p2m type from the gfn?
> 
> No, the question isn't the choice of method to retrieve the
> current type, but the lack of measures against the retrieved
> type becoming stale by the time you actually use it.
> 

I don't think that issue is specific to this code. AFAIK nothing in the I/O 
emulation system protects against a type change whilst a request is in flight.
Also, what are the consequences of a change? Only that the wrong range type is 
selected and the emulation goes to the wrong place. This may be a problem for 
the VM but should cause no other problems.

  Paul

> >>> --- a/xen/include/asm-x86/hvm/domain.h
> >>> +++ b/xen/include/asm-x86/hvm/domain.h
> >>> @@ -48,8 +48,8 @@ struct hvm_ioreq_vcpu {
> >>>       bool_t           pending;
> >>>   };
> >>>
> >>> -#define NR_IO_RANGE_TYPES (HVMOP_IO_RANGE_PCI + 1)
> >>> -#define MAX_NR_IO_RANGES  256
> >>> +#define NR_IO_RANGE_TYPES (HVMOP_IO_RANGE_WP_MEM + 1)
> >>> +#define MAX_NR_IO_RANGES  8192
> >>
> >> I'm sure I've objected before to this universal bumping of the limit:
> >> Even if I were to withdraw my objection to the higher limit on the
> >> new kind of tracked resource, I would continue to object to all
> >> other resources getting their limits bumped too.
> >>
> >
> > Hah. So how about we keep MAX_NR_IO_RANGES as 256, and use a new
> value,
> > say MAX_NR_WR_MEM_RANGES, set to 8192 in this patch? :)
> 
> That would at least limit the damage to the newly introduced type.
> But I suppose you realize it would still be a resource consumption
> concern. In order for this to not become a security issue, you
> might e.g. stay with the conservative old limit and allow a command
> line or even better guest config file override to it (effectively making
> the admin state his consent with the higher resource use).
> 
> Jan


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