Re: [Xen-devel] [PATCH v15 01/11] multicall: add no preemption ability between two calls

[Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>]

On Tue, Sep 09, 2014 at 02:15:59PM +0100, Jan Beulich wrote:
> >>> On 09.09.14 at 14:44, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 09/09/14 12:51, Jan Beulich wrote:
> >>>>> On 09.09.14 at 12:51, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>> On 09/09/14 11:39, Jan Beulich wrote:
> >>>>>>> On 09.09.14 at 08:43, <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> >>>>> On Fri, Sep 05, 2014 at 11:46:20AM +0100, Andrew Cooper wrote:
> >>>>>> On 05/09/14 09:37, Chao Peng wrote:
> >>>>>>> Add a flag to indicate if the execution can be preempted between two
> >>>>>>> calls. If not specified, stay preemptable.
> >>>>>>>
> >>>>>>> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> >>>>>>> ---
> >>>>>>>  xen/common/multicall.c   |    5 ++++-
> >>>>>>>  xen/include/public/xen.h |    4 ++++
> >>>>>>>  2 files changed, 8 insertions(+), 1 deletion(-)
> >>>>>>>
> >>>>>>> diff --git a/xen/common/multicall.c b/xen/common/multicall.c
> >>>>>>> index fa9d910..83b96eb 100644
> >>>>>>> --- a/xen/common/multicall.c
> >>>>>>> +++ b/xen/common/multicall.c
> >>>>>>> @@ -40,6 +40,7 @@ do_multicall(
> >>>>>>>      struct mc_state *mcs = &current->mc_state;
> >>>>>>>      uint32_t         i;
> >>>>>>>      int              rc = 0;
> >>>>>>> +    bool_t           preemptable = 0;
> >>>>>>>  
> >>>>>>>      if ( unlikely(__test_and_set_bit(_MCSF_in_multicall, 
> >>>>>>> &mcs->flags)) )
> >>>>>>>      {
> >>>>>>> @@ -52,7 +53,7 @@ do_multicall(
> >>>>>>>  
> >>>>>>>      for ( i = 0; !rc && i < nr_calls; i++ )
> >>>>>>>      {
> >>>>>>> -        if ( i && hypercall_preempt_check() )
> >>>>>>> +        if ( preemptable && hypercall_preempt_check() )
> >>>>>>>              goto preempted;
> >>>>>>>  
> >>>>>>>          if ( unlikely(__copy_from_guest(&mcs->call, call_list, 1)) )
> >>>>>>> @@ -61,6 +62,8 @@ do_multicall(
> >>>>>>>              break;
> >>>>>>>          }
> >>>>>>>  
> >>>>>>> +        preemptable = mcs->call.flags & MC_NO_PREEMPT;
> >>>>>>> +
> >>>>>> Please consider what would happen if a malicious guest set NO_PREEMPT 
> >>>>>> on
> >>>>>> every multicall entry.
> >>>>> OK, I see. My direct purpose here is to support batch operations for
> >>>>> XENPF_resource_op added in next patch. Recall what Jan suggested in v14
> >>>>> comments, we have 3 possible ways to support XENPF_resource_op batch:
> >>>>> 1) Add a field in the xenpf_resource_op to indicate the iteration;
> >>>>> 2) Fiddle multicall mechanism, just like this patch;
> >>>>> 3) Add a brand new hypercall.
> >>>>>
> >>>>> So perhaps I will give up option 2) before I can see any improvement
> >>>>> here. While option 3) is aggressive, so I'd go option 1) through I also
> >>>>> don't quite like it (Totally because the iteration is transparent for 
> >>>>> user).
> >>>> The I suppose you didn't really understand Andrew's comment: I
> >>>> don't think he was suggesting to drop the approach, but instead
> >>>> to implement it properly (read: securely).
> >>> That is certainly one part of it.
> >>>
> >>> However, there is the other open question (dropped from this context) of
> >>> how to deal with a multicall which has NO_PREEMT set, which itself
> >>> preempts, and I don't have a good answer for this.
> >> The pretty natural answer to this is - the specific handler knows
> >> best what to do.
> > 
> > Given our past history at retrofitting preempting into existing
> > hypercalls, the multicaller has no idea whether the ops they have
> > selected will preempt or not, and no way to guarentee that the behaviour
> > will stay the same in future.
> > 
> > The multicall dispatches to the regular hypercall handlers, which
> > (cant?)
> 
> They can - current->mc_state.flags has _MCSF_in_multicall
> set.
> 
> > and certainly shouldn't distinguish between a regular hypercall
> > and multicall.
> 
> I agree with this. Yet it's a bug in the caller to request no
> preemption at this layer for a constituent hypercall that can itself
> preempt. But that's only a problem for the caller, not for the
> hypervisor.
> 
> > As I have been looking through this code, I have noticed that the NDEBUG
> > parameter corruption will break half of our existing preemption logic,
> > which does use some of the parameters to hold preemption information.
> 
> Certainly not - call_list is being copied over a second time a few
> lines after that NDEBUG section.
> 
> Jan
> 
Clear, thank you two for your discussion.
So the only thing need to be done here is fixing the potential security
issue, right? I will follow this.

Thanks,
Chao
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