Re: [Xen-devel] [PATCH v2 3/3] [GSOC14] FIX:- Race condition between initializing shared ring and mempaging.

[Dushyant Behl <myselfdushyantbehl@xxxxxxxxx>]

Ian,

Sorry for this extremely late response.

On Fri, Jun 27, 2014 at 4:03 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Mon, 2014-06-16 at 23:50 +0530, Dushyant Behl wrote:
>> This patch is part of the work done under the gsoc project -
>> Lazy Restore Using Memory Paging.
>>
>> This patch is meant to fix a known race condition bug in mempaging
>> ring setup routines. The race conditon was between initializing
>
> "condition"
>
>> mem paging and initializing shared ring, earlier the code initialized
>> mem paging before removing the ring page from guest's physical map
>> which could enable the guest to interfere with the ring initialisation.
>> Now the code removes the page from the guest's physical map before
>> enabling mempaging so that the guest cannot clobber the ring after
>> we initialise it.
>>
>> Signed-off-by: Dushyant Behl <myselfdushyantbehl@xxxxxxxxx>
>> Reviewed-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
>> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>>  tools/libxc/xc_mem_paging_setup.c | 24 ++++++++++++++++++++----
>>  1 file changed, 20 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/libxc/xc_mem_paging_setup.c 
>> b/tools/libxc/xc_mem_paging_setup.c
>> index 679c606..a4c4798 100644
>> --- a/tools/libxc/xc_mem_paging_setup.c
>> +++ b/tools/libxc/xc_mem_paging_setup.c
>> @@ -73,6 +73,24 @@ int xc_mem_paging_ring_setup(xc_interface *xch,
>>          }
>>      }
>>
>> +    /*
>> +     * Remove the page from guest's physical map so that the guest will not
>> +     * be able to break security by pretending to be toolstack for a while.
>> +     */
>> +    if ( xc_domain_decrease_reservation_exact(xch, domain_id, 1, 0, 
>> ring_page) )
>
> ring_page here is a void * mapping of the page, isn't it? Not a
> xen_pfn_t[] array of pfns, so surely this isn't right.

Yes, you are correct and the invocation should be with ring_pfn. But
if I change this with
the invocation done below as -
if ( xc_domain_decrease_reservation_exact(xch, domain_id, 1, 0, &ring_pfn) )

then the code fails to enable mem paging.
Can anyone help as to why this would happen?

Thanks and Regards,
Dushyant Behl

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