Re: [Xen-devel] [PATCH v2 2/3] xen/pvh: check permissions when adding MMIO regions

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

El 20/01/15 a les 19.19, Andrew Cooper ha escrit:
> On 20/01/15 17:05, Roger Pau Monne wrote:
>> Check that MMIO regions added to PVH Dom0 are allowed. Previously a PVH Dom0
>> would have access to the full MMIO range.
>>
>> Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> Changes since v1:
>>  - Use the newly introduced p2m_access_t to set the access type.
>>  - Don't add a next label.
>> ---
>>  xen/arch/x86/domain_build.c | 17 +++++++++++++++--
>>  1 file changed, 15 insertions(+), 2 deletions(-)
>>
>> diff --git a/xen/arch/x86/domain_build.c b/xen/arch/x86/domain_build.c
>> index f687c78..41d2541 100644
>> --- a/xen/arch/x86/domain_build.c
>> +++ b/xen/arch/x86/domain_build.c
>> @@ -319,12 +319,25 @@ static __init void pvh_add_mem_mapping(struct domain 
>> *d, unsigned long gfn,
>>                                         unsigned long mfn, unsigned long 
>> nr_mfns)
>>  {
>>      unsigned long i;
>> +    mfn_t omfn;
>> +    p2m_type_t t;
>> +    p2m_access_t a;
>>      int rc;
>>  
>>      for ( i = 0; i < nr_mfns; i++ )
>>      {
>> -        if ( (rc = set_mmio_p2m_entry(d, gfn + i, _mfn(mfn + i),
>> -                                      p2m_get_hostp2m(d)->default_access)) )
>> +        if ( !iomem_access_permitted(d, mfn + i, mfn + i) ) {
>> +            omfn = get_gfn_query_unlocked(d, gfn + i, &t);
>> +            guest_physmap_remove_page(d, gfn + i, mfn_x(omfn), 
>> PAGE_ORDER_4K);
>> +            continue;
>> +        }
> 
> This suggests a design flaw (possibly pre-existing).  We should not be
> removing physmap entries in pvh_add_mem_mapping(), nor should we be a
> position to need to revoke physmap entries during domain build.
> 
> If there is anything needing revoking at this stage, it should not have
> been added earlier.  How did you come to introduce this code?

This code was introduced with the PVH Dom0 support series done by
Mukesh. Basically we let construct_dom0 build the physmap as it would be
done for a PV Dom0 (no holes at all, plain physmap from 0 to maxmem) and
then we punch the MMIO holes as needed. After punching the holes, we add
the leftover memory to the end of the memory map.

IMHO this seems better than having two different ways of building the
Dom0 memory map interleaved in the code, one for PV and one for PVH,
specially taking into account that the code in construct_dom0 is already
quite convoluted.

Roger.

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