Re: [Xen-devel] [PATCH] Extra check in grant table code for mapping of shared frame

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Sep 19, 2012, at 11:35 AM, Jan Beulich wrote:

>>>> On 13.09.12 at 17:27, Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> wrote:
>> --- a/xen/common/grant_table.c
>> +++ b/xen/common/grant_table.c
>> @@ -649,9 +649,12 @@ __gnttab_map_grant_ref(
>>     }
>>     else if ( owner == rd || owner == dom_cow )
>>     {
>> -        if ( gnttab_host_mapping_get_page_type(op, ld, rd) &&
>> -             !get_page_type(pg, PGT_writable_page) )
>> -            goto could_not_pin;
>> +        if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
>> +        {
>> +            if ( (owner == dom_cow) ||
>> +                 !get_page_type(pg, PGT_writable_page) )
>> +                goto could_not_pin;
>> +        }
>> 
>>         nr_gets++;
>>         if ( op->flags & GNTMAP_host_map )
> 
> Isn't that only half of it, in that the error/unmap paths need to
> also consider that get_page_type() wasn't called? There's
> quite a few calls to gnttab_host_mapping_get_page_type()/
> put_page_type() sequences there.

I think this is covered. could_not_pin will cascade into undo_out, and nr_gets 
remains at zero at this point. Then:
 undo_out:
    if ( nr_gets > 1 )
    {
       …
    }
    if ( nr_gets > 0 )
    {
        if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
            put_page_type(pg);
            ...

i.e. put_page_type will not be called. This is really tricky code!

Andres
> 
> Jan
> 


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