Re: [Qemu-devel] [PATCH v2 02/11] memory: destroy phys_sections one by one

[Paolo Bonzini]

Il 01/07/2013 16:14, Jan Kiszka ha scritto:
> On 2013-06-28 18:58, Paolo Bonzini wrote:
>> phys_sections_clear is invoked after the dispatch tree has been
>> destroyed.
> 
> You mean destroy_all_mappings called by mem_begin vs.
> phys_sections_clear called by core_begin, right?
> 
>>  This leaves a window where phys_sections_nb > 0 but the
>> subpages are not valid anymore, which is a recipe for use-after-free
>> bugs.
>>
>> Move the destruction of subpages in phys_sections_clear.
> 
> What ensures that we still destroy the subpages when an address space is
> cleaned up (address_space_destroy_dispatch)? The fact that an address
> space has to be / is de facto empty when destroying it?

Yes, the address space is emptied at destruction time.  See

    memory_region_transaction_begin();
    as->root = NULL;
    memory_region_transaction_commit();

in address_space_destroy.

Paolo

> Jan
> 
>>
>> Signed-off-by: Paolo Bonzini <address@hidden>
>> ---
>>  exec.c | 27 +++++++++++++--------------
>>  1 file changed, 13 insertions(+), 14 deletions(-)
>>
>> diff --git a/exec.c b/exec.c
>> index 1118587..e7eadf5 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -762,17 +762,6 @@ hwaddr memory_region_section_get_iotlb(CPUArchState 
>> *env,
>>  static int subpage_register (subpage_t *mmio, uint32_t start, uint32_t end,
>>                               uint16_t section);
>>  static subpage_t *subpage_init(AddressSpace *as, hwaddr base);
>> -static void destroy_page_desc(uint16_t section_index)
>> -{
>> -    MemoryRegionSection *section = &phys_sections[section_index];
>> -    MemoryRegion *mr = section->mr;
>> -
>> -    if (mr->subpage) {
>> -        subpage_t *subpage = container_of(mr, subpage_t, iomem);
>> -        memory_region_destroy(&subpage->iomem);
>> -        g_free(subpage);
>> -    }
>> -}
>>  
>>  static void destroy_l2_mapping(PhysPageEntry *lp, unsigned level)
>>  {
>> @@ -787,8 +776,6 @@ static void destroy_l2_mapping(PhysPageEntry *lp, 
>> unsigned level)
>>      for (i = 0; i < L2_SIZE; ++i) {
>>          if (!p[i].is_leaf) {
>>              destroy_l2_mapping(&p[i], level - 1);
>> -        } else {
>> -            destroy_page_desc(p[i].ptr);
>>          }
>>      }
>>      lp->is_leaf = 0;
>> @@ -818,9 +805,21 @@ static uint16_t phys_section_add(MemoryRegionSection 
>> *section)
>>      return phys_sections_nb++;
>>  }
>>  
>> +static void phys_section_destroy(MemoryRegion *mr)
>> +{
>> +    if (mr->subpage) {
>> +        subpage_t *subpage = container_of(mr, subpage_t, iomem);
>> +        memory_region_destroy(&subpage->iomem);
>> +        g_free(subpage);
>> +    }
>> +}
>> +
>>  static void phys_sections_clear(void)
>>  {
>> -    phys_sections_nb = 0;
>> +    while (phys_sections_nb > 0) {
>> +        MemoryRegionSection *section = &phys_sections[--phys_sections_nb];
>> +        phys_section_destroy(section->mr);
>> +    }
>>  }
>>  
>>  static void register_subpage(AddressSpaceDispatch *d, MemoryRegionSection 
>> *section)
>>
>