Re: [PATCH] memory: Initialize MemoryRegionOps for RAM memory regions

[P J P]

+-- On Mon, 17 Aug 2020, Philippe Mathieu-Daudé wrote --+
| On 8/16/20 8:26 PM, Philippe Mathieu-Daudé wrote:
| > There is an issue when using memory_region_dispatch_read() or
| > memory_region_dispatch_write() on RAM memory regions.
| > 
| > RAM memory regions are initialized as:
| > 
| >   memory_region_init_ram()
| >   -> memory_region_init_ram_nomigrate()
| >      -> memory_region_init_ram_shared_nomigrate()
| >         -> memory_region_init()
| >            -> object_initialize(TYPE_MEMORY_REGION)
| >               -> memory_region_initfn()
| >                  -> mr->ops = &unassigned_mem_ops;
| > 
| > Later when accessing the alias, the memory_region_dispatch_read()
| > flow is:
| > 
| >   memory_region_dispatch_read()
| >   -> memory_region_dispatch_read1()
| >      -> if (mr->ops->read) { ... }
| >                    ^^^^^^
| >                    NULL deref as unassigned_mem_ops.read is NULL.
| > 
| >   memory_region_dispatch_write()
| >   -> if (mr->ops->write) { ... }
| >                 ^^^^^^^
| >                 NULL deref as unassigned_mem_ops.read is NULL.

* This check should pass/fail as normal I think. NULL dereference would happen 
  on invoking mr->ops->read/write() call.

* When mr->ops->read/write==NULL, the dispatch_read/write function would go to 
  the else section to call read/write_with_attrs()

    const MemoryRegionOps unassigned_mem_ops = {                                
    
      .valid.accepts = unassigned_mem_accepts,                                  
  
      .endianness = DEVICE_NATIVE_ENDIAN,                                       
  
    };

  Maybe we define read/write_with_attrs in 'unassigned_mem_ops' above?


| > Fix by initializing the MemoryRegionOps to ram_device_mem_ops,
| > this way the memory accesses are properly dispatched using
| > memory_region_ram_device_read() / memory_region_ram_device_write().
| > 
| > Fixes: 4a2e242bbb ("memory: Don't use memcpy for ram_device regions")

Yes, this should be fine too.


Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
8685 545E B54C 486B C6EB 271E E285 8B5A F050 DE8D