Re: [Qemu-devel] [PATCH for-4.0-maybe] device_tree: Fix integer overflowing in load_device_tree()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 4/10/19 7:28 AM, Markus Armbruster wrote:
> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
>> On 4/9/19 7:40 PM, Markus Armbruster wrote:
>>> If the value of get_image_size() exceeds INT_MAX / 2 - 10000, the
>>> computation of @dt_size overflows to a negative number, which then
>>> gets converted to a very large size_t for g_malloc0() and
>>> load_image_size().  In the (fortunately improbable) case g_malloc0()
>>> succeeds and load_image_size() survives, we'd assign the negative
>>> number to *sizep.  What that would do to the callers I can't say, but
>>> it's unlikely to be good.
>>>
>>> Fix by rejecting images whose size would overflow.
>>>
>>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>>> ---
>>>  device_tree.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/device_tree.c b/device_tree.c
>>> index 296278e12a..f8b46b3c73 100644
>>> --- a/device_tree.c
>>> +++ b/device_tree.c
>>> @@ -84,6 +84,10 @@ void *load_device_tree(const char *filename_path, int *sizep)
>>>                       filename_path);
>>>          goto fail;
>>>      }
>>> +    if (dt_size > INT_MAX / 2 - 10000) {
>>
>> We should avoid magic number duplication.
>> That said, this patch looks safe.
>>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> Thanks!
> 
>> BTW how did you figure that out?
> 
> Downstream handling of upstream commit da885fe1ee8 led me to the
> function.  I spotted dt_size = get_image_size(filename_path).
> Experience has taught me to check the left hand side's type.  Bad.  Then
> I saw how dt_size gets increased.  Worse.

So you genuinely neglected to mention Kurtis Miller then :)

>>> +        error_report("Device tree file '%s' is too large", filename_path);
>>> +        goto fail;
>>> +    }
>>>  
>>>      /* Expand to 2x size to give enough room for manipulation.  */
>>>      dt_size += 10000;
>>>