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

[Markus Armbruster <armbru@redhat.com>]

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.

>> +        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;
>>