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 4/10/19 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 :)

Explanation, not excuse: the only occurence of the name in my downstream
reading was a two-liner BZ comment, which I totally missed in my haste
to give the fix a chance to make 4.0.  I certainly didn't mean to
deprive him of credit!

[...]