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

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

On 4/9/19 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>

BTW how did you figure that out?

> +        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;
>