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

[Alistair Francis]

On Tue, Apr 9, 2019 at 10:59 PM Markus Armbruster <address@hidden> wrote:
>
> Philippe Mathieu-Daudé <address@hidden> writes:
>
> > On 4/10/19 7:28 AM, Markus Armbruster wrote:
> >> Philippe Mathieu-Daudé <address@hidden> 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 <address@hidden>
> >>>> ---
> >>>>  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 <address@hidden>
> >>
> >> 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!

No worries. I have sent the pull request and it includes the reported by.

Alistair

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