[PATCH v2] drm/i915: bounds check execbuffer relocation count

[Kees Cook]

On Mon, Mar 11, 2013 at 3:00 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Mon, Mar 11, 2013 at 02:23:29PM -0700, Kees Cook wrote:
>> It is possible to wrap the counter used to allocate the buffer for
>> relocation copies. This could lead to heap writing overflows.
>
> I'd keep the return value as EINVAL so that we can continue to
> distinguish between the user passing garbage and hitting an oom. And
> total_relocs is preferrable to total, which also leads us to think more
> carefully about the error condition. I think the check should be against
> INT_MAX / sizeof(struct reloc_entry) for consistency with our other
> guard against overflows whilst allocating.

I've ended up with this:

        int max_alloc = INT_MAX / sizeof(struct drm_i915_gem_relocation_entry);
...
                /* First check for malicious input causing overflow */
                if (exec[i].relocation_count > max_alloc)
                        return -EINVAL;
                if (exec[i].relocation_count > max_alloc - total_relocs)
                        return -EINVAL;
                total_relocs += exec[i].relocation_count;

And looking at that, I wonder if we should just eliminate the first if entirely?

-Kees

--
Kees Cook
Chrome OS Security