[PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

[Thomas Zimmermann]

Hi

Am 03.11.21 um 01:12 schrieb Anand K. Mistry:
> Any movement on merging this patch? Not urgent on my end (we have this
> patch in our tree), but I think other amd users might run into this
> UAF.

Thanks for reminding. I've merged your patch into drm-misc-fixes.

Best regards
Thomas

> 
> On Thu, 30 Sept 2021 at 17:21, Thomas Zimmermann <tzimmermann at suse.de> wrote:
>>
>> Hi
>>
>> Am 30.09.21 um 01:00 schrieb Anand K Mistry:
>>> drm_gem_ttm_mmap() drops a reference to the gem object on success. If
>>> the gem object's refcount == 1 on entry to drm_gem_prime_mmap(), that
>>> drop will free the gem object, and the subsequent drm_gem_object_get()
>>> will be a UAF. Fix by grabbing a reference before calling the mmap
>>> helper.
>>>
>>> This issue was forseen when the reference dropping was adding in
>>> commit 9786b65bc61ac ("drm/ttm: fix mmap refcounting"):
>>>     "For that to work properly the drm_gem_object_get() call in
>>>     drm_gem_ttm_mmap() must be moved so it happens before calling
>>>     obj->funcs->mmap(), otherwise the gem refcount would go down
>>>     to zero."
>>>
>>> Signed-off-by: Anand K Mistry <amistry at google.com>
>>
>> Acked-by: Thomas Zimmermann <tzimmermann at suse.de>
>>
>> This looks fine to me, but it affects many drivers. Let's maybe wait a
>> bit if more reviews come it.
>>
>> Best regards
>> Thomas
>>
>>> ---
>>>
>>>    drivers/gpu/drm/drm_prime.c | 6 ++++--
>>>    1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>>> index 2a54f86856af..e1854fd24bb0 100644
>>> --- a/drivers/gpu/drm/drm_prime.c
>>> +++ b/drivers/gpu/drm/drm_prime.c
>>> @@ -719,11 +719,13 @@ int drm_gem_prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>>>        if (obj->funcs && obj->funcs->mmap) {
>>>                vma->vm_ops = obj->funcs->vm_ops;
>>>
>>> +             drm_gem_object_get(obj);
>>>                ret = obj->funcs->mmap(obj, vma);
>>> -             if (ret)
>>> +             if (ret) {
>>> +                     drm_gem_object_put(obj);
>>>                        return ret;
>>> +             }
>>>                vma->vm_private_data = obj;
>>> -             drm_gem_object_get(obj);
>>>                return 0;
>>>        }
>>>
>>>
>>
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Felix Imendörffer
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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