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

[Thomas Zimmermann]

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
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