[PATCH] drm/vgem: Close use-after-free race in vgem_gem_create

[Chris Wilson]

Quoting Daniel Vetter (2020-02-02 13:21:33)
> There's two references floating around here (for the object reference,
> not the handle_count reference, that's a different thing):
> 
> - The temporary reference held by vgem_gem_create, acquired by
>   creating the object and released by calling
>   drm_gem_object_put_unlocked.
> 
> - The reference held by the object handle, created by
>   drm_gem_handle_create. This one generally outlives the function,
>   except if a 2nd thread races with a GEM_CLOSE ioctl call.
> 
> So usually everything is correct, except in that race case, where the
> access to gem_object->size could be looking at freed data already.
> Which again isn't a real problem (userspace shot its feet off already
> with the race, we could return garbage), but maybe someone can exploit
> this as an information leak.
> 
> Cc: Dan Carpenter <dan.carpenter at oracle.com>
> Cc: Hillf Danton <hdanton at sina.com>
> Cc: Reported-by: syzbot+0dc4444774d419e916c8 at syzkaller.appspotmail.com
> Cc: stable at vger.kernel.org
> Cc: Emil Velikov <emil.velikov at collabora.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Sean Paul <seanpaul at chromium.org>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Eric Anholt <eric at anholt.net>
> Cc: Sam Ravnborg <sam at ravnborg.org>
> Cc: Rob Clark <robdclark at chromium.org>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> ---
>  drivers/gpu/drm/vgem/vgem_drv.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
> index 5bd60ded3d81..909eba43664a 100644
> --- a/drivers/gpu/drm/vgem/vgem_drv.c
> +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> @@ -196,9 +196,10 @@ static struct drm_gem_object *vgem_gem_create(struct drm_device *dev,
>                 return ERR_CAST(obj);
>  
>         ret = drm_gem_handle_create(file, &obj->base, handle);
> -       drm_gem_object_put_unlocked(&obj->base);
> -       if (ret)
> +       if (ret) {
> +               drm_gem_object_put_unlocked(&obj->base);
>                 return ERR_PTR(ret);
> +       }
>  
>         return &obj->base;
>  }
> @@ -221,7 +222,9 @@ static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
>         args->size = gem_object->size;
>         args->pitch = pitch;
>  
> -       DRM_DEBUG("Created object of size %lld\n", size);
> +       drm_gem_object_put_unlocked(gem_object);
> +
> +       DRM_DEBUG("Created object of size %llu\n", args->size);

I was thinking we either should return size from vgem_gem_create (the
strategy we took in i915) or simply remove the vgem_gem_create() as that
doesn't improve readability.

-static struct drm_gem_object *vgem_gem_create(struct drm_device *dev,
-                                             struct drm_file *file,
-                                             unsigned int *handle,
-                                             unsigned long size)
+static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
+                               struct drm_mode_create_dumb *args)
 {
        struct drm_vgem_gem_object *obj;
-       int ret;
+       u64 pitch, size;
+       u32 handle;
+
+       pitch = args->width * DIV_ROUND_UP(args->bpp, 8);
+       size = mul_u32_u32(args->height, pitch);
+       if (size == 0 || pitch < args->width)
+               return -EINVAL;

        obj = __vgem_gem_create(dev, size);
        if (IS_ERR(obj))
-               return ERR_CAST(obj);
+               return PTR_ERR(obj);
+
+       size = obj->base.size;

-       ret = drm_gem_handle_create(file, &obj->base, handle);
+       ret = drm_gem_handle_create(file, &obj->base, &handle);
        drm_gem_object_put_unlocked(&obj->base);
        if (ret)
                return ERR_PTR(ret);

-       return &obj->base;
-}
-
-static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
-                               struct drm_mode_create_dumb *args)
-{
-       struct drm_gem_object *gem_object;
-       u64 pitch, size;
-
-       pitch = args->width * DIV_ROUND_UP(args->bpp, 8);
-       size = args->height * pitch;
-       if (size == 0)
-               return -EINVAL;
-
-       gem_object = vgem_gem_create(dev, file, &args->handle, size);
-       if (IS_ERR(gem_object))
-               return PTR_ERR(gem_object);
-
-       args->size = gem_object->size;
+       args->size = size;
        args->pitch = pitch;
+       args->handle = handle;


At the end of the day, it makes no difference,
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris