Hi Daniel.
On Sun, Feb 02, 2020 at 02:21:33PM +0100, Daniel Vetter wrote:
> 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@xxxxxxxxxx>
> Cc: Hillf Danton <hdanton@xxxxxxxx>
> Cc: Reported-by: syzbot+0dc4444774d419e916c8@xxxxxxxxxxxxxxxxxxxxxxxxx
^^ Small typo
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Emil Velikov <emil.velikov@xxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Eric Anholt <eric@xxxxxxxxxx>
> Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
> Cc: Rob Clark <robdclark@xxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
> 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);
>
> return 0;
> }
> --
> 2.24.1
>
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