Hi Steve and others.
On Fri, Jul 10, 2020 at 06:40:26PM -0400, Steve Cohen wrote:
> BUG: KASAN: use-after-free in drm_gem_open_ioctl
>
> There is potential for use-after-free here if the GEM object
> handle is closed between the idr lookup and retrieving the size
> from the object since a local reference is not being held at that
> point. Hold the local reference while the object can still be
> accessed to resolve this.
>
> Signed-off-by: Steve Cohen <cohens@xxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/drm_gem.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 7bf628e..4b2891c 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -898,14 +898,15 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
>
> /* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
> ret = drm_gem_handle_create_tail(file_priv, obj, &handle);
> - drm_gem_object_put_unlocked(obj);
> if (ret)
> - return ret;
> + goto out;
>
> args->handle = handle;
> args->size = obj->size;
>
> - return 0;
> +out:
> + drm_gem_object_put_unlocked(obj);
> + return ret;
Lookign at drm_gem_flink_ioctl() that is implmented just above this
functions there are two things that I noted.
1) In drm_gem_flink_ioctl() the label is named "err:" - and my OCD likes
that similar labels have the same name.
2) The function takes the object_name_lock but fails to release it in
the error situation.
Danile Vetter updated the locking in
20228c447846da9399ead53fdbbc8ab69b47788a ("drm/gem: completely close gem_open vs. gem_close races")
but I failed to follow it all.
Sam
> }
>
> /**
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