[PATCH 2/2] drm: Protect drm_master pointers in drm_lease.c

[Emil Velikov]

On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<desmondcheongzx at gmail.com> wrote:
>
> This patch ensures that the device's master mutex is acquired before
> accessing pointers to struct drm_master that are subsequently
> dereferenced. Without the mutex, the struct drm_master may be freed
> concurrently by another process calling drm_setmaster_ioctl(). This
> could then lead to use-after-free errors.
>
> Reported-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>
> ---

<snip>

> @@ -578,6 +594,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>         /* Hook up the fd */
>         fd_install(fd, lessee_file);
>
> +       mutex_unlock(&dev->master_mutex);

I was going to suggest pushing the unlock call further up - after the
drm_lease_create call. Although that would make the error path rather
messy, so let's keep it as-is.

<snip>

> @@ -662,7 +684,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>         struct drm_mode_get_lease *arg = data;
>         __u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
>         __u32 count_objects = arg->count_objects;
> -       struct drm_master *lessee = lessee_priv->master;
> +       struct drm_master *lessee;
>         struct idr *object_idr;
>         int count;
>         void *entry;
> @@ -678,8 +700,10 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>
>         DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>
> +       mutex_lock(&dev->master_mutex);

As-is we're dereferencing an uninitialised pointer in DRM_DEBUG_LEASE.
Move the lock and assignment before the DRM_DEBUG_LEASE, just like
you've done in the list ioctl.

With this fix, the patch is;
Reviewed-by: Emil Velikov <emil.l.velikov at gmail.com>

-Emil