On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<desmondcheongzx@xxxxxxxxx> 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@xxxxxxxx>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> ---
<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@xxxxxxxxx>
-Emil
