On Tue, Jun 08, 2021 at 07:04:36PM +0800, Desmond Cheong Zhi Xi wrote: > There is a time-of-check-to-time-of-use error in drm_getunique() due > to retrieving file_priv->master prior to locking the device's master > mutex. > > An example can be seen in the crash report of the use-after-free error > found by Syzbot: > https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803 > > In the report, the master pointer was used after being freed. This is > because another process had acquired the device's master mutex in > drm_setmaster_ioctl(), then overwrote fpriv->master in > drm_new_set_master(). The old value of fpriv->master was subsequently > freed before the mutex was unlocked. > > To fix this, we lock the device's master mutex before retrieving the > pointer from from fpriv->master. This patch passes the Syzbot > reproducer test. > > Reported-by: syzbot+c3a706cec1ea99e1c693@xxxxxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> Very nice debug work, thanks a lot! > --- > > In this patch, I made the assumption that dev and file_priv->master->dev > refer to the same struct drm_device*, and from my test runs this seemed > to be the case. If this was a wrong assumption to make, please let me > know. Yup that's correct. Patch applied to drm-misc-fixes and cc: stable added so it gets backported. Cheers, Daniel > > drivers/gpu/drm/drm_ioctl.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > index d273d1a8603a..495a4767a443 100644 > --- a/drivers/gpu/drm/drm_ioctl.c > +++ b/drivers/gpu/drm/drm_ioctl.c > @@ -118,17 +118,18 @@ int drm_getunique(struct drm_device *dev, void *data, > struct drm_file *file_priv) > { > struct drm_unique *u = data; > - struct drm_master *master = file_priv->master; > + struct drm_master *master; > > - mutex_lock(&master->dev->master_mutex); > + mutex_lock(&dev->master_mutex); > + master = file_priv->master; > if (u->unique_len >= master->unique_len) { > if (copy_to_user(u->unique, master->unique, master->unique_len)) { > - mutex_unlock(&master->dev->master_mutex); > + mutex_unlock(&dev->master_mutex); > return -EFAULT; > } > } > u->unique_len = master->unique_len; > - mutex_unlock(&master->dev->master_mutex); > + mutex_unlock(&dev->master_mutex); > > return 0; > } > -- > 2.25.1 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
