From: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> commit b436acd1cf7fac0ba987abd22955d98025c80c2b upstream. 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=148d2f1dfac64af52ffd27b661981a540724f80... 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@syzkaller.appspotmail.com Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> Cc: stable@vger.kernel.org Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: https://patchwork.freedesktop.org/patch/msgid/20210608110436.239583-1-desmon... Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Yang Yingliang <yangyingliang@huawei.com> --- 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 b92682f037b22..2449b357f524b 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -112,17 +112,18 @@ 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