CVE-2021-47280

In the Linux kernel, the following vulnerability has been resolved: drm:
Fix use-after-free read in drm_getunique() 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.