CVE-2022-48636

In the Linux kernel, the following vulnerability has been resolved:

s390/dasd: fix Oops in dasd_alias_get_start_dev due to missing pavgroup

Fix Oops in dasd_alias_get_start_dev() function caused by the pavgroup pointer being NULL.

The pavgroup pointer is checked on the entrance of the function but without the lcu->lock being held. Therefore there is a race window between dasd_alias_get_start_dev() and _lcu_update() which sets pavgroup to NULL with the lcu->lock held.

Fix by checking the pavgroup pointer with lcu->lock held.