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

stable inclusion
from stable-v4.19.260
commit aaba5ff2742043705bc4c02fd0b2b246e2e16da1
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9KHGT
CVE: CVE-2022-48636

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaba5ff2742043705bc4c02fd0b2b246e2e16da1



--------------------------------

commit db7ba071 upstream.

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.

Cc: <stable@vger.kernel.org> # 2.6.25+
Fixes: 8e09f215 ("[S390] dasd: add hyper PAV support to DASD device driver, part 1")
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Link: https://lore.kernel.org/r/20220919154931.4123002-2-sth@linux.ibm.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Li Zetao <lizetao1@huawei.com>