s390/qeth: fix use-after-free in hsci

KASAN found that addr was dereferenced after br2dev_event_work was freed.

==================================================================
BUG: KASAN: use-after-free in qeth_l2_br2dev_worker+0x5ba/0x6b0
Read of size 1 at addr 00000000fdcea440 by task kworker/u760:4/540
CPU: 17 PID: 540 Comm: kworker/u760:4 Tainted: G            E      6.1.0-20221128.rc7.git1.5aa3bed4ce83.300.fc36.s390x+kasan #1
Hardware name: IBM 8561 T01 703 (LPAR)
Workqueue: 0.0.8000_event qeth_l2_br2dev_worker
Call Trace:
 [<000000016944d4ce>] dump_stack_lvl+0xc6/0xf8
 [<000000016942cd9c>] print_address_description.constprop.0+0x34/0x2a0
 [<000000016942d118>] print_report+0x110/0x1f8
 [<0000000167a7bd04>] kasan_report+0xfc/0x128
 [<000000016938d79a>] qeth_l2_br2dev_worker+0x5ba/0x6b0
 [<00000001673edd1e>] process_one_work+0x76e/0x1128
 [<00000001673ee85c>] worker_thread+0x184/0x1098
 [<000000016740718a>] kthread+0x26a/0x310
 [<00000001672c606a>] __ret_from_fork+0x8a/0xe8
 [<00000001694711da>] ret_from_fork+0xa/0x40
Allocated by task 108338:
 kasan_save_stack+0x40/0x68
 kasan_set_track+0x36/0x48
 __kasan_kmalloc+0xa0/0xc0
 qeth_l2_switchdev_event+0x25a/0x738
 atomic_notifier_call_chain+0x9c/0xf8
 br_switchdev_fdb_notify+0xf4/0x110
 fdb_notify+0x122/0x180
 fdb_add_entry.constprop.0.isra.0+0x312/0x558
 br_fdb_add+0x59e/0x858
 rtnl_fdb_add+0x58a/0x928
 rtnetlink_rcv_msg+0x5f8/0x8d8
 netlink_rcv_skb+0x1f2/0x408
 netlink_unicast+0x570/0x790
 netlink_sendmsg+0x752/0xbe0
 sock_sendmsg+0xca/0x110
 ____sys_sendmsg+0x510/0x6a8
 ___sys_sendmsg+0x12a/0x180
 __sys_sendmsg+0xe6/0x168
 __do_sys_socketcall+0x3c8/0x468
 do_syscall+0x22c/0x328
 __do_syscall+0x94/0xf0
 system_call+0x82/0xb0
Freed by task 540:
 kasan_save_stack+0x40/0x68
 kasan_set_track+0x36/0x48
 kasan_save_free_info+0x4c/0x68
 ____kasan_slab_free+0x14e/0x1a8
 __kasan_slab_free+0x24/0x30
 __kmem_cache_free+0x168/0x338
 qeth_l2_br2dev_worker+0x154/0x6b0
 process_one_work+0x76e/0x1128
 worker_thread+0x184/0x1098
 kthread+0x26a/0x310
 __ret_from_fork+0x8a/0xe8
 ret_from_fork+0xa/0x40
Last potentially related work creation:
 kasan_save_stack+0x40/0x68
 __kasan_record_aux_stack+0xbe/0xd0
 insert_work+0x56/0x2e8
 __queue_work+0x4ce/0xd10
 queue_work_on+0xf4/0x100
 qeth_l2_switchdev_event+0x520/0x738
 atomic_notifier_call_chain+0x9c/0xf8
 br_switchdev_fdb_notify+0xf4/0x110
 fdb_notify+0x122/0x180
 fdb_add_entry.constprop.0.isra.0+0x312/0x558
 br_fdb_add+0x59e/0x858
 rtnl_fdb_add+0x58a/0x928
 rtnetlink_rcv_msg+0x5f8/0x8d8
 netlink_rcv_skb+0x1f2/0x408
 netlink_unicast+0x570/0x790
 netlink_sendmsg+0x752/0xbe0
 sock_sendmsg+0xca/0x110
 ____sys_sendmsg+0x510/0x6a8
 ___sys_sendmsg+0x12a/0x180
 __sys_sendmsg+0xe6/0x168
 __do_sys_socketcall+0x3c8/0x468
 do_syscall+0x22c/0x328
 __do_syscall+0x94/0xf0
 system_call+0x82/0xb0
Second to last potentially related work creation:
 kasan_save_stack+0x40/0x68
 __kasan_record_aux_stack+0xbe/0xd0
 kvfree_call_rcu+0xb2/0x760
 kernfs_unlink_open_file+0x348/0x430
 kernfs_fop_release+0xc2/0x320
 __fput+0x1ae/0x768
 task_work_run+0x1bc/0x298
 exit_to_user_mode_prepare+0x1a0/0x1a8
 __do_syscall+0x94/0xf0
 system_call+0x82/0xb0
The buggy address belongs to the object at 00000000fdcea400
 which belongs to the cache kmalloc-96 of size 96
The buggy address is located 64 bytes inside of
 96-byte region [00000000fdcea400, 00000000fdcea460)
The buggy address belongs to the physical page:
page:000000005a9c26e8 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0xfdcea
flags: 0x3ffff00000000200(slab|node=0|zone=1|lastcpupid=0x1ffff)
raw: 3ffff00000000200 0000000000000000 0000000100000122 000000008008cc00
raw: 0000000000000000 0020004100000000 ffffffff00000001 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address:
 00000000fdcea300: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
 00000000fdcea380: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
>00000000fdcea400: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
                                           ^
 00000000fdcea480: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
 00000000fdcea500: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
==================================================================

Fixes: f7936b7b ("s390/qeth: Update MACs of LEARNING_SYNC device")
Reported-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Reviewed-by: Thorsten Winkler <twinkler@linux.ibm.com>
Link: https://lore.kernel.org/r/20221207105304.20494-1-wintera@linux.ibm.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/s390/net/qeth_l2_main.c

@@ -758,7 +758,6 @@ static void qeth_l2_br2dev_worker(struct work_struct *work)
 	struct list_head *iter;
 	int err = 0;
 
-	kfree(br2dev_event_work);
 	QETH_CARD_TEXT_(card, 4, "b2dw%04lx", event);
 	QETH_CARD_TEXT_(card, 4, "ma%012llx", ether_addr_to_u64(addr));
 
@@ -815,6 +814,7 @@ static void qeth_l2_br2dev_worker(struct work_struct *work)
 	dev_put(brdev);
 	dev_put(lsyncdev);
 	dev_put(dstdev);
+	kfree(br2dev_event_work);
 }
 
 static int qeth_l2_br2dev_queue_work(struct net_device *brdev,