KASAN: use-after-free Write in afs_manage_cell

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    fffe3ae0 Merge tag 'for-linus-hmm' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1053efea900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=18bb86f2e4ebfda2
dashboard link: https://syzkaller.appspot.com/bug?extid=f59c67285cb61166a0cf
compiler:       gcc (GCC) 10.1.0-syz 20200507

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f59c67285cb61166a0cf@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: use-after-free in afs_activate_cell fs/afs/cell.c:626 [inline]
BUG: KASAN: use-after-free in afs_manage_cell+0xf72/0x11c0 fs/afs/cell.c:697
Write of size 8 at addr ffff88808ecf5888 by task kworker/1:2/2576

CPU: 1 PID: 2576 Comm: kworker/1:2 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: afs afs_manage_cell
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x18f/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x436 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 afs_activate_cell fs/afs/cell.c:626 [inline]
 afs_manage_cell+0xf72/0x11c0 fs/afs/cell.c:697
 process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Allocated by task 16538:
 save_stack+0x1b/0x40 mm/kasan/common.c:48
 set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xc2/0xd0 mm/kasan/common.c:494
 kmem_cache_alloc_trace+0x14f/0x2d0 mm/slab.c:3551
 kmalloc include/linux/slab.h:555 [inline]
 kzalloc include/linux/slab.h:669 [inline]
 afs_alloc_cell fs/afs/cell.c:151 [inline]
 afs_lookup_cell+0x5c8/0x1440 fs/afs/cell.c:262
 afs_parse_source fs/afs/super.c:290 [inline]
 afs_parse_param+0x404/0x8c0 fs/afs/super.c:326
 vfs_parse_fs_param fs/fs_context.c:117 [inline]
 vfs_parse_fs_param+0x203/0x550 fs/fs_context.c:98
 vfs_parse_fs_string+0xe6/0x150 fs/fs_context.c:161
 do_new_mount fs/namespace.c:2869 [inline]
 do_mount+0x658/0x1fe0 fs/namespace.c:3200
 __do_sys_mount fs/namespace.c:3410 [inline]
 __se_sys_mount fs/namespace.c:3387 [inline]
 __x64_sys_mount+0x18f/0x230 fs/namespace.c:3387
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 7033:
 save_stack+0x1b/0x40 mm/kasan/common.c:48
 set_track mm/kasan/common.c:56 [inline]
 kasan_set_free_info mm/kasan/common.c:316 [inline]
 __kasan_slab_free+0xf5/0x140 mm/kasan/common.c:455
 __cache_free mm/slab.c:3426 [inline]
 kfree+0x103/0x2c0 mm/slab.c:3757
 rcu_do_batch kernel/rcu/tree.c:2427 [inline]
 rcu_core+0x5c7/0x1190 kernel/rcu/tree.c:2655
 __do_softirq+0x2de/0xa24 kernel/softirq.c:298

The buggy address belongs to the object at ffff88808ecf5800
 which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 136 bytes inside of
 1024-byte region [ffff88808ecf5800, ffff88808ecf5c00)
The buggy address belongs to the page:
page:ffffea00023b3d40 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea0002896a48 ffffea0002784e88 ffff8880aa000c40
raw: 0000000000000000 ffff88808ecf5000 0000000100000002 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88808ecf5780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88808ecf5800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88808ecf5880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                      ^
 ffff88808ecf5900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88808ecf5980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


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