Re: KASAN: use-after-free Write in afs_manage_cell

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    abb3438d Merge tag 'm68knommu-for-v5.9-rc3' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1753522e900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=978db74cb30aa994
dashboard link: https://syzkaller.appspot.com/bug?extid=f59c67285cb61166a0cf
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13a9888e900000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=157cb346900000

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

FS-Cache: N-cookie c=00000000a6a6ef54 [p=000000005b0520e2 fl=2 nc=0 na=1]
FS-Cache: N-cookie d=00000000a15022ee n=0000000040eaeaa1
FS-Cache: N-key=[5] '23ee2ea1fb'
==================================================================
BUG: KASAN: use-after-free in afs_activate_cell fs/afs/cell.c:609 [inline]
BUG: KASAN: use-after-free in afs_manage_cell+0xea0/0x11c0 fs/afs/cell.c:697
Write of size 8 at addr ffff888098036890 by task kworker/0:0/5

CPU: 0 PID: 5 Comm: kworker/0:0 Not tainted 5.9.0-rc2-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/0x497 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:609 [inline]
 afs_manage_cell+0xea0/0x11c0 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 8859:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:461
 kmem_cache_alloc_trace+0x16e/0x2c0 mm/slab.c:3550
 kmalloc include/linux/slab.h:554 [inline]
 kzalloc include/linux/slab.h:666 [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
 generic_parse_monolithic+0x16f/0x1f0 fs/fs_context.c:201
 do_new_mount fs/namespace.c:2871 [inline]
 path_mount+0x133f/0x2070 fs/namespace.c:3192
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount fs/namespace.c:3390 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3390
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 3903:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
 kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
 __kasan_slab_free+0xd8/0x120 mm/kasan/common.c:422
 __cache_free mm/slab.c:3418 [inline]
 kfree+0x103/0x2c0 mm/slab.c:3756
 rcu_do_batch kernel/rcu/tree.c:2428 [inline]
 rcu_core+0x5c7/0x1190 kernel/rcu/tree.c:2656
 __do_softirq+0x2de/0xa24 kernel/softirq.c:298

Last call_rcu():
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_record_aux_stack+0x82/0xb0 mm/kasan/generic.c:346
 __call_rcu kernel/rcu/tree.c:2894 [inline]
 call_rcu+0x14f/0x7e0 kernel/rcu/tree.c:2968
 afs_manage_cell+0x250/0x11c0 fs/afs/cell.c:751
 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

Second to last call_rcu():
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_record_aux_stack+0x82/0xb0 mm/kasan/generic.c:346
 __call_rcu kernel/rcu/tree.c:2894 [inline]
 call_rcu+0x14f/0x7e0 kernel/rcu/tree.c:2968
 afs_manage_cell+0x250/0x11c0 fs/afs/cell.c:751
 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

The buggy address belongs to the object at ffff888098036800
 which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 144 bytes inside of
 1024-byte region [ffff888098036800, ffff888098036c00)
The buggy address belongs to the page:
page:00000000a84846b4 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x98036
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea000225ef48 ffffea000225e008 ffff8880aa040700
raw: 0000000000000000 ffff888098036000 0000000100000002 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888098036780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888098036800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888098036880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                         ^
 ffff888098036900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888098036980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================