Re: Use after free in cgroup_migrate_execute

[Mukesh Ojha <quic_mojha@quicinc.com>]

+@paulmc

It would be difficult to reproduce this with clean kernel.

However, i have put trace_printk on the cset object and found that task 
reference is dropped from rcu_do_batch() as below and later trying to 
get same cset reference resulted in the warning and later BUG_ON().

rcuop/0-16    [002]   255.430731: bprint: 
put_css_set_locked:cset:refzero:put_css_set_locked:cset:0xffffff80274eb800:

Callers 
(cgroup_free<-delayed_put_task_struct<-rcu_do_batch<-nocb_cb_wait<-rcu_nocb_cb_kthread<-kthread<-ret_from_fork)

PERFD-SERVER-1387  [002]   255.432631: bprint: 
get_css_set: get_css_set: 0xffffff80274eb800: 
Callers:(cgroup_migrate_execute<-cgroup_attach_task<-__cgroup1_procs_write<-cgroup1_procs_write<-cgroup_file_write<-kernfs_fop_write_iter<-vfs_write)

PERFD-SERVER-1387  [002]   255.456360: bprint: 
get_css_set: get_css_set: 0xffffff80274eb800: 
Callers:(cgroup_migrate_execute<-cgroup_attach_task<-__cgroup1_procs_write<-cgroup1_procs_write<-cgroup_file_write<-kernfs_fop_write_iter<-vfs_write)


Thanks,
-Mukesh

On 3/2/2022 9:15 PM, Tejun Heo wrote:
> On Wed, Mar 02, 2022 at 08:42:32PM +0530, Mukesh Ojha wrote:
>> Hi ,
>>
>> We are facing one issue like below in cgroup .
>> Not able to find which race could lead to this.
>> Any idea, would be helpful.
>>
>> [136233.086904][ T1457] ------------[ cut here ]------------
>> *[136233.086912][ T1457] refcount_t: addition on 0; use-after-free.*
>> [136233.086943][ T1457] WARNING: CPU: 4 PID: 1457 at lib/refcount.c:25
>> cgroup_migrate_execute+0x188/0x528
>> [136233.087527][ T1457] CPU: 4 PID: 1457 Comm: PERFD-SERVER Tainted: G
>> S      WC O      5.10.66 #1
>> [136233.087532][ T1457] pstate: 62400085 (nZCv daIf +PAN -UAO +TCO BTYPE=--)
>> [136233.087536][ T1457] pc : cgroup_migrate_execute+0x188/0x528
>> [136233.087539][ T1457] lr : cgroup_migrate_execute+0x188/0x528
>> [136233.087541][ T1457] sp : ffffffc01ff23a60
>> [136233.087543][ T1457] x29: ffffffc01ff23a60 x28: 00000000c0000000
>> [136233.087547][ T1457] x27: ffffffffffffeaa8 x26: ffffff88cbc55668
>> [136233.087551][ T1457] x25: ffffff878424d458 x24: ffffff891fdd5e00
>> [136233.087557][ T1457] x23: ffffff88cbc55600 x22: ffffff8784d673d8
>> [136233.087565][ T1457] x21: ffffff88cbc55758 x20: ffffffc01ff23b20
>> [136233.087572][ T1457] x19: ffffffc01ff23b00 x18: ffffffc019475068
>> [136233.087580][ T1457] x17: 0000000000000000 x16: 0000000000162ba8
>> [136233.087587][ T1457] x15: 0000000000000004 x14: 000000000000407f
>> [136233.087594][ T1457] x13: ffffff8ae5d48be8 x12: 00000000ffffffff
>> [136233.087602][ T1457] x11: ffffff8785a79f98 x10: 0000000000000002
>> [136233.087609][ T1457] x9 : 759287265d79e000 x8 : 759287265d79e000
>> [136233.087616][ T1457] x7 : 206e6f206e6f6974 x6 : ffffffd7616121b4
>> [136233.087623][ T1457] x5 : ffffffffffffffff x4 : 0000000000000000
>> [136233.087629][ T1457] x3 : ffffffd7635ce996 x2 : 0000000000000000
>> [136233.087633][ T1457] x1 : ffffffd7635ce996 x0 : 000000000000002a
>> [136233.087636][ T1457] Call trace:
>> [136233.087640][ T1457]  cgroup_migrate_execute+0x188/0x528
>> [136233.087643][ T1457]  cgroup_migrate+0xb4/0xe4
>> [136233.087646][ T1457]  cgroup_attach_task+0x128/0x20c
>> [136233.087650][ T1457]  __cgroup1_procs_write+0x1d8/0x290
>> [136233.087653][ T1457]  cgroup1_procs_write+0x18/0x28
>> [136233.087656][ T1457]  cgroup_file_write+0xa4/0x544
>> [136233.087661][ T1457]  kernfs_fop_write_iter+0x1b0/0x2f8
>> [136233.087665][ T1457]  vfs_write+0x300/0x37c
>> [136233.087668][ T1457]  ksys_write+0x84/0x12c
>> [136233.087672][ T1457]  __arm64_sys_write+0x20/0x30
>> [136233.087676][ T1457]  el0_svc_common+0xdc/0x294
>> [136233.087681][ T1457]  el0_svc+0x38/0x9c
>> [136233.087684][ T1457]  el0_sync_handler+0x8c/0xf0
>> [136233.087688][ T1457]  el0_sync+0x1b4/0x1c0
>> [136233.087690][ T1457] ---[ end trace 9e592742965258ba ]---
>> [136233.087693][ T1457] ------------[ cut here ]------------
>> *[136233.087695][ T1457] refcount_t: saturated; leaking memory.*
> 
> Looks like the target css_set ref underglowed but you have five taint flags
> set and this isn't even the first warning message. Any chance you can
> reproduce this in a cleaner environment?
> 
> Thanks.
>