Re: [PATCH] cgroup: don't queue css_release_work if one already pending

[Tadeusz Struk <tadeusz.struk@linaro.org>]

Hi Hillf,
On 4/13/22 02:56, Hillf Danton wrote:
> On Tue, 12 Apr 2022 12:24:59 -0700 Tadeusz Struk wrote:
>> Syzbot found a corrupted list bug scenario that can be triggered from
>> cgroup css_create(). The reproduces writes to cgroup.subtree_control
>> file, which invokes cgroup_apply_control_enable(), css_create(), and
>> css_populate_dir(), which then randomly fails with a fault injected -ENOMEM.
>> In such scenario the css_create() error path rcu enqueues css_free_rwork_fn
>> work for an css->refcnt initialized with css_release() destructor,
>> and there is a chance that the css_release() function will be invoked
> 
> Could you tip-point the percpu_ref_kill needed to trigger the css_release()?

What I think happens is that the write triggers:
cgroup_subtree_control_write()->cgroup_apply_control()->cgroup_apply_control_enable()->css_create()

which, allocates and initializes the css, then fails in cgroup_idr_alloc(), 
bails out and calls queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);

then cgroup_subtree_control_write bails out to out_unlock, which then goes:

cgroup_kn_unlock()->cgroup_put()->css_put()->percpu_ref_put(&css->refcnt)->percpu_ref_put_many(ref)
which then calls ref->data->release(ref); and tries to enqueue the same causing 
list corruption in insert_work.


>> for a cgroup_subsys_state, for which a destroy_work has already been
>> queued via css_create() error path. This causes a list_add corruption
>> as can be seen in the syzkaller report [1].
>> This can be avoided by adding a check to css_release() that checks
>> if it has already been enqueued.
>>
>> [1] https://syzkaller.appspot.com/bug?id=e26e54d6eac9d9fb50b221ec3e4627b327465dbd
> 
> Given my failure of finding the lore URL to what was
> Reported-by: syzbot+e42ae441c3b10acf9e9d@syzkaller.appspotmail.com
> such a link is welcome to see the list corruption.
> 

syzkaller doesn't send reports to any kernel mailing lists that are rachived in 
lore. The relevant links can be found in the dashboard, link [1] in the patch.

https://syzkaller.appspot.com/bug?id=e26e54d6eac9d9fb50b221ec3e4627b327465dbd

See the links in:
Crash: BUG: corrupted list in insert_work (log)

crash: https://syzkaller.appspot.com/text?tag=CrashReport&x=11bd5e9b700000
log: https://syzkaller.appspot.com/text?tag=CrashLog&x=16bd5e9b700000

>>
>> Cc: Tejun Heo <tj@kernel.org>
>> Cc: Zefan Li <lizefan.x@bytedance.com>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Christian Brauner <brauner@kernel.org>
>> Cc: Alexei Starovoitov <ast@kernel.org>
>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>> Cc: Andrii Nakryiko <andrii@kernel.org>
>> Cc: Martin KaFai Lau <kafai@fb.com>
>> Cc: Song Liu <songliubraving@fb.com>
>> Cc: Yonghong Song <yhs@fb.com>
>> Cc: John Fastabend <john.fastabend@gmail.com>
>> Cc: KP Singh <kpsingh@kernel.org>
>> Cc: <cgroups@vger.kernel.org>
>> Cc: <netdev@vger.kernel.org>
>> Cc: <bpf@vger.kernel.org>
>> Cc: <stable@vger.kernel.org>
>> Cc: <linux-kernel@vger.kernel.org>
>>
>> Reported-by: syzbot+e42ae441c3b10acf9e9d@syzkaller.appspotmail.com
>> Fixes: 8f36aaec9c92 ("cgroup: Use rcu_work instead of explicit rcu and work item")
>> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
>> ---
>>   kernel/cgroup/cgroup.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index adb820e98f24..9ae2de29f8c9 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -5210,8 +5210,11 @@ static void css_release(struct percpu_ref *ref)
>>   	struct cgroup_subsys_state *css =
>>   		container_of(ref, struct cgroup_subsys_state, refcnt);
>>   
>> -	INIT_WORK(&css->destroy_work, css_release_work_fn);
>> -	queue_work(cgroup_destroy_wq, &css->destroy_work);
>> +	if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT,
>> +			      work_data_bits(&css->destroy_work))) {
>> +		INIT_WORK(&css->destroy_work, css_release_work_fn);
>> +		queue_work(cgroup_destroy_wq, &css->destroy_work);
>> +	}
>>   }
>>   
>>   static void init_and_link_css(struct cgroup_subsys_state *css,
>> -- 
>> 2.35.1
>>

-- 
Thanks,
Tadeusz