netdev - Re: [BUG] bpf: syscall: a possible sleep-in-atomic-context bug in map_update

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Date:   Fri, 10 Aug 2018 22:01:33 -0700
From:   Y Song <ys114321@...il.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [BUG] bpf: syscall: a possible sleep-in-atomic-context bug in map_update_elem()

On Fri, Aug 10, 2018 at 6:57 PM, Jia-Ju Bai <baijiaju1990@...il.com> wrote:
>
>
> On 2018/8/10 22:22, Daniel Borkmann wrote:
>>
>> On 08/10/2018 04:07 PM, Jia-Ju Bai wrote:
>>>
>>> The kernel may sleep with holding a rcu read lock.
>>>
>>> The function call paths (from bottom to top) in Linux-4.16 are:
>>>
>>> [FUNC] kmalloc(GFP_KERNEL)
>>> kernel/kthread.c, 283: kmalloc in __kthread_create_on_node
>>> kernel/kthread.c, 365: __kthread_create_on_node in kthread_create_on_node
>>> kernel/bpf/cpumap.c, 368: kthread_create_on_node in __cpu_map_entry_alloc
>>> kernel/bpf/cpumap.c, 490: __cpu_map_entry_alloc in cpu_map_update_elem
>>> kernel/bpf/syscall.c, 724: [FUNC_PTR]cpu_map_update_elem in
>>> map_update_elem
>>> kernel/bpf/syscall.c, 723: rcu_read_lock in map_update_elem
>>>
>>> Note that [FUNC_PTR] means a function pointer call is used.
>>>
>>> I do not find a good way to fix it, so I only report.
>>> This is found by my static analysis tool (DSAC).

Maybe your static analysis tool have false positives in this case?

>>
>> Thanks for the report Jia-Ju! In the map_update_elem() from syscall
>> path there's a check map->map_type == BPF_MAP_TYPE_CPUMAP, where we
>> call the cpumap's map->ops->map_update_elem() while /not/ being under
>> rcu_read_lock() as in other cases, so looks okay to me. Could you point
>> out the case for being under rcu_read_lock() more specifically which
>> the tool found?
>
>
> Thanks for your reply :)
> My tool cannot accurately track the case of map->map_type at present...
>
> According to my code review, there is a indeed check on line 697 in
> Linux-4.16:
>     else if (map->map_type == BPF_MAP_TYPE_CPUMAP) {
>         err = map->ops->map_update_elem(map, key, value, attr->flags);
>         goto out;
>     }
> But there is a call to map->ops->map_update_elem() that is under
> rcu_read_lock on line 724:
>         rcu_read_lock();
>         err = map->ops->map_update_elem(map, key, value, attr->flags);
>         rcu_read_unlock();
>
> So I think if map->map_type is not equal to BPF_MAP_TYPE_CPUMAP,
> map->ops->map_update_elem() can still be called under rcu_read_lock, is it
> right?

map->ops->map_update_elem() can be called under rcu_read_lock(), but
since it is not type cpumap, the function should not be cpu_map_update_elem().
Could you double check your static analysis tool?

>
>
> Best wishes,
> Jia-Ju Bai