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Subject: Re: [BUG] bpf: syscall: a possible sleep-in-atomic-context bug in
 map_update_elem()
To:     Daniel Borkmann <daniel@iogearbox.net>, ast@kernel.org
Cc:     netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        brouer@redhat.com
References: <65830741-bf35-4d32-e365-c32fc17c25cb@gmail.com>
 <97aa2e6b-1863-f123-af2c-24a2e6333ba5@iogearbox.net>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <4f9a083d-1498-8cfb-b54c-614abccb193e@gmail.com>
Date:   Sat, 11 Aug 2018 09:57:20 +0800
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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).
> 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?


Best wishes,
Jia-Ju Bai