Messages in this thread | | | From | Y Song <> | Date | Fri, 10 Aug 2018 22:01:33 -0700 | Subject | Re: [BUG] bpf: syscall: a possible sleep-in-atomic-context bug in map_update_elem() |
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On Fri, Aug 10, 2018 at 6:57 PM, Jia-Ju Bai <baijiaju1990@gmail.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
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