Re: Re: [PATCH bpf] bpf: Fix deadlock when freeing cgroup storage

Abel Wu
2024-12-21T02:26:46Z

On 12/20/24 2:39 AM, Yonghong Song Wrote:
>
>
>
> On 12/19/24 4:38 AM, Abel Wu wrote:
>> Hi Yonghong,
>>
>> On 12/19/24 10:45 AM, Yonghong Song Wrote:
>>>
>>>
>>>
>>> On 12/18/24 1:21 AM, Abel Wu wrote:
>>>> The following commit
>>>> bc235cdb423a ("bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]")
>>>> first introduced deadlock prevention for fentry/fexit programs attaching
>>>> on bpf_task_storage helpers. That commit also employed the logic in map
>>>> free path in its v6 version.
>>>>
>>>> Later bpf_cgrp_storage was first introduced in
>>>> c4bcfb38a95e ("bpf: Implement cgroup storage available to non-cgroup-attached bpf progs")
>>>> which faces the same issue as bpf_task_storage, instead of its busy
>>>> counter, NULL was passed to bpf_local_storage_map_free() which opened
>>>> a window to cause deadlock:
>>>>
>>>>     <TASK>
>>>>     _raw_spin_lock_irqsave+0x3d/0x50
>>>>     bpf_local_storage_update+0xd1/0x460
>>>>     bpf_cgrp_storage_get+0x109/0x130
>>>>     bpf_prog_72026450ec387477_cgrp_ptr+0x38/0x5e
>>>>     bpf_trace_run1+0x84/0x100
>>>>     cgroup_storage_ptr+0x4c/0x60
>>>>     bpf_selem_unlink_storage_nolock.constprop.0+0x135/0x160
>>>>     bpf_selem_unlink_storage+0x6f/0x110
>>>>     bpf_local_storage_map_free+0xa2/0x110
>>>>     bpf_map_free_deferred+0x5b/0x90
>>>>     process_one_work+0x17c/0x390
>>>>     worker_thread+0x251/0x360
>>>>     kthread+0xd2/0x100
>>>>     ret_from_fork+0x34/0x50
>>>>     ret_from_fork_asm+0x1a/0x30
>>>>     </TASK>
>>>>
>>>>     [ Since the verifier treats 'void *' as scalar which
>>>>       prevents me from getting a pointer to 'struct cgroup *',
>>>>       I added a raw tracepoint in cgroup_storage_ptr() to
>>>>       help reproducing this issue. ]
>>>>
>>>> Although it is tricky to reproduce, the risk of deadlock exists and
>>>> worthy of a fix, by passing its busy counter to the free procedure so
>>>> it can be properly incremented before storage/smap locking.
>>>
>>> The above stack trace and explanation does not show that we will have
>>> a potential dead lock here. You mentioned that it is tricky to reproduce,
>>> does it mean that you have done some analysis or coding to reproduce it?
>>> Could you share the details on why you think we may have deadlock here?
>>
>> The stack is A-A deadlocked: cgroup_storage_ptr() is called with
>> storage->lock held, while the bpf_prog attaching on this function
>> also tries to acquire the same lock by calling bpf_cgrp_storage_get()
>> thus leading to a AA deadlock.
>>
>> The tricky part is, instead of attaching on cgroup_storage_ptr()
>> directly, I added a tracepoint inside it to hook:
>>
>> ------
>> diff --git a/kernel/bpf/bpf_cgrp_storage.c b/kernel/bpf/bpf_cgrp_storage.c
>> index 20f05de92e9c..679209d4f88f 100644
>> --- a/kernel/bpf/bpf_cgrp_storage.c
>> +++ b/kernel/bpf/bpf_cgrp_storage.c
>> @@ -40,6 +40,8 @@ static struct bpf_local_storage __rcu **cgroup_storage_ptr(void *owner)
>>  {
>>         struct cgroup *cg = owner;
>>
>> +       trace_cgroup_ptr(cg);
>> +
>>         return &cg->bpf_cgrp_storage;
>>  }
>>
>> ------
>>
>> The reason doing so is that typecasting from 'void *owner' to
>> 'struct cgroup *' will be rejected by the verifier. But there
>> could be other ways to obtain a pointer to the @owner cgroup
>> too, making the deadlock possible.
>
> I checked the callstack and what you described indeed the case.
> In function bpf_selem_unlink_storage(), local_storage->lock is
> held before calling bpf_selem_unlink_storage_nolock/cgroup_storage_ptr.
> If there is a fentry/tracepoint on the cgroup_storage_ptr and then we could
> have a deadlock as you described in the above.
>
> As you mentioned, it is tricky to reproduce. fentry on cgroup_storage_ptr
> does not work due to func signature:
>   struct bpf_local_storage __rcu **cgroup_storage_ptr(void *owner)
> Even say we support 'void *' for fentry and we do bpf_rdonly_cast()
> to cast 'void *owner' to 'struct cgroup *owner', and owner cannot be
> passed to helper/kfunc.
>
> Your fix looks good but it would be great to have a reproducer.
> One possibility is to find a function which can be fentried within
> local_storage->lock. If you know the cgroup id, in bpf program you
> can use bpf_cgroup_from_id() to get a trusted cgroup ptr from the id.
> and then you can use that cgroup ptr to do bpf_cgrp_storage_get() etc.
> which should be able to triger deadlock. Could you give a try?

Sure.

>
> Also, in your commit message, it will be great if you can illustrage
> where each lock happens, e.g.
>    local_storage->lock
>      bpf_selem_unlink_storage_nolock.constprop.0
>        ...
>        bpf_local_storage_update
>          ...
>          local_storage->lock
>          ...
>

OK, will update.

>>
>> Thanks,
>>     Abel
>>
>>>
>>>>
>>>> Fixes: c4bcfb38a95e ("bpf: Implement cgroup storage available to non-cgroup-attached bpf progs")
>>>> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
>>>> ---
>>>>   kernel/bpf/bpf_cgrp_storage.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/bpf/bpf_cgrp_storage.c b/kernel/bpf/bpf_cgrp_storage.c
>>>> index 20f05de92e9c..7996fcea3755 100644
>>>> --- a/kernel/bpf/bpf_cgrp_storage.c
>>>> +++ b/kernel/bpf/bpf_cgrp_storage.c
>>>> @@ -154,7 +154,7 @@ static struct bpf_map *cgroup_storage_map_alloc(union bpf_attr *attr)
>>>>   static void cgroup_storage_map_free(struct bpf_map *map)
>>>>   {
>>>> -    bpf_local_storage_map_free(map, &cgroup_cache, NULL);
>>>> +    bpf_local_storage_map_free(map, &cgroup_cache, &bpf_cgrp_storage_busy);
>>>>   }
>>>>   /* *gfp_flags* is a hidden argument provided by the verifier */
>>>
>>
>