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

Yonghong Song
2024-12-19T18:57:41Z

On 12/19/24 10:43 AM, Alexei Starovoitov wrote:
> On Thu, Dec 19, 2024 at 10:39 AM Yonghong Song <yonghong.song@linux.dev> 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?
> I'd rather mark a set of functions as notrace to avoid this situation
> or add:
> CFLAGS_REMOVE_bpf_cgrp_storage.o = $(CC_FLAGS_FTRACE)

If we go through CFLAGS_REMOVE path, we need to do

CFLAGS_REMOVE_bpf_local_storage.o = $(CC_FLAGS_FTRACE)

as well since bpf_selem_unlink_storage_nolock() calls a few functions
which, if fentry traced, could trigger similar issue (with bpf_cgroup_from_id() approach).