linux-kernel - Re: Re: [PATCH bpf v2] bpf: Fix deadlock when freeing cgroup storage

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Message-ID: <7139ed64-55be-4b70-a03f-8b2414fc93d3@bytedance.com>
Date: Sun, 26 Jan 2025 17:31:40 +0800
From: Abel Wu <wuyun.abel@...edance.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, David Vernet <void@...ifault.com>,
 "open list:BPF [STORAGE & CGROUPS]" <bpf@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH bpf v2] bpf: Fix deadlock when freeing cgroup storage

On 1/25/25 4:20 AM, Martin KaFai Lau Wrote:
> On 12/20/24 10:10 PM, 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>
>>         (acquiring local_storage->lock)
>>     _raw_spin_lock_irqsave+0x3d/0x50
>>     bpf_local_storage_update+0xd1/0x460
>>     bpf_cgrp_storage_get+0x109/0x130
>>     bpf_prog_a4d4a370ba857314_cgrp_ptr+0x139/0x170
>>     ? __bpf_prog_enter_recur+0x16/0x80
>>     bpf_trampoline_6442485186+0x43/0xa4
>>     cgroup_storage_ptr+0x9/0x20
>>         (holding local_storage->lock)
>>     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>
>>
>> Progs:
>>   - A: SEC("fentry/cgroup_storage_ptr")
> 
> The v1 thread has suggested using notrace in a few functions. I didn't see any counterarguments that wouldn't be sufficient.
> 
> imo, that should be a better option instead of having more unnecessary failures in all other normal use cases which will not be interested in tracing cgroup_storage_ptr().

Hi Martin, thank you very much reminding me this. I found my reply to
that message hadn't been sent out for unknown reasons.. I have re-send
just now, please refer to:

https://lore.kernel.org/bpf/46947563-c2e8-4346-84ca-c0774fa0ce39@bytedance.com/

Best Regards,
	Abel

> 
> pw-bot: cr
> 
>>     - cgid (BPF_MAP_TYPE_HASH)
>>     Record the id of the cgroup the current task belonging
>>     to in this hash map, using the address of the cgroup
>>     as the map key.
>>     - cgrpa (BPF_MAP_TYPE_CGRP_STORAGE)
>>     If current task is a kworker, lookup the above hash
>>     map using function parameter @owner as the key to get
>>     its corresponding cgroup id which is then used to get
>>     a trusted pointer to the cgroup through
>>     bpf_cgroup_from_id(). This trusted pointer can then
>>     be passed to bpf_cgrp_storage_get() to finally trigger
>>     the deadlock issue.
>>   - B: SEC("tp_btf/sys_enter")
>>     - cgrpb (BPF_MAP_TYPE_CGRP_STORAGE)
>>     The only purpose of this prog is to fill Prog A's
>>     hash map by calling bpf_cgrp_storage_get() for as
>>     many userspace tasks as possible.
>>
>> Steps to reproduce:
>>   - Run A;
>>   - while (true) { Run B; Destroy B; }
>>
>> Fix this issue by passing its busy counter to the free procedure so
>> it can be properly incremented before storage/smap locking.
>>
>> Fixes: c4bcfb38a95e ("bpf: Implement cgroup storage available to non-cgroup-attached bpf progs")
>> Signed-off-by: Abel Wu <wuyun.abel@...edance.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 */
>