Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage_[get|delete]

From: Song Liu <songliubraving@fb.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	Kernel Team <Kernel-team@fb.com>,
	"Peter Ziljstra" <peterz@infradead.org>
Subject: Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage_[get|delete]
Date: Tue, 23 Feb 2021 07:16:18 +0000	[thread overview]
Message-ID: <6A4F1927-AF73-4AC8-AE44-5878ACEDF944@fb.com> (raw)
In-Reply-To: <CAEf4BzaZ0ATbJsLoQu_SRUYgzkak9zv61N+T=gijOQ+X=57ErA@mail.gmail.com>

> On Feb 22, 2021, at 10:21 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Mon, Feb 22, 2021 at 5:23 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> BPF helpers bpf_task_storage_[get|delete] could hold two locks:
>> bpf_local_storage_map_bucket->lock and bpf_local_storage->lock. Calling
>> these helpers from fentry/fexit programs on functions in bpf_*_storage.c
>> may cause deadlock on either locks.
>> 
>> Prevent such deadlock with a per cpu counter, bpf_task_storage_busy, which
>> is similar to bpf_prog_active. We need this counter to be global, because
>> the two locks here belong to two different objects: bpf_local_storage_map
>> and bpf_local_storage. If we pick one of them as the owner of the counter,
>> it is still possible to trigger deadlock on the other lock. For example,
>> if bpf_local_storage_map owns the counters, it cannot prevent deadlock
>> on bpf_local_storage->lock when two maps are used.
>> 
>> Signed-off-by: Song Liu <songliubraving@fb.com>
>> ---
>> kernel/bpf/bpf_task_storage.c | 57 ++++++++++++++++++++++++++++++-----
>> 1 file changed, 50 insertions(+), 7 deletions(-)
>> 
> 
> [...]
> 
>> @@ -109,7 +136,9 @@ static void *bpf_pid_task_storage_lookup_elem(struct bpf_map *map, void *key)
>>                goto out;
>>        }
>> 
>> +       bpf_task_storage_lock();
>>        sdata = task_storage_lookup(task, map, true);
>> +       bpf_task_storage_unlock();
>>        put_pid(pid);
>>        return sdata ? sdata->data : NULL;
>> out:
>> @@ -141,8 +170,10 @@ static int bpf_pid_task_storage_update_elem(struct bpf_map *map, void *key,
>>                goto out;
>>        }
>> 
>> +       bpf_task_storage_lock();
>>        sdata = bpf_local_storage_update(
>>                task, (struct bpf_local_storage_map *)map, value, map_flags);
> 
> this should probably be container_of() instead of casting

bpf_task_storage.c uses casting in multiple places. How about we fix it in a 
separate patch?

Thanks,
Song

> 
>> +       bpf_task_storage_unlock();
>> 
>>        err = PTR_ERR_OR_ZERO(sdata);
>> out:
>> @@ -185,7 +216,9 @@ static int bpf_pid_task_storage_delete_elem(struct bpf_map *map, void *key)
>>                goto out;
>>        }
>> 
>> +       bpf_task_storage_lock();
>>        err = task_storage_delete(task, map);
>> +       bpf_task_storage_unlock();
>> out:
>>        put_pid(pid);
>>        return err;
> 
> [...]