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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Mon, 22 Feb 2021 22:21:32 -0800
Message-ID: <CAEf4BzaZ0ATbJsLoQu_SRUYgzkak9zv61N+T=gijOQ+X=57ErA@mail.gmail.com>
Subject: Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage_[get|delete]
To:     Song Liu <songliubraving@fb.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>
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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_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;

[...]