Date:   Wed, 10 Oct 2018 09:48:01 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     mauricio.vasquez@...ito.it
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 3/7] bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall

On Wed, Oct 10, 2018 at 7:06 AM Mauricio Vasquez B
<mauricio.vasquez@...ito.it> wrote:
>
> The following patch implements a bpf queue/stack maps that
> provides the peek/pop/push functions.  There is not a direct
> relationship between those functions and the current maps
> syscalls, hence a new MAP_LOOKUP_AND_DELETE_ELEM syscall is added,
> this is mapped to the pop operation in the queue/stack maps
> and it is still to implement in other kind of maps.
>
> Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@...ito.it>
> ---
>  include/linux/bpf.h      |    1 +
>  include/uapi/linux/bpf.h |    1 +
>  kernel/bpf/syscall.c     |   82 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 84 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 9b558713447f..5793f0c7fbb5 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -39,6 +39,7 @@ struct bpf_map_ops {
>         void *(*map_lookup_elem)(struct bpf_map *map, void *key);
>         int (*map_update_elem)(struct bpf_map *map, void *key, void *value, u64 flags);
>         int (*map_delete_elem)(struct bpf_map *map, void *key);
> +       void *(*map_lookup_and_delete_elem)(struct bpf_map *map, void *key);
>
>         /* funcs called by prog_array and perf_event_array map */
>         void *(*map_fd_get_ptr)(struct bpf_map *map, struct file *map_file,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f9187b41dff6..3bb94aa2d408 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -103,6 +103,7 @@ enum bpf_cmd {
>         BPF_BTF_LOAD,
>         BPF_BTF_GET_FD_BY_ID,
>         BPF_TASK_FD_QUERY,
> +       BPF_MAP_LOOKUP_AND_DELETE_ELEM,
>  };
>
>  enum bpf_map_type {
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index f36c080ad356..6907d661dea5 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -980,6 +980,85 @@ static int map_get_next_key(union bpf_attr *attr)
>         return err;
>  }
>
> +#define BPF_MAP_LOOKUP_AND_DELETE_ELEM_LAST_FIELD value
> +
> +static int map_lookup_and_delete_elem(union bpf_attr *attr)
> +{
> +       void __user *ukey = u64_to_user_ptr(attr->key);
> +       void __user *uvalue = u64_to_user_ptr(attr->value);
> +       int ufd = attr->map_fd;
> +       struct bpf_map *map;
> +       void *key, *value, *ptr;
> +       u32 value_size;
> +       struct fd f;
> +       int err;
> +
> +       if (CHECK_ATTR(BPF_MAP_LOOKUP_AND_DELETE_ELEM))
> +               return -EINVAL;
> +
> +       f = fdget(ufd);
> +       map = __bpf_map_get(f);
> +       if (IS_ERR(map))
> +               return PTR_ERR(map);
> +
> +       if (!(f.file->f_mode & FMODE_CAN_WRITE)) {
> +               err = -EPERM;
> +               goto err_put;
> +       }
> +
> +       key = __bpf_copy_key(ukey, map->key_size);
> +       if (IS_ERR(key)) {
> +               err = PTR_ERR(key);
> +               goto err_put;
> +       }
> +
> +       value_size = map->value_size;
> +
> +       err = -ENOMEM;
> +       value = kmalloc(value_size, GFP_USER | __GFP_NOWARN);
> +       if (!value)
> +               goto free_key;
> +
> +       err = -EFAULT;
> +       if (copy_from_user(value, uvalue, value_size) != 0)
> +               goto free_value;
> +
> +       /* must increment bpf_prog_active to avoid kprobe+bpf triggering from
> +        * inside bpf map update or delete otherwise deadlocks are possible
> +        */
> +       preempt_disable();
> +       __this_cpu_inc(bpf_prog_active);
> +       if (map->ops->map_lookup_and_delete_elem) {
> +               rcu_read_lock();
> +               ptr = map->ops->map_lookup_and_delete_elem(map, key);
> +               if (ptr)
> +                       memcpy(value, ptr, value_size);
I think we are exposed to race condition with push and pop in parallel.
map_lookup_and_delete_elem() only updates the head/tail, so it gives
no protection for the buffer pointed by ptr.

Thanks,
Song

> +               rcu_read_unlock();
> +               err = ptr ? 0 : -ENOENT;
> +       } else {
> +               err = -ENOTSUPP;
> +       }
> +
> +       __this_cpu_dec(bpf_prog_active);
> +       preempt_enable();
> +
> +       if (err)
> +               goto free_value;
> +
> +       if (copy_to_user(uvalue, value, value_size) != 0)
> +               goto free_value;
> +
> +       err = 0;
> +
> +free_value:
> +       kfree(value);
> +free_key:
> +       kfree(key);
> +err_put:
> +       fdput(f);
> +       return err;
> +}
> +
>  static const struct bpf_prog_ops * const bpf_prog_types[] = {
>  #define BPF_PROG_TYPE(_id, _name) \
>         [_id] = & _name ## _prog_ops,
> @@ -2453,6 +2532,9 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>         case BPF_TASK_FD_QUERY:
>                 err = bpf_task_fd_query(&attr, uattr);
>                 break;
> +       case BPF_MAP_LOOKUP_AND_DELETE_ELEM:
> +               err = map_lookup_and_delete_elem(&attr);
> +               break;
>         default:
>                 err = -EINVAL;
>                 break;
>

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