linux-kernel - Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get

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Date:   Thu, 4 Aug 2022 10:16:59 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lee Jones <lee@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <lee@...nel.org> wrote:
>
> The documentation for find_pid() clearly states:
>
>   "Must be called with the tasklist_lock or rcu_read_lock() held."
>
> Presently we do neither.
>
> Let's use find_get_pid() which searches for the vpid, then takes a
> reference to it preventing early free, all within the safety of
> rcu_read_lock().  Once we have our reference we can safely make use of
> it up until the point it is put.
>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Martin KaFai Lau <martin.lau@...ux.dev>
> Cc: Song Liu <song@...nel.org>
> Cc: Yonghong Song <yhs@...com>
> Cc: KP Singh <kpsingh@...nel.org>
> Cc: Stanislav Fomichev <sdf@...gle.com>
> Cc: Hao Luo <haoluo@...gle.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: bpf@...r.kernel.org
> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> Signed-off-by: Lee Jones <lee@...nel.org>
> ---
>
> v1 => v2:
>   * Commit log update - no code differences
>
>  kernel/bpf/syscall.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 83c7136c5788d..c20cff30581c4 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4385,6 +4385,7 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
>         const struct perf_event *event;
>         struct task_struct *task;
>         struct file *file;
> +       struct pid *ppid;
>         int err;
>
>         if (CHECK_ATTR(BPF_TASK_FD_QUERY))
> @@ -4396,7 +4397,9 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
>         if (attr->task_fd_query.flags != 0)
>                 return -EINVAL;
>
> -       task = get_pid_task(find_vpid(pid), PIDTYPE_PID);
> +       ppid = find_get_pid(pid);
> +       task = get_pid_task(ppid, PIDTYPE_PID);
> +       put_pid(ppid);

rcu_read_lock/unlock around this line
would be a cheaper and faster alternative than pid's
refcount inc/dec.