linux-kernel - Re: [PATCH v3 1/1] bpf: Drop unprotected find_vpid() in favour of find_get

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Date:   Thu, 8 Sep 2022 08:35:26 +0100
From:   Lee Jones <lee@...nel.org>
To:     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>, bpf@...r.kernel.org,
        Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v3 1/1] bpf: Drop unprotected find_vpid() in favour of
 find_get_pid()

On Tue, 09 Aug 2022, Lee Jones wrote:

> The documentation for find_vpid() 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: bpf@...r.kernel.org
> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> Acked-by: Jiri Olsa <jolsa@...nel.org>
> Signed-off-by: Lee Jones <lee@...nel.org>
> ---
> 
> v1 => v2:
>   * Commit log update - description - no code differences
> 
> v2 => v3:
>   * Commit log update - spelling of find_vpid() - no code differences

Did anyone get a chance to look at this please?

Would you like a [RESEND]?

>  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);
>  	if (!task)
>  		return -ENOENT;
>  

-- 
Lee Jones [李琼斯]