Re: [PATCH 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jul 21, 2022 at 12:14:30PM +0100, Lee Jones wrote:
> The documentation for find_pid() clearly states:
> 
>   "Must be called with the tasklist_lock or rcu_read_lock() held."
> 
> Presently we do neither.
> 
> In an ideal world we would wrap the in-lined call to find_vpid() along
> with get_pid_task() in the suggested rcu_read_lock() and have done.
> However, looking at get_pid_task()'s internals, it already does that
> independently, so this would lead to deadlock.

hm, we can have nested rcu_read_lock calls, right?

jirka

> 
> Instead, we'll 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@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Song Liu <song@kernel.org>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: Stanislav Fomichev <sdf@google.com>
> Cc: Hao Luo <haoluo@google.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: bpf@vger.kernel.org
> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> Signed-off-by: Lee Jones <lee@kernel.org>
> ---
>  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;
>  
> -- 
> 2.37.0.170.g444d1eabd0-goog
>