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

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Date:   Thu, 21 Jul 2022 12:59:09 +0100
From:   Lee Jones <lee@...nel.org>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     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
Subject: Re: [PATCH 1/1] bpf: Drop unprotected find_vpid() in favour of
 find_get_pid()

On Thu, 21 Jul 2022, Jiri Olsa wrote:

> 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?

I assumed not, but that might be an oversight on my part.

Would that be your preference?

> > 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@...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>
> > ---
> >  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]