linux-kernel - Re: [PATCH v1] pid: annotate data-races around pid_ns->pid

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Message-ID: <20250423163818.GB28646@redhat.com>
Date: Wed, 23 Apr 2025 18:38:18 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: linux-kernel@...r.kernel.org, mrpre@....com, mkoutny@...e.com,
	syzbot+adcaa842b762a1762e7d@...kaller.appspotmail.com,
	syzbot+fab52e3459fa2f95df57@...kaller.appspotmail.com,
	syzbot+0718f65353d72efaac1e@...kaller.appspotmail.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Wei Yang <richard.weiyang@...il.com>,
	David Hildenbrand <david@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Mateusz Guzik <mjguzik@...il.com>,
	Joel Granados <joel.granados@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Wei Liu <wei.liu@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v1] pid: annotate data-races around pid_ns->pid_allocated

On 04/23, Jiayuan Chen wrote:
>
> April 23, 2025 at 21:51, "Oleg Nesterov" <oleg@...hat.com> wrote:
>
>
>
> >
> > On 04/23, Jiayuan Chen wrote:
> >
> > >
> > > Suppress syzbot reports by annotating these accesses using
> > >
> > >  READ_ONCE() / WRITE_ONCE().
> > >
> >
> > ...
> >
> > >
> > > --- a/kernel/pid.c
> > >
> > >  +++ b/kernel/pid.c
> > >
> > >  @@ -122,7 +122,8 @@ void free_pid(struct pid *pid)
> > >
> > >  for (i = 0; i <= pid->level; i++) {
> > >
> > >  struct upid *upid = pid->numbers + i;
> > >
> > >  struct pid_namespace *ns = upid->ns;
> > >
> > >  - switch (--ns->pid_allocated) {
> > >
> > >  + WRITE_ONCE(ns->pid_allocated, READ_ONCE(ns->pid_allocated) - 1);
> > >
> > >  + switch (READ_ONCE(ns->pid_allocated)) {
> > >
> >
> > I keep forgetting how kcsan works, but we don't need
> >
> > READ_ONCE(ns->pid_allocated) under pidmap_lock?
> >
> > Same for other functions which read/modify ->pid_allocated with
> >
> > this lock held.
> >
> > Oleg.
> >
>
> However, not all places that read/write pid_allocated are locked,
> for example:
> https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/kernel/pid_namespace.c#n271
> https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/kernel/fork.c#n2602
>
> So, in fact, the pidmap_lock is not effective. And if we were to add locks
> to all these places, it would be too heavy.

It seems you misunderstood me. I didn't argue with the lockless READ_ONCE()s
outside of pidmap_lock.

Oleg.