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

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Message-ID: <20250424-marsch-radio-fb7bf6484c05@brauner>
Date: Thu, 24 Apr 2025 11:38:08 +0200
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Jiayuan Chen <jiayuan.chen@...ux.dev>, 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>, 
	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 Wed, Apr 23, 2025 at 06:38:18PM +0200, Oleg Nesterov wrote:
> 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.

Agreed. We should only add those annotations where they're really
needed (someone once taught me ;).