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

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

April 23, 2025 at 21:51, "Oleg Nesterov" <oleg@redhat.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.

There's no actual impact on usage without locks, so I think it might be more
suitable to add these macros, KASAN can recognize READ_ONCE and WRITE_ONCE
and suppress warnings.

Thanks.