Re: [PATCH] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock

[Dennis Zhou <dennis@kernel.org>]

On Thu, Jul 03, 2025 at 02:19:34PM +0900, Jeongjun Park wrote:
> Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Wed, Jul 02, 2025 at 05:27:49PM +0900, Jeongjun Park wrote:
> > > Read/Write to pcpu_nr_populated should be performed while protected
> > > by pcpu_lock. However, pcpu_nr_pages() reads pcpu_nr_populated without any
> > > protection, which causes a data race between read/write.
> > >
> > > Therefore, when reading pcpu_nr_populated in pcpu_nr_pages(), it should be
> > > modified to be protected by pcpu_lock.
> > >
> > > Reported-by: syzbot+e5bd32b79413e86f389e@syzkaller.appspotmail.com
> > > Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
> > > Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> > > ---
> > >  mm/percpu.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/percpu.c b/mm/percpu.c
> > > index b35494c8ede2..0f98b857fb36 100644
> > > --- a/mm/percpu.c
> > > +++ b/mm/percpu.c
> > > @@ -3355,7 +3355,13 @@ void __init setup_per_cpu_areas(void)
> > >   */
> > >  unsigned long pcpu_nr_pages(void)
> > >  {
> > > -     return pcpu_nr_populated * pcpu_nr_units;
> >
> > No need for the lock as I think race is fine here. Use something like
> > the following and add a comment.
> >
> >         data_race(READ_ONCE(pcpu_nr_populated)) * pcpu_nr_units;
> >
> 
> This race itself is not a critical security vuln, but it is a read/write
> race that actually occurs. Writing to pcpu_nr_populated is already
> systematically protected through pcpu_lock, so why do you think you can
> ignore the data race only when reading?
> 

As mentioned in the other thread, the reader of this value is
/proc/meminfo and reading a stale value isn't the end of the world
either.

Thanks,
Dennis