linux-kernel - Re: [PATCH] mm/vmalloc: fix data race in show_numa

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Message-ID: <CAO9qdTHU6xGYFYVigMuUPeFZeMN+i9d-rRm+4Nx6vECOhPOtpw@mail.gmail.com>
Date: Tue, 6 May 2025 14:44:15 +0900
From: Jeongjun Park <aha310510@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: akpm@...ux-foundation.org, urezki@...il.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmalloc: fix data race in show_numa_info()

Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, May 5, 2025 at 11:06 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Mon, May 5, 2025 at 10:20 AM Jeongjun Park <aha310510@...il.com> wrote:
> > >
> > > The following data-race was found in show_numa_info():
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in vmalloc_info_show / vmalloc_info_show
> > >
> > > r
> > > value changed: 0x0000008f -> 0x00000000
> > >
> > > Reported by Kernel Concurrency Sanitizer on:
> > > CPU: 1 UID: 0 PID: 8287 Comm: syz.0.411 Not tainted 6.15.0-rc4-00256-g95d3481af6dc-dirty #1 PREEMPT(voluntary)
> > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> > > ==================================================================
> > >
> > > There is a read/write data-race in counter[]. This seems to be happening
> > > because only read memory barriers are currently applied, so we need to
> > > modify the write operation to counters[] to be handled atomically.
> > >
> > > Fixes: a47a126ad5ea ("vmallocinfo: add NUMA information")
> > > Signed-off-by: Jeongjun Park <aha310510@...il.com>
> > > ---
> > >  mm/vmalloc.c | 9 +++++----
> > >  1 file changed, 5 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 3ed720a787ec..d93fa535bc21 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -4917,7 +4917,8 @@ bool vmalloc_dump_obj(void *object)
> > >  static void show_numa_info(struct seq_file *m, struct vm_struct *v)
> > >  {
> > >         if (IS_ENABLED(CONFIG_NUMA)) {
> > > -               unsigned int nr, *counters = m->private;
> > > +               atomic_t *counters = m->private;
> > > +               unsigned int nr;
> > >                 unsigned int step = 1U << vm_area_page_order(v);
> > >
> > >                 if (!counters)
> > > @@ -4931,10 +4932,10 @@ static void show_numa_info(struct seq_file *m, struct vm_struct *v)
> > >                 memset(counters, 0, nr_node_ids * sizeof(unsigned int));
> > >
> > >                 for (nr = 0; nr < v->nr_pages; nr += step)
> > > -                       counters[page_to_nid(v->pages[nr])] += step;
> > > +                       atomic_add(step, &counters[page_to_nid(v->pages[nr])]);
> > >                 for_each_node_state(nr, N_HIGH_MEMORY)
> > > -                       if (counters[nr])
> > > -                               seq_printf(m, " N%u=%u", nr, counters[nr]);
> > > +                       if (atomic_read(&counters[nr]))
> > > +                               seq_printf(m, " N%u=%u", nr, atomic_read(&counters[nr]));
> > >         }
> > >  }
> > >
> > > --
> >
> > This patch looks bogus to me.
> >
> > The race is about using m->private for storage, while the same file
> > can be read from multiple threads.
> >
> > Using atomic_t is going to silence syzbot, but the bug is still there.
>
> A more correct fix would be :
>

Thanks for your suggestion! It definitely looks much better this way.
I'll do some refactoring on show_numa_info() and send out a
v2 patch.

Regards,

Jeongjun Park

> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a6e7acebe9adf5e6c8abd52dcf7d02a6a1bc3030..cb69b44587d2032a6192f3ceb518490a05eff541
> 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4921,24 +4921,24 @@ bool vmalloc_dump_obj(void *object)
>  static void show_numa_info(struct seq_file *m, struct vm_struct *v)
>  {
>         if (IS_ENABLED(CONFIG_NUMA)) {
> -               unsigned int nr, *counters = m->private;
> +               unsigned int nr, *counters;
>                 unsigned int step = 1U << vm_area_page_order(v);
>
> +               if (v->flags & VM_UNINITIALIZED)
> +                       return;
> +               counters = kcalloc(nr_node_ids, sizeof(unsigned int),
> GFP_KERNEL);
>                 if (!counters)
>                         return;
>
> -               if (v->flags & VM_UNINITIALIZED)
> -                       return;
>                 /* Pair with smp_wmb() in clear_vm_uninitialized_flag() */
>                 smp_rmb();
>
> -               memset(counters, 0, nr_node_ids * sizeof(unsigned int));
> -
>                 for (nr = 0; nr < v->nr_pages; nr += step)
>                         counters[page_to_nid(v->pages[nr])] += step;
>                 for_each_node_state(nr, N_HIGH_MEMORY)
>                         if (counters[nr])
>                                 seq_printf(m, " N%u=%u", nr, counters[nr]);
> +               kfree(counters);
>         }
>  }
>
> @@ -5032,13 +5032,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
>
>  static int __init proc_vmalloc_init(void)
>  {
> -       void *priv_data = NULL;
> -
> -       if (IS_ENABLED(CONFIG_NUMA))
> -               priv_data = kmalloc(nr_node_ids * sizeof(unsigned
> int), GFP_KERNEL);
> -
> -       proc_create_single_data("vmallocinfo",
> -               0400, NULL, vmalloc_info_show, priv_data);
> +       proc_create_single("vmallocinfo", 0400, NULL, vmalloc_info_show);
>
>         return 0;
>  }