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References: <20191005112806.13960-1-christian.brauner@ubuntu.com> <20191006235216.7483-1-christian.brauner@ubuntu.com>
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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 7 Oct 2019 09:31:16 +0200
Message-ID: <CACT4Y+Z95M0co_vLTvbNDxb5YjuXwMcOBwNxZnJkMb_fLCDXXg@mail.gmail.com>
Subject: Re: [PATCH] taskstats: fix data-race
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     bsingharora@gmail.com, Marco Elver <elver@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        syzbot <syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Mon, Oct 7, 2019 at 1:52 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> When assiging and testing taskstats in taskstats_exit() there's a race
> when writing and reading sig->stats when a thread-group with more than
> one thread exits:
>
> cpu0:
> thread catches fatal signal and whole thread-group gets taken down
>  do_exit()
>  do_group_exit()
>  taskstats_exit()
>  taskstats_tgid_alloc()
> The tasks reads sig->stats holding sighand lock seeing garbage.
>
> cpu1:
> task calls exit_group()
>  do_exit()
>  do_group_exit()
>  taskstats_exit()
>  taskstats_tgid_alloc()
> The task takes sighand lock and assigns new stats to sig->stats.
>
> Fix this by using READ_ONCE() and smp_store_release().
>
> Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> ---
> /* v1 */
> Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com
>
> /* v2 */
> - Dmitry Vyukov <dvyukov@google.com>, Marco Elver <elver@google.com>:
>   - fix the original double-checked locking using memory barriers
> ---
>  kernel/taskstats.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Thanks!

> diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> index 13a0f2e6ebc2..8ee046e8a792 100644
> --- a/kernel/taskstats.c
> +++ b/kernel/taskstats.c
> @@ -554,24 +554,25 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
>  static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
>  {
>         struct signal_struct *sig = tsk->signal;
> -       struct taskstats *stats;
> +       struct taskstats *stats_new, *stats;
>
> -       if (sig->stats || thread_group_empty(tsk))
> -               goto ret;
> +       stats = READ_ONCE(sig->stats);
> +       if (stats || thread_group_empty(tsk))
> +               return stats;
>
>         /* No problem if kmem_cache_zalloc() fails */
> -       stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
> +       stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
>
>         spin_lock_irq(&tsk->sighand->siglock);
>         if (!sig->stats) {
> -               sig->stats = stats;
> -               stats = NULL;
> +               smp_store_release(&sig->stats, stats_new);
> +               stats_new = NULL;
>         }
>         spin_unlock_irq(&tsk->sighand->siglock);
>
> -       if (stats)
> -               kmem_cache_free(taskstats_cache, stats);
> -ret:
> +       if (stats_new)
> +               kmem_cache_free(taskstats_cache, stats_new);
> +
>         return sig->stats;
>  }
>
> --
> 2.23.0
>