On Mon, Oct 7, 2019 at 1:52 AM Christian Brauner
<christian.brau...@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+c5d03165a1bd1dead...@syzkaller.appspotmail.com
> Cc: Dmitry Vyukov <dvyu...@google.com>
> Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com>
> ---
> /* v1 */
> Link:
> https://lore.kernel.org/r/20191005112806.13960-1-christian.brau...@ubuntu.com
>
> /* v2 */
> - Dmitry Vyukov <dvyu...@google.com>, Marco Elver <el...@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 <dvyu...@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
>
