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SubjectRe: [PATCH] taskstats: fix data-race
On Wed, 9 Oct 2019 at 13:31, 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 without 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 smp_load_acquire() and smp_store_release().
>
> Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com
> Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> ---
> /* v1 */
> Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com
>
> /* v2 */
> Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com
> - Dmitry Vyukov <dvyukov@google.com>, Marco Elver <elver@google.com>:
> - fix the original double-checked locking using memory barriers
>
> /* v3 */
> Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@ubuntu.com/
> - Andrea Parri <parri.andrea@gmail.com>:
> - document memory barriers to make checkpatch happy
>
> /* v4 */
> - Andrea Parri <parri.andrea@gmail.com>:
> - use smp_load_acquire(), not READ_ONCE()
> - update commit message

Acked-by: Marco Elver <elver@google.com>

Note that this now looks almost like what I suggested, except the
return at the end of the function is accessing sig->stats again. In
this case, it seems it's fine assuming sig->stats cannot be written
elsewhere. Just wanted to point it out to make sure it's considered.

Thanks!

> ---
> kernel/taskstats.c | 24 +++++++++++++++---------
> 1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> index 13a0f2e6ebc2..e6b45315607a 100644
> --- a/kernel/taskstats.c
> +++ b/kernel/taskstats.c
> @@ -554,24 +554,30 @@ 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;
> + /* Pairs with smp_store_release() below. */
> + stats = smp_load_acquire(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;
> + /*
> + * Pairs with smp_store_release() above and order the
> + * kmem_cache_zalloc().
> + */
> + 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
>

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