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 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 Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com --- /* v1 */ Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.co...
/* v2 */ - Dmitry Vyukov dvyukov@google.com, Marco Elver elver@google.com: - fix the original double-checked locking using memory barriers
/* v3 */ - Andrea Parri parri.andrea@gmail.com: - document memory barriers to make checkpatch happy --- kernel/taskstats.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-)
diff --git a/kernel/taskstats.c b/kernel/taskstats.c index 13a0f2e6ebc2..978d7931fb65 100644 --- a/kernel/taskstats.c +++ b/kernel/taskstats.c @@ -554,24 +554,27 @@ 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 = 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; + /* Pairs with READ_ONCE() above. */ + 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; }