[PATCH] taskstats: fix data-race

[Christian Brauner]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
---
/* v1 */
Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@xxxxxxxxxx

/* v2 */
- Dmitry Vyukov <dvyukov@xxxxxxxxxx>, Marco Elver <elver@xxxxxxxxxx>:
  - fix the original double-checked locking using memory barriers
---
 kernel/taskstats.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

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