[PATCH v6] 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 without holding sighand lock.

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.

The first approach used smp_load_acquire() and smp_store_release().
However, after having discussed this it seems that the data dependency
for kmem_cache_alloc() would be fixed by WRITE_ONCE().
Furthermore, the smp_load_acquire() would only manage to order the stats
check before the thread_group_empty() check. So it seems just using
READ_ONCE() and WRITE_ONCE() will do the job and I wanted to bring this
up for discussion at least.

Reported-by: syzbot+c5d03165a1bd1dead0c1@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation")
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
---
/* v1 */
Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@xxxxxxxxxx

/* v2 */
Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@xxxxxxxxxx
- Dmitry Vyukov <dvyukov@xxxxxxxxxx>, Marco Elver <elver@xxxxxxxxxx>:
  - fix the original double-checked locking using memory barriers

/* v3 */
Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@xxxxxxxxxx
- Andrea Parri <parri.andrea@xxxxxxxxx>:
  - document memory barriers to make checkpatch happy

/* v4 */
Link: https://lore.kernel.org/r/20191009113134.5171-1-christian.brauner@xxxxxxxxxx
- Andrea Parri <parri.andrea@xxxxxxxxx>:
  - use smp_load_acquire(), not READ_ONCE()
  - update commit message

/* v5 */
Link: https://lore.kernel.org/r/20191009114809.8643-1-christian.brauner@xxxxxxxxxx
- Andrea Parri <parri.andrea@xxxxxxxxx>:
  - fix typo in smp_load_acquire()

/* v6 */
- Christian Brauner <christian.brauner@xxxxxxxxxx>:
  - bring up READ_ONCE()/WRITE_ONCE() approach for discussion
---
 kernel/taskstats.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 13a0f2e6ebc2..111bb4139aa2 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -554,25 +554,29 @@ 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 WRITE_ONCE() 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;
+	if (!stats) {
+		stats = stats_new;
+		/* Pairs with READ_ONCE() above. */
+		WRITE_ONCE(sig->stats, stats_new);
+		stats_new = NULL;
 	}
 	spin_unlock_irq(&tsk->sighand->siglock);
 
-	if (stats)
-		kmem_cache_free(taskstats_cache, stats);
-ret:
-	return sig->stats;
+	if (stats_new)
+		kmem_cache_free(taskstats_cache, stats_new);
+
+	return stats;
 }
 
 /* Send pid data out on exit */
-- 
2.23.0