taskstats: fix data-race
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
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.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.com
/* v2 */
- Dmitry Vyukov <dvyukov@google.com>, Marco Elver <elver@google.com>:
- fix the original double-checked locking using memory barriers
diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 13a0f2e..8ee046e 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -554,24 +554,25 @@
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;
}
