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
Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20191006235216.7483-1-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
/* v3 */
- Andrea Parri <parri.andrea@xxxxxxxxx>:
- 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;
}
--
2.23.0
