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+c5d03165a1bd1dead...@syzkaller.appspotmail.com Cc: Dmitry Vyukov <dvyu...@google.com> Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com> --- /* v1 */ Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brau...@ubuntu.com /* v2 */ - Dmitry Vyukov <dvyu...@google.com>, Marco Elver <el...@google.com>: - 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
