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@syzkaller.appspotmail.com Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") Cc: Will Deacon will@kernel.org Cc: Marco Elver elver@google.com Cc: Andrea Parri parri.andrea@gmail.com Cc: Dmitry Vyukov dvyukov@google.com Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner christian.brauner@ubuntu.com --- /* v1 */ Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.co...
/* v2 */ Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com - Dmitry Vyukov dvyukov@google.com, Marco Elver elver@google.com: - fix the original double-checked locking using memory barriers
/* v3 */ Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@ubuntu.com - Andrea Parri parri.andrea@gmail.com: - document memory barriers to make checkpatch happy
/* v4 */ Link: https://lore.kernel.org/r/20191009113134.5171-1-christian.brauner@ubuntu.com - Andrea Parri parri.andrea@gmail.com: - use smp_load_acquire(), not READ_ONCE() - update commit message
/* v5 */ Link: https://lore.kernel.org/r/20191009114809.8643-1-christian.brauner@ubuntu.com - Andrea Parri parri.andrea@gmail.com: - fix typo in smp_load_acquire()
/* v6 */ - Christian Brauner christian.brauner@ubuntu.com: - 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 */