From 63f818f46af9f8b3f17b9695501e8d08959feb60 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Tue, 7 Apr 2020 09:43:04 -0500 Subject: proc: Use a dedicated lock in struct pid syzbot wrote: > ======================================================== > WARNING: possible irq lock inversion dependency detected > 5.6.0-syzkaller #0 Not tainted > -------------------------------------------------------- > swapper/1/0 just changed the state of lock: > ffffffff898090d8 (tasklist_lock){.+.?}-{2:2}, at: send_sigurg+0x9f/0x320 fs/fcntl.c:840 > but this lock took another, SOFTIRQ-unsafe lock in the past: > (&pid->wait_pidfd){+.+.}-{2:2} > > > and interrupts could create inverse lock ordering between them. > > > other info that might help us debug this: > Possible interrupt unsafe locking scenario: > > CPU0 CPU1 > ---- ---- > lock(&pid->wait_pidfd); > local_irq_disable(); > lock(tasklist_lock); > lock(&pid->wait_pidfd); > > lock(tasklist_lock); > > *** DEADLOCK *** > > 4 locks held by swapper/1/0: The problem is that because wait_pidfd.lock is taken under the tasklist lock. It must always be taken with irqs disabled as tasklist_lock can be taken from interrupt context and if wait_pidfd.lock was already taken this would create a lock order inversion. Oleg suggested just disabling irqs where I have added extra calls to wait_pidfd.lock. That should be safe and I think the code will eventually do that. It was rightly pointed out by Christian that sharing the wait_pidfd.lock was a premature optimization. It is also true that my pre-merge window testing was insufficient. So remove the premature optimization and give struct pid a dedicated lock of it's own for struct pid things. I have verified that lockdep sees all 3 paths where we take the new pid->lock and lockdep does not complain. It is my current day dream that one day pid->lock can be used to guard the task lists as well and then the tasklist_lock won't need to be held to deliver signals. That will require taking pid->lock with irqs disabled. Acked-by: Christian Brauner Link: https://lore.kernel.org/lkml/00000000000011d66805a25cd73f@google.com/ Cc: Oleg Nesterov Cc: Christian Brauner Reported-by: syzbot+343f75cdeea091340956@syzkaller.appspotmail.com Reported-by: syzbot+832aabf700bc3ec920b9@syzkaller.appspotmail.com Reported-by: syzbot+f675f964019f884dbd0f@syzkaller.appspotmail.com Reported-by: syzbot+a9fb1457d720a55d6dc5@syzkaller.appspotmail.com Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc") Signed-off-by: "Eric W. Biederman" --- kernel/pid.c | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel') diff --git a/kernel/pid.c b/kernel/pid.c index efd34874b3d1..517d0855d4cf 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -246,6 +246,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid, get_pid_ns(ns); refcount_set(&pid->count, 1); + spin_lock_init(&pid->lock); for (type = 0; type < PIDTYPE_MAX; ++type) INIT_HLIST_HEAD(&pid->tasks[type]); -- cgit v1.2.3