Re: [syzbot] [net?] WARNING: refcount bug in inet_twsk_kill

[Florian Westphal <fw@strlen.de>]

Florian Westphal <fw@strlen.de> wrote:
> https://syzkaller.appspot.com/x/log.txt?x=117f3182980000
> 
> ... shows at two cores racing:
> 
> [ 3127.234402][ T1396] CPU: 3 PID: 1396 Comm: syz-executor.3 Not
> and
> [ 3127.257864][   T13] CPU: 1 PID: 13 Comm: kworker/u32:1 Not tainted 6.9.0-syzkalle (netns cleanup net).
> 
> 
> first splat backtrace shows invocation of tcp_sk_exit_batch() from
> netns error unwinding code.
> 
> Second one lacks backtrace, but its also in tcp_sk_exit_batch(),

... which doesn't work.  Does this look like a plausible
theory/exlanation?

Given:
1 exiting netns, has >= 1 tw sk.
1 (unrelated) netns that failed in setup_net

... we run into following race:

exiting netns, from cleanup wq, calls tcp_sk_exit_batch(), which calls
inet_twsk_purge(&tcp_hashinfo).

At same time, from error unwinding code, we also call tcp_sk_exit_batch().

Both threads walk tcp_hashinfo ehash buckets.

From work queue (normal netns exit path), we hit

303                         if (state == TCP_TIME_WAIT) {
304                                 inet_twsk_deschedule_put(inet_twsk(sk));

Because both threads operate on tcp_hashinfo, the unrelated
struct net (exiting net) is also visible to error-unwinding thread.

So, error unwinding code will call

303                         if (state == TCP_TIME_WAIT) {
304                                 inet_twsk_deschedule_put(inet_twsk(sk));

for the same tw sk and both threads do

218 void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
219 {
220         if (del_timer_sync(&tw->tw_timer))
221                 inet_twsk_kill(tw);

Error unwind path cancel timer, calls inet_twsk_kill, while
work queue sees timer as already shut-down so it ends up
returning to tcp_sk_exit_batch(), where it will WARN here:

  WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));

... because the supposedly-last tw_refcount decrement did not drop
it down to 0.

Meanwhile, error unwiding thread calls refcount_dec() on
tw_refcount, which now drops down to 0 instead of 1, which
provides another warn splat.

I'll ponder on ways to fix this tomorrow unless someone
else already has better theory/solution.