netdev - Re: [PATCH net] tcp: prevent concurrent execution of tcp_sk_exit

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Message-ID: <20240812232842.92219-1-kuniyu@amazon.com>
Date: Mon, 12 Aug 2024 16:28:42 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <fw@...len.de>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kerneljasonxing@...il.com>,
	<kuba@...nel.org>, <kuniyu@...zon.com>, <netdev@...r.kernel.org>,
	<pabeni@...hat.com>, <syzbot+8ea26396ff85d23a8929@...kaller.appspotmail.com>
Subject: Re: [PATCH net] tcp: prevent concurrent execution of tcp_sk_exit_batch

From: Florian Westphal <fw@...len.de>
Date: Tue, 13 Aug 2024 00:28:25 +0200
> Its possible that two threads call tcp_sk_exit_batch() concurrently,
> once from the cleanup_net workqueue, once from a task that failed to clone
> a new netns.  In the latter case, error unwinding calls the exit handlers
> in reverse order for the 'failed' netns.
> 
> tcp_sk_exit_batch() calls tcp_twsk_purge().
> Problem is that since commit b099ce2602d8 ("net: Batch inet_twsk_purge"),
> this function picks up twsk in any dying netns, not just the one passed
> in via exit_batch list.
> 
> This means that the error unwind of setup_net() can "steal" and destroy
> timewait sockets belonging to the exiting netns.
> 
> This allows the netns exit worker to proceed to call
> 
> WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
> 
> without the expected 1 -> 0 transition, which then splats.
> 
> At same time, error unwind path that is also running inet_twsk_purge()
> will splat as well:
> 
> WARNING: .. at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210
> ...
>  refcount_dec include/linux/refcount.h:351 [inline]
>  inet_twsk_kill+0x758/0x9c0 net/ipv4/inet_timewait_sock.c:70
>  inet_twsk_deschedule_put net/ipv4/inet_timewait_sock.c:221
>  inet_twsk_purge+0x725/0x890 net/ipv4/inet_timewait_sock.c:304
>  tcp_sk_exit_batch+0x1c/0x170 net/ipv4/tcp_ipv4.c:3522
>  ops_exit_list+0x128/0x180 net/core/net_namespace.c:178
>  setup_net+0x714/0xb40 net/core/net_namespace.c:375
>  copy_net_ns+0x2f0/0x670 net/core/net_namespace.c:508
>  create_new_namespaces+0x3ea/0xb10 kernel/nsproxy.c:110
> 
> ... because refcount_dec() of tw_refcount unexpectedly dropped to 0.
> 
> This doesn't seem like an actual bug (no tw sockets got lost and I don't
> see a use-after-free) but as erroneous trigger of debug check.

I guess the reason you don't move the check back to tcp_sk_exit() is
to catch a potential issue explained in solution 4 in the link, right ?

Then,

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>

> 
> Add a mutex to force strict ordering: the task that calls tcp_twsk_purge()
> blocks other task from doing final _dec_and_test before mutex-owner has
> removed all tw sockets of dying netns.
> 
> Fixes: e9bd0cca09d1 ("tcp: Don't allocate tcp_death_row outside of struct netns_ipv4.")
> Cc: Kuniyuki Iwashima <kuniyu@...zon.com>
> Cc: Jason Xing <kerneljasonxing@...il.com>
> Reported-by: syzbot+8ea26396ff85d23a8929@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/0000000000003a5292061f5e4e19@google.com/
> Link: https://lore.kernel.org/netdev/20240812140104.GA21559@breakpoint.cc/
> Signed-off-by: Florian Westphal <fw@...len.de>