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

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Message-ID: <170606da-5d5d-40f5-a67c-9c8624a5e913@redhat.com>
Date: Thu, 15 Aug 2024 12:47:14 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Kuniyuki Iwashima <kuniyu@...zon.com>, Jason Xing
 <kerneljasonxing@...il.com>,
 syzbot+8ea26396ff85d23a8929@...kaller.appspotmail.com
Subject: Re: [PATCH net] tcp: prevent concurrent execution of
 tcp_sk_exit_batch

On 8/13/24 00:28, Florian Westphal wrote:
> 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.
> 
> 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>

The fixes LGTM, but I prefer to keep the patch in PW a little longer to 
allow Eric having a look here.

Cheers,

Paolo