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Message-ID: <CANn89i+y32YsDyUSyAvVswZTUaYTNjA=zGYmV5JXrCqa5EEHJA@mail.gmail.com>
Date: Thu, 23 May 2024 09:19:13 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com,
davem@...emloft.net, kuniyu@...zon.com, netdev@...r.kernel.org,
Jason Xing <kernelxing@...cent.com>,
syzbot+2eca27bdcb48ed330251@...kaller.appspotmail.com
Subject: Re: [PATCH v2 net] tcp: fix a race when purging the netns and
allocating tw socket
On Thu, May 23, 2024 at 7:04 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> Syzbot[1] reported the drecrement of reference count hits leaking memory.
>
> Race condition:
> CPU 0 CPU 1
> ----- -----
> inet_twsk_purge tcp_time_wait
> inet_twsk_deschedule_put __inet_twsk_schedule
> mod_timer(tw_timer...
> del_timer_sync
> inet_twsk_kill
> refcount_dec(tw_refcount)[1]
> refcount_inc(tw_refcount)[2]
>
> Race case happens because [1] decrements refcount first before [2].
>
> After we reorder the mod_timer() and refcount_inc() in the initialization
> phase, we can use the status of timer as an indicator to test if we want
> to destroy the tw socket in inet_twsk_purge() or postpone it to
> tw_timer_handler().
>
> After this patch applied, we get four possible cases:
> 1) if we can see the armed timer during the initialization phase
> CPU 0 CPU 1
> ----- -----
> inet_twsk_purge tcp_time_wait
> inet_twsk_deschedule_put __inet_twsk_schedule
> refcount_inc(tw_refcount)
> mod_timer(tw_timer...
> test if the timer is queued
> //timer is queued
> del_timer_sync
> inet_twsk_kill
> refcount_dec(tw_refcount)
> Note: we finish it up in the purge process.
>
> 2) if we fail to see the armed timer during the initialization phase
> CPU 0 CPU 1
> ----- -----
> inet_twsk_purge tcp_time_wait
> inet_twsk_deschedule_put __inet_twsk_schedule
> refcount_inc(tw_refcount)
> test if the timer is queued
> //timer isn't queued
> postpone
> mod_timer(tw_timer...
> Note: later, in another context, expired timer will finish up tw socket
>
> 3) if we're seeing a running timer after the initialization phase
> CPU 0 CPU 1 CPU 2
> ----- ----- -----
> tcp_time_wait
> __inet_twsk_schedule
> refcount_inc(tw_refcount)
> mod_timer(tw_timer...
> ...(around 1 min)...
> inet_twsk_purge
> inet_twsk_deschedule_put
> test if the timer is queued
> // timer is running
> skip tw_timer_handler
> Note: CPU 2 is destroying the timewait socket
>
> 4) if we're seeing a pending timer after the initialization phase
> CPU 0 CPU 1
> ----- -----
> tcp_time_wait
> __inet_twsk_schedule
> refcount_inc(tw_refcount)
> mod_timer(tw_timer...
> ...(< 1 min)...
> inet_twsk_purge
> inet_twsk_deschedule_put
> test if the timer is queued
> // timer is queued
> del_timer_sync
> inet_twsk_kill
>
> Therefore, only making sure that we either call inet_twsk_purge() or
> call tw_timer_handler() to destroy the timewait socket, we can
> handle all the cases as above.
>
> [1]
> refcount_t: decrement hit 0; leaking memory.
> WARNING: CPU: 3 PID: 1396 at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31
> Modules linked in:
> CPU: 3 PID: 1396 Comm: syz-executor.3 Not tainted 6.9.0-syzkaller-07370-g33e02dc69afb #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> RIP: 0010:refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31
> RSP: 0018:ffffc9000480fa70 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc9002ce28000
> RDX: 0000000000040000 RSI: ffffffff81505406 RDI: 0000000000000001
> RBP: ffff88804d8b3f80 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000002 R12: ffff88804d8b3f80
> R13: ffff888031c601c0 R14: ffffc900013c04f8 R15: 000000002a3e5567
> FS: 00007f56d897c6c0(0000) GS:ffff88806b300000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000001b3182b000 CR3: 0000000034ed6000 CR4: 0000000000350ef0
> Call Trace:
> <TASK>
> __refcount_dec include/linux/refcount.h:336 [inline]
> 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 [inline]
> inet_twsk_purge+0x725/0x890 net/ipv4/inet_timewait_sock.c:304
> tcp_twsk_purge+0x115/0x150 net/ipv4/tcp_minisocks.c:402
> 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
> unshare_nsproxy_namespaces+0xc0/0x1f0 kernel/nsproxy.c:228
> ksys_unshare+0x419/0x970 kernel/fork.c:3323
> __do_sys_unshare kernel/fork.c:3394 [inline]
> __se_sys_unshare kernel/fork.c:3392 [inline]
> __x64_sys_unshare+0x31/0x40 kernel/fork.c:3392
> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> do_syscall_64+0xcf/0x260 arch/x86/entry/common.c:83
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f56d7c7cee9
>
> Fixes: 2a750d6a5b36 ("rds: tcp: Fix use-after-free of net in reqsk_timer_handler().")
> Reported-by: syzbot+2eca27bdcb48ed330251@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2eca27bdcb48ed330251
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> v2
> Link: https://lore.kernel.org/all/20240521144930.23805-1-kerneljasonxing@gmail.com/
> 1. Use timer as a flag to test if we can safely destroy the timewait socket
> based on top of the patch Eric wrote.
> 2. change the title and add more explanation into body message.
> ---
> net/ipv4/inet_timewait_sock.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index e28075f0006e..b890d1c280a1 100644
> --- a/net/ipv4/inet_timewait_sock.c
> +++ b/net/ipv4/inet_timewait_sock.c
> @@ -255,8 +255,8 @@ void __inet_twsk_schedule(struct inet_timewait_sock *tw, int timeo, bool rearm)
>
> __NET_INC_STATS(twsk_net(tw), kill ? LINUX_MIB_TIMEWAITKILLED :
> LINUX_MIB_TIMEWAITED);
> - BUG_ON(mod_timer(&tw->tw_timer, jiffies + timeo));
> refcount_inc(&tw->tw_dr->tw_refcount);
> + BUG_ON(mod_timer(&tw->tw_timer, jiffies + timeo));
> } else {
> mod_timer_pending(&tw->tw_timer, jiffies + timeo);
> }
> @@ -301,7 +301,14 @@ void inet_twsk_purge(struct inet_hashinfo *hashinfo)
> rcu_read_unlock();
> local_bh_disable();
> if (state == TCP_TIME_WAIT) {
> - inet_twsk_deschedule_put(inet_twsk(sk));
> + struct inet_timewait_sock *tw = inet_twsk(sk);
> +
> + /* If the timer is armed, we can safely destroy
> + * it, or else we postpone the process of destruction
> + * to tw_timer_handler().
> + */
> + if (timer_pending(&tw->tw_timer))
> + inet_twsk_deschedule_put(tw);
This patch is not needed, and a timer_pending() would be racy anywau.
As already explained, del_timer_sync() takes care of this with proper locking.
inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
{
if (del_timer_sync(&tw->tw_timer))
inet_twsk_kill(tw);
inet_twsk_put(tw);
}
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