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Date: Thu, 23 May 2024 17:23:47 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.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 3:19 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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);
> }
Sorry, I'm lost. But now I understand what I wrote in the morning is
not correct...
After rethinking about this, only reordering the mod_timer() and
refcount_inc() in the initialization phase (just like the patch you
wrote) can ensure safety.
CPU 0 uses del_timer_sync() to detect if the timer is running while
CPU 1 mod_timer() and then increments the refcount.
1) If CPU 0 sees the queued timer which means the tw socket has done
incrementing refcount, then CPU 0 can kill (inet_twsk_kill()) the
socket.
2) If CPU 0 cannot see the queued timer (and will not kill the
socket), then CPU 1 will call mod_timer() and expire in one minute.
Another cpu will call inet_twsk_kill() to clear tw socket at last.
The patch you provided seems to solve the race issue in the right
way... I cannot see the problem of it :(
So what is the problem to be solved with your patch? Thanks for your
patience and help.
Thanks,
Jason
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