tcp: fix a race when purging the netns and allocating tw socket

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: 2a750d6 ("rds: tcp: Fix use-after-free of net in reqsk_timer_handler().")
Reported-by: syzbot+2eca27bdcb48ed330251@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2eca27bdcb48ed330251
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: NipaLocal <nipa@local>

Author: JasonXing

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);
 			} else {
 				struct request_sock *req = inet_reqsk(sk);