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Message-ID: <CANn89iKAGeR9CX1cOSXFK8CH-d9bS_sHvU1DhGvhvt7CmCSsAg@mail.gmail.com>
Date: Wed, 22 May 2024 08:51:37 +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 net] Revert "rds: tcp: Fix use-after-free of net in reqsk_timer_handler()."
On Wed, May 22, 2024 at 8:28 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> Hello Eric,
>
> On Tue, May 21, 2024 at 11:49 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, May 21, 2024 at 4:49 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > >
> > > From: Jason Xing <kernelxing@...cent.com>
> > >
> > > This reverts commit 2a750d6a5b365265dbda33330a6188547ddb5c24.
> > >
> > > Syzbot[1] reported the drecrement of reference count hits leaking memory.
> > >
> > > If we failed in setup_net() and try to undo the setup process, the
> > > reference now is 1 which shouldn't be decremented. However, it happened
> > > actually.
> > >
> > > After applying this patch which allows us to check the reference first,
> > > it will not hit zero anymore in tcp_twsk_purge() without calling
> > > inet_twsk_purge() one more time.
> > >
> > > [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>
> > > ---
> > > The reverted patch trying to solve another issue causes unexpected error as above. I
> > > think that issue can be properly analyzed and handled later. So can we revert it first?
> > > ---
> > > net/ipv4/tcp_minisocks.c | 4 ++++
> > > 1 file changed, 4 insertions(+)
> > >
> > > diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> > > index b93619b2384b..46e6f9db4227 100644
> > > --- a/net/ipv4/tcp_minisocks.c
> > > +++ b/net/ipv4/tcp_minisocks.c
> > > @@ -399,6 +399,10 @@ void tcp_twsk_purge(struct list_head *net_exit_list)
> > > /* Even if tw_refcount == 1, we must clean up kernel reqsk */
> > > inet_twsk_purge(net->ipv4.tcp_death_row.hashinfo);
> > > } else if (!purged_once) {
> > > + /* The last refcount is decremented in tcp_sk_exit_batch() */
> > > + if (refcount_read(&net->ipv4.tcp_death_row.tw_refcount) == 1)
> > > + continue;
> > > +
> > > inet_twsk_purge(&tcp_hashinfo);
> > > purged_once = true;
> > > }
> > > --
> >
> > This can not be a fix for a race condition.
> >
> > By definition a TW has a refcount on tcp_death_row.tw_refcount only
> > if its timer is armed.
> >
> > And inet_twsk_deschedule_put() does
> >
> > if (del_timer_sync(&tw->tw_timer))
> > inet_twsk_kill(tw);
> >
> > I think you need to provide a full explanation, instead of a shot in the dark.
> >
> > Before releasing this syzbot, I thought that maybe the refcount_inc()
> > was done too late,
> > but considering other invariants, this should not matter.
> >
> > diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> > index e28075f0006e333897ad379ebc8c87fc3f9643bd..d8f4d93c45331be64d70af96de33f783870e1dcc
> > 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);
> > }
>
> Thanks for your information.
>
> What you wrote is right, I think for a while. In
> inet_twsk_deschedule_put(), there are two possible cases:
> 1) if it finds the timer is armed, then it can kill the tw socket by
> decrementing the refcount in the right way. So it's a good/lucky thing
> for us.
> or
> 2) if it misses the point, then the tw socket arms the timer which
> will expire in 60 seconds in the initialization phase. The tw socket
> would have a chance to be destroyed when the timer expires.
>
> It seems that you don't think your code could solve the race issue?
inet_twsk_schedule(tw) is called while the tw refcount is still 0, so
this tw can be be found in inet_twsk_purge()
at the same time. Look at inet_twsk_hashdance() for details.
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