Message-ID: <20221012193630.92200-1-kuniyu@amazon.com>
Date:   Wed, 12 Oct 2022 12:36:30 -0700
From:   Kuniyuki Iwashima <kuniyu@...zon.com>
To:     <edumazet@...gle.com>
CC:     <davem@...emloft.net>, <dsahern@...nel.org>, <kuba@...nel.org>,
        <kuni1840@...il.com>, <kuniyu@...zon.com>,
        <netdev@...r.kernel.org>, <pabeni@...hat.com>,
        <syzkaller@...glegroups.com>, <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH v1 net] tcp: Clean up kernel listener's reqsk in inet_twsk_purge()

From:   Eric Dumazet <edumazet@...gle.com>
Date:   Wed, 12 Oct 2022 11:54:42 -0700
> On Wed, Oct 12, 2022 at 10:28 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > From:   Eric Dumazet <edumazet@...gle.com>
> > Date:   Wed, 12 Oct 2022 09:31:44 -0700
> > > On Wed, Oct 12, 2022 at 7:51 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > > >
> > > > Eric Dumazet reported a use-after-free related to the per-netns ehash
> > > > series. [0]
> > > >
> > > > When we create a TCP socket from userspace, the socket always holds a
> > > > refcnt of the netns.  This guarantees that a reqsk timer is always fired
> > > > before netns dismantle.  Each reqsk has a refcnt of its listener, so the
> > > > listener is not freed before the reqsk, and the net is not freed before
> > > > the listener as well.
> > > >
> > > > OTOH, when in-kernel users create a TCP socket, it might not hold a refcnt
> > > > of its netns.  Thus, a reqsk timer can be fired after the netns dismantle
> > > > and access freed per-netns ehash.
> > >
> > > Patch seems good, but changelog is incorrect.
> > >
> > > 1) we have a TCP listener (or more) on a netns
> > > 2) We receive SYN packets, creating SYN_RECV request sockets, added in
> > > ehash table.
> > > 3) job is killed, TCP listener closed.
> > > 4) When a TCP listener is closed, we do not purge all SYN_RECV
> > > requests sockets, because we rely
> > >    on normal per-request timer firing, then finding the listener is no
> > > longer in LISTEN state -> drop the request socket.
> > >    (We do not maintain a per-listener list of request sockets, and
> > > going through ehash would be quite expensive on busy servers)
> > > 5) netns is deleted (and optional TCP ehashinfo freed)
> > > 6) request socket timer fire, and wecrash while trying to unlink the
> > > request socket from the freed ehash table.
> > >
> > > In short, I think the case could happen with normal TCP sockets,
> > > allocated from user space.
> >
> > Hmm.. I think 5) always happens after reqsk_timer for TCP socket
> > allocated from user space because reqsk has a refcnt for its netns
> > indirectly via the listener.
> >
> > reqsk has its listener's sk_refcnt, so when reqsk timer is fired
> > the last reqsk_put() in inet_csk_reqsk_queue_drop_and_put() calls
> > sock_put() for the listener, and then listener release the last
> > refcnt for the net, and finally net is queued up to the free-list.
> >
> > ---8<---
> > static void __sk_destruct(struct rcu_head *head)
> > {
> > ...
> >         if (likely(sk->sk_net_refcnt))
> >                 put_net(sock_net(sk));
> >         sk_prot_free(sk->sk_prot_creator, sk);
> > }
> > ---8<---
> >
> >
> > I did some tests with this script, but KASAN did not detect UAF
> > and I checked the timer is always executed before netns dismantle
> > for userspace listener.
> >
> > ---8<---
> > set -e
> >
> > sysctl -w net.ipv4.tcp_child_ehash_entries=128
> > sysctl -w net.ipv4.tcp_max_orphans=0
> >
> > cat <<EOF>test.py
> > from socket import *
> > from subprocess import run
> >
> > s = socket()
> > s.bind(('localhost', 80))
> > s.listen()
> >
> > c = socket()
> > c.connect(('localhost', 80))
> > run('netstat -tan'.split())
> > EOF
> >
> > cat <<EOF>test_net.sh
> > set -e
> >
> > sysctl net.ipv4.tcp_child_ehash_entries
> >
> > ip link set lo up
> >
> > iptables -A OUTPUT -o lo -p tcp --dport 80 --tcp-flags ACK ACK -j DROP
> >
> > python3 test.py
> >
> > netstat -tan
> >
> > EOF
> >
> > unshare -n bash test_net.sh
> > ---8<---
> >
> 
> Hmm... maybe , but I have other syzbot reports that suggest the netns
> refcounting is wrong...
> 
> Maybe this is time I add a ref tracker dir for 'kernel sockets',
> making sure this tracker is empty right before netns is destroyed.

Yes, that makes sense.
It seems a similar issue (kernel sk vs TCP full sk timer) happend before.
https://lore.kernel.org/all/00000000000045dc96059f4d7b02@google.com/

Actually I was thinking of adding a kernel reqsk counter like
tw_refcount to short-circuit inet_twsk_purge(), but considering
per-netns ehash size is usually small, it might not be worth adding.


> BUG: KASAN: use-after-free in tcp_probe_timer net/ipv4/tcp_timer.c:378 [inline]
> BUG: KASAN: use-after-free in tcp_write_timer_handler+0x7e2/0x920
> net/ipv4/tcp_timer.c:624
> Read of size 1 at addr ffff88807d6ba385 by task syz-executor.2/3638
> 
> CPU: 0 PID: 3638 Comm: syz-executor.2 Not tainted
> 6.0.0-syzkaller-09589-g55be6084c8e0 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine,
> BIOS Google 09/22/2022
> Call Trace:
>  <IRQ>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0x1e3/0x2cb lib/dump_stack.c:106
>  print_address_description+0x65/0x4b0 mm/kasan/report.c:317
>  print_report+0x108/0x220 mm/kasan/report.c:433
>  kasan_report+0xfb/0x130 mm/kasan/report.c:495
>  tcp_probe_timer net/ipv4/tcp_timer.c:378 [inline]
>  tcp_write_timer_handler+0x7e2/0x920 net/ipv4/tcp_timer.c:624
>  tcp_write_timer+0x176/0x280 net/ipv4/tcp_timer.c:637
>  call_timer_fn+0xf5/0x210 kernel/time/timer.c:1474
>  expire_timers kernel/time/timer.c:1519 [inline]
>  __run_timers+0x76a/0x980 kernel/time/timer.c:1790
>  run_timer_softirq+0x63/0xf0 kernel/time/timer.c:1803
>  __do_softirq+0x277/0x75b kernel/softirq.c:571
>  __irq_exit_rcu+0xec/0x170 kernel/softirq.c:650
>  irq_exit_rcu+0x5/0x20 kernel/softirq.c:662
>  sysvec_apic_timer_interrupt+0x91/0xb0 arch/x86/kernel/apic/apic.c:1107
> 
> 
> 
> 
> >
> >
> > > > To avoid the use-after-free, we need to clean up TCP_NEW_SYN_RECV sockets
> > > > in inet_twsk_purge() if the netns uses a per-netns ehash.
> > > >
> > > > [0]: https://lore.kernel.org/netdev/CANn89iLXMup0dRD_Ov79Xt8N9FM0XdhCHEN05sf3eLwxKweM6w@mail.gmail.com/
> > > >
> > > > BUG: KASAN: use-after-free in tcp_or_dccp_get_hashinfo
> > > > include/net/inet_hashtables.h:181 [inline]
> > > > BUG: KASAN: use-after-free in reqsk_queue_unlink+0x320/0x350
> > > > net/ipv4/inet_connection_sock.c:913
> > > > Read of size 8 at addr ffff88807545bd80 by task syz-executor.2/8301
> > > >
> > > > CPU: 1 PID: 8301 Comm: syz-executor.2 Not tainted
> > > > 6.0.0-syzkaller-02757-gaf7d23f9d96a #0
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine,
> > > > BIOS Google 09/22/2022
> > > > Call Trace:
> > > > <IRQ>
> > > > __dump_stack lib/dump_stack.c:88 [inline]
> > > > dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> > > > print_address_description mm/kasan/report.c:317 [inline]
> > > > print_report.cold+0x2ba/0x719 mm/kasan/report.c:433
> > > > kasan_report+0xb1/0x1e0 mm/kasan/report.c:495
> > > > tcp_or_dccp_get_hashinfo include/net/inet_hashtables.h:181 [inline]
> > > > reqsk_queue_unlink+0x320/0x350 net/ipv4/inet_connection_sock.c:913
> > > > inet_csk_reqsk_queue_drop net/ipv4/inet_connection_sock.c:927 [inline]
> > > > inet_csk_reqsk_queue_drop_and_put net/ipv4/inet_connection_sock.c:939 [inline]
> > > > reqsk_timer_handler+0x724/0x1160 net/ipv4/inet_connection_sock.c:1053
> > > > call_timer_fn+0x1a0/0x6b0 kernel/time/timer.c:1474
> > > > expire_timers kernel/time/timer.c:1519 [inline]
> > > > __run_timers.part.0+0x674/0xa80 kernel/time/timer.c:1790
> > > > __run_timers kernel/time/timer.c:1768 [inline]
> > > > run_timer_softirq+0xb3/0x1d0 kernel/time/timer.c:1803
> > > > __do_softirq+0x1d0/0x9c8 kernel/softirq.c:571
> > > > invoke_softirq kernel/softirq.c:445 [inline]
> > > > __irq_exit_rcu+0x123/0x180 kernel/softirq.c:650
> > > > irq_exit_rcu+0x5/0x20 kernel/softirq.c:662
> > > > sysvec_apic_timer_interrupt+0x93/0xc0 arch/x86/kernel/apic/apic.c:1107
> > > > </IRQ>
> > > >
> > > > Fixes: d1e5e6408b30 ("tcp: Introduce optional per-netns ehash.")
> > > > Reported-by: syzbot <syzkaller@...glegroups.com>
> > > > Reported-by: Eric Dumazet <edumazet@...gle.com>
> > > > Suggested-by: Eric Dumazet <edumazet@...gle.com>
> > > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > > ---
> > > >  net/ipv4/inet_timewait_sock.c | 15 ++++++++++++++-
> > > >  net/ipv4/tcp_minisocks.c      |  9 +++++----
> > > >  2 files changed, 19 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> > > > index 71d3bb0abf6c..66fc940f9521 100644
> > > > --- a/net/ipv4/inet_timewait_sock.c
> > > > +++ b/net/ipv4/inet_timewait_sock.c
> > > > @@ -268,8 +268,21 @@ void inet_twsk_purge(struct inet_hashinfo *hashinfo, int family)
> > > >                 rcu_read_lock();
> > > >  restart:
> > > >                 sk_nulls_for_each_rcu(sk, node, &head->chain) {
> > > > -                       if (sk->sk_state != TCP_TIME_WAIT)
> > > > +                       if (sk->sk_state != TCP_TIME_WAIT) {
> > > > +                               /* A kernel listener socket might not hold refcnt for net,
> > > > +                                * so reqsk_timer_handler() could be fired after net is
> > > > +                                * freed.  Userspace listener and reqsk never exist here.
> > > > +                                */
> > > > +                               if (unlikely(sk->sk_state == TCP_NEW_SYN_RECV &&
> > > > +                                            hashinfo->pernet)) {
> > > > +                                       struct request_sock *req = inet_reqsk(sk);
> > > > +
> > > > +                                       inet_csk_reqsk_queue_drop_and_put(req->rsk_listener, req);
> > > > +                               }
> > > > +
> > > >                                 continue;
> > > > +                       }
> > > > +
> > > >                         tw = inet_twsk(sk);
> > > >                         if ((tw->tw_family != family) ||
> > > >                                 refcount_read(&twsk_net(tw)->ns.count))
> > > > diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> > > > index 79f30f026d89..c375f603a16c 100644
> > > > --- a/net/ipv4/tcp_minisocks.c
> > > > +++ b/net/ipv4/tcp_minisocks.c
> > > > @@ -353,13 +353,14 @@ void tcp_twsk_purge(struct list_head *net_exit_list, int family)
> > > >         struct net *net;
> > > >
> > > >         list_for_each_entry(net, net_exit_list, exit_list) {
> > > > -               /* The last refcount is decremented in tcp_sk_exit_batch() */
> > > > -               if (refcount_read(&net->ipv4.tcp_death_row.tw_refcount) == 1)
> > > > -                       continue;
> > > > -
> > > >                 if (net->ipv4.tcp_death_row.hashinfo->pernet) {
> > > > +                       /* Even if tw_refcount == 1, we must clean up kernel reqsk */
> > > >                         inet_twsk_purge(net->ipv4.tcp_death_row.hashinfo, family);
> > > >                 } 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, family);
> > > >                         purged_once = true;
> > > >                 }
> > > > --
> > > > 2.30.2

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