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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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