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Date:   Thu, 21 May 2020 22:55:11 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jon Maloy <jon.maloy@...csson.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] tipc: block BH before using dst_cache

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On Thu, May 21, 2020 at 10:53 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
>
>
> On Thu, May 21, 2020 at 10:50 PM Xin Long <lucien.xin@...il.com> wrote:
>>
>> On Fri, May 22, 2020 at 2:30 AM Eric Dumazet <edumazet@...gle.com> wrote:
>> >
>> > dst_cache_get() documents it must be used with BH disabled.
>> Interesting, I thought under rcu_read_lock() is enough, which calls
>> preempt_disable().
>
>
> rcu_read_lock() does not disable BH, never.
>
> And rcu_read_lock() does not necessarily disable preemption.
>
>
>>
>> have you checked other places where dst_cache_get() are used?
>
>
>
> Yes, other paths are fine.
>
>>
>>
>> >
>> > sysbot reported :
>> >
>> > BUG: using smp_processor_id() in preemptible [00000000] code: /21697
>> > caller is dst_cache_get+0x3a/0xb0 net/core/dst_cache.c:68
>> > CPU: 0 PID: 21697 Comm:  Not tainted 5.7.0-rc6-syzkaller #0
>> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> > Call Trace:
>> >  __dump_stack lib/dump_stack.c:77 [inline]
>> >  dump_stack+0x188/0x20d lib/dump_stack.c:118
>> >  check_preemption_disabled lib/smp_processor_id.c:47 [inline]
>> >  debug_smp_processor_id.cold+0x88/0x9b lib/smp_processor_id.c:57
>> >  dst_cache_get+0x3a/0xb0 net/core/dst_cache.c:68
>> >  tipc_udp_xmit.isra.0+0xb9/0xad0 net/tipc/udp_media.c:164
>> >  tipc_udp_send_msg+0x3e6/0x490 net/tipc/udp_media.c:244
>> >  tipc_bearer_xmit_skb+0x1de/0x3f0 net/tipc/bearer.c:526
>> >  tipc_enable_bearer+0xb2f/0xd60 net/tipc/bearer.c:331
>> >  __tipc_nl_bearer_enable+0x2bf/0x390 net/tipc/bearer.c:995
>> >  tipc_nl_bearer_enable+0x1e/0x30 net/tipc/bearer.c:1003
>> >  genl_family_rcv_msg_doit net/netlink/genetlink.c:673 [inline]
>> >  genl_family_rcv_msg net/netlink/genetlink.c:718 [inline]
>> >  genl_rcv_msg+0x627/0xdf0 net/netlink/genetlink.c:735
>> >  netlink_rcv_skb+0x15a/0x410 net/netlink/af_netlink.c:2469
>> >  genl_rcv+0x24/0x40 net/netlink/genetlink.c:746
>> >  netlink_unicast_kernel net/netlink/af_netlink.c:1303 [inline]
>> >  netlink_unicast+0x537/0x740 net/netlink/af_netlink.c:1329
>> >  netlink_sendmsg+0x882/0xe10 net/netlink/af_netlink.c:1918
>> >  sock_sendmsg_nosec net/socket.c:652 [inline]
>> >  sock_sendmsg+0xcf/0x120 net/socket.c:672
>> >  ____sys_sendmsg+0x6bf/0x7e0 net/socket.c:2362
>> >  ___sys_sendmsg+0x100/0x170 net/socket.c:2416
>> >  __sys_sendmsg+0xec/0x1b0 net/socket.c:2449
>> >  do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
>> >  entry_SYSCALL_64_after_hwframe+0x49/0xb3
>> > RIP: 0033:0x45ca29
>> >
>> > Fixes: e9c1a793210f ("tipc: add dst_cache support for udp media")
>> > Cc: Xin Long <lucien.xin@...il.com>
>> > Cc: Jon Maloy <jon.maloy@...csson.com>
>> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> > Reported-by: syzbot <syzkaller@...glegroups.com>
>> > ---
>> >  net/tipc/udp_media.c | 6 +++++-
>> >  1 file changed, 5 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/net/tipc/udp_media.c b/net/tipc/udp_media.c
>> > index d6620ad535461a4d04ed5ba90569ce8b7df9f994..28a283f26a8dff24d613e6ed57e5e69d894dae66 100644
>> > --- a/net/tipc/udp_media.c
>> > +++ b/net/tipc/udp_media.c
>> > @@ -161,9 +161,11 @@ static int tipc_udp_xmit(struct net *net, struct sk_buff *skb,
>> >                          struct udp_bearer *ub, struct udp_media_addr *src,
>> >                          struct udp_media_addr *dst, struct dst_cache *cache)
>> >  {
>> > -       struct dst_entry *ndst = dst_cache_get(cache);
>> > +       struct dst_entry *ndst;
>> >         int ttl, err = 0;
>> >
>> > +       local_bh_disable();
>> > +       ndst = dst_cache_get(cache);
>> >         if (dst->proto == htons(ETH_P_IP)) {
>> >                 struct rtable *rt = (struct rtable *)ndst;
>> >
>> > @@ -210,9 +212,11 @@ static int tipc_udp_xmit(struct net *net, struct sk_buff *skb,
>> >                                            src->port, dst->port, false);
>> >  #endif
>> >         }
>> > +       local_bh_enable();
>> >         return err;
>> >
>> >  tx_error:
>> > +       local_bh_enable();
>> >         kfree_skb(skb);
>> >         return err;
>> >  }
>> > --
>> > 2.27.0.rc0.183.gde8f92d652-goog
>> >

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