netdev - Re: [PATCH net-next] phonet: do not call synchronize_rcu() from phonet_route

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Message-ID: <CANn89iKN2HsFRYiboWn38zfptAsFRiRo89p7EGHFQu90=-O+3w@mail.gmail.com>
Date: Tue, 5 Nov 2024 13:05:22 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Rémi Denis-Courmont <remi@...lab.net>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	syzbot <syzkaller@...glegroups.com>, Kuniyuki Iwashima <kuniyu@...zon.com>, 
	Remi Denis-Courmont <courmisch@...il.com>
Subject: Re: [PATCH net-next] phonet: do not call synchronize_rcu() from phonet_route_del()

On Tue, Nov 5, 2024 at 12:05 PM Rémi Denis-Courmont <remi@...lab.net> wrote:
>
>
>
> Le 4 novembre 2024 17:26:22 GMT+02:00, Eric Dumazet <edumazet@...gle.com> a écrit :
> >Calling synchronize_rcu() while holding rcu_read_lock() is not
> >permitted [1]
> >
> >Move the synchronize_rcu() to route_doit().
> >
> >[1]
> >WARNING: suspicious RCU usage
> >6.12.0-rc5-syzkaller-01056-gf07a6e6ceb05 #0 Not tainted
> >-----------------------------
> >kernel/rcu/tree.c:4092 Illegal synchronize_rcu() in RCU read-side critical section!
> >
> >other info that might help us debug this:
> >
> >rcu_scheduler_active = 2, debug_locks = 1
> >1 lock held by syz-executor427/5840:
> >  #0: ffffffff8e937da0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:337 [inline]
> >  #0: ffffffff8e937da0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
> >  #0: ffffffff8e937da0 (rcu_read_lock){....}-{1:2}, at: route_doit+0x3d6/0x640 net/phonet/pn_netlink.c:264
> >
> >stack backtrace:
> >CPU: 1 UID: 0 PID: 5840 Comm: syz-executor427 Not tainted 6.12.0-rc5-syzkaller-01056-gf07a6e6ceb05 #0
> >Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
> >Call Trace:
> > <TASK>
> >  __dump_stack lib/dump_stack.c:94 [inline]
> >  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
> >  lockdep_rcu_suspicious+0x226/0x340 kernel/locking/lockdep.c:6821
> >  synchronize_rcu+0xea/0x360 kernel/rcu/tree.c:4089
> >  phonet_route_del+0xc6/0x140 net/phonet/pn_dev.c:409
> >  route_doit+0x514/0x640 net/phonet/pn_netlink.c:275
> >  rtnetlink_rcv_msg+0x791/0xcf0 net/core/rtnetlink.c:6790
> >  netlink_rcv_skb+0x1e3/0x430 net/netlink/af_netlink.c:2551
> >  netlink_unicast_kernel net/netlink/af_netlink.c:1331 [inline]
> >  netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1357
> >  netlink_sendmsg+0x8e4/0xcb0 net/netlink/af_netlink.c:1901
> >  sock_sendmsg_nosec net/socket.c:729 [inline]
> >  __sock_sendmsg+0x221/0x270 net/socket.c:744
> >  sock_write_iter+0x2d7/0x3f0 net/socket.c:1165
> >  new_sync_write fs/read_write.c:590 [inline]
> >  vfs_write+0xaeb/0xd30 fs/read_write.c:683
> >  ksys_write+0x183/0x2b0 fs/read_write.c:736
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
> > entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> >Fixes: 17a1ac0018ae ("phonet: Don't hold RTNL for route_doit().")
> >Reported-by: syzbot <syzkaller@...glegroups.com>
> >Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> >Cc: Kuniyuki Iwashima <kuniyu@...zon.com>
> >Cc: Remi Denis-Courmont <courmisch@...il.com>
> >---
> > net/phonet/pn_dev.c     |  4 +++-
> > net/phonet/pn_netlink.c | 10 ++++++++--
> > 2 files changed, 11 insertions(+), 3 deletions(-)
> >
> >diff --git a/net/phonet/pn_dev.c b/net/phonet/pn_dev.c
> >index 19234d664c4fb537eba0267266efbb226cf103c3..578d935f2b11694fd1004c5f854ec344b846eeb2 100644
> >--- a/net/phonet/pn_dev.c
> >+++ b/net/phonet/pn_dev.c
> >@@ -406,7 +406,9 @@ int phonet_route_del(struct net_device *dev, u8 daddr)
> >
> >       if (!dev)
> >               return -ENOENT;
> >-      synchronize_rcu();
> >+
> >+      /* Note : our caller must call synchronize_rcu() */
> >+
> >       dev_put(dev);
> >       return 0;
> > }
> >diff --git a/net/phonet/pn_netlink.c b/net/phonet/pn_netlink.c
> >index ca1f04e4a2d9eb3b2a6d6cc5b299aee28d569b08..24930733ac572ed3ec5fd142d347c115346a28fa 100644
> >--- a/net/phonet/pn_netlink.c
> >+++ b/net/phonet/pn_netlink.c
> >@@ -233,6 +233,7 @@ static int route_doit(struct sk_buff *skb, struct nlmsghdr *nlh,
> > {
> >       struct net *net = sock_net(skb->sk);
> >       struct nlattr *tb[RTA_MAX+1];
> >+      bool sync_needed = false;
> >       struct net_device *dev;
> >       struct rtmsg *rtm;
> >       u32 ifindex;
> >@@ -269,16 +270,21 @@ static int route_doit(struct sk_buff *skb, struct nlmsghdr *nlh,
> >               return -ENODEV;
> >       }
> >
> >-      if (nlh->nlmsg_type == RTM_NEWROUTE)
> >+      if (nlh->nlmsg_type == RTM_NEWROUTE) {
> >               err = phonet_route_add(dev, dst);
> >-      else
> >+      } else {
> >               err = phonet_route_del(dev, dst);
> >+              if (!err)
> >+                      sync_needed = true;
> >+      }
> >
> >       rcu_read_unlock();
> >
> >       if (!err)
> >               rtm_phonet_notify(net, nlh->nlmsg_type, ifindex, dst);
> >
> >+      if (sync_needed)
> >+              synchronize_rcu();
>
> Synchronising after sending notifications sounds a bit iffy. Whatever a given notification is about should be fully committed so we don't create a user-visible race here.
>
> Can't we reorder here?

Fair enough, I will move dev_put() here in V2.