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When the route uses a nexthop object, rt6_nlmsg_size() and rt6_fill_node() access the nexthop's RCU protected nexthop info and group pointers via rcu_dereference_rtnl(), resulting in suspicious RCU usage splats [1]. Fix by holding RCU while sizing and filling the notification, like fib_alias_hw_flags_set() already does. Switch to GFP_ATOMIC for the skb allocation accordingly. Reproducer: # echo "10 0" > /sys/bus/netdevsim/new_device # sysctl -wq net.ipv6.fib_notify_on_flag_change=1 # sysctl -wq net.ipv4.nexthop_compat_mode=1 # ip -6 nexthop add id 1 blackhole # ip -6 route add 2001:db8:1::1/128 nhid 1 [1] WARNING: suspicious RCU usage 7.3.0-rc7-virtme-g191aa41548b2 #38 Not tainted net/ipv4/nexthop.c:1539 suspicious rcu_dereference_check() usage! [...] Workqueue: events nsim_fib_event_work Call Trace: dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120) lockdep_rcu_suspicious.cold (kernel/locking/lockdep.c:6972) nexthop_for_each_fib6_nh (net/ipv4/nexthop.c:1539 (discriminator 16)) rt6_nlmsg_size (net/ipv6/route.c:5728) fib6_info_hw_flags_set (net/ipv6/route.c:6503) nsim_fib6_rt_add (drivers/net/netdevsim/fib.c:671 drivers/net/netdevsim/fib.c:694) nsim_fib6_rt_insert (drivers/net/netdevsim/fib.c:759) nsim_fib_event_work (drivers/net/netdevsim/fib.c:856 drivers/net/netdevsim/fib.c:889 drivers/net/netdevsim/fib.c:1493) process_one_work (kernel/workqueue.c:3396) worker_thread (kernel/workqueue.c:3479 kernel/workqueue.c:3560) kthread (kernel/kthread.c:436) ret_from_fork (arch/x86/kernel/process.c:158) ret_from_fork_asm (arch/x86/entry/entry_64.S:245) Fixes: 907eea486888 ("net: ipv6: Emit notification when fib hardware flags are changed") Reported-by: Sashiko Closes: https://lore.kernel.org/netdev/179152182668.434549.18232968419539498055@kernel.org/ Assisted-by: LLM Signed-off-by: Ido Schimmel --- net/ipv6/route.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/net/ipv6/route.c b/net/ipv6/route.c index a5e955e56e79..ab355e86ea54 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -6500,7 +6500,9 @@ void fib6_info_hw_flags_set(struct net *net, struct fib6_info *f6i, if (!fib_notify_on_flag_change) return; - skb = nlmsg_new(rt6_nlmsg_size(f6i), GFP_KERNEL); + rcu_read_lock(); + + skb = nlmsg_new(rt6_nlmsg_size(f6i), GFP_ATOMIC); if (!skb) { err = -ENOBUFS; goto errout; @@ -6513,10 +6515,13 @@ void fib6_info_hw_flags_set(struct net *net, struct fib6_info *f6i, goto errout; } + rcu_read_unlock(); + rtnl_notify(skb, net, 0, RTNLGRP_IPV6_ROUTE, NULL, GFP_KERNEL); return; errout: + rcu_read_unlock(); rtnl_set_sk_err(net, RTNLGRP_IPV6_ROUTE, err); } EXPORT_SYMBOL(fib6_info_hw_flags_set); -- 2.56.0