On PREEMPT_RT kernels, after rt6_get_pcpu_route() returns NULL, the current task can be preempted. Another task running on the same CPU may then execute rt6_make_pcpu_route() and successfully install a pcpu_rt entry. When the first task resumes execution, its cmpxchg() in rt6_make_pcpu_route() will fail because rt6i_pcpu is no longer NULL, triggering the BUG_ON(prev). It's easy to reproduce it by adding mdelay() after rt6_get_pcpu_route(). Using preempt_disable/enable is not appropriate here because ip6_rt_pcpu_alloc() may sleep. Fix this by: 1. Removing the BUG_ON and instead handling the race gracefully by freeing our allocation and returning the existing pcpu_rt when cmpxchg() fails. 2. Keeping the BUG_ON for non-PREEMPT_RT kernels, since preemption should not occur in this context and a cmpxchg failure would indicate a real bug. Link: https://syzkaller.appspot.com/bug?extid=9b35e9bc0951140d13e6 Fixes: 951f788a80ff ("ipv6: fix a BUG in rt6_get_pcpu_route()") Reported-by: syzbot+9b35e9bc0951140d13e6@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6918cd88.050a0220.1c914e.0045.GAE@google.com/T/ Signed-off-by: Jiayuan Chen --- v1 -> v2: Drop migrate_{disable, enabled} suggested by Steven and Paolo. https://lore.kernel.org/all/20251209124805.379112-1-jiayuan.chen@linux.dev/T/ --- net/ipv6/route.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/net/ipv6/route.c b/net/ipv6/route.c index aee6a10b112a..9e7afda7cba2 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -1470,7 +1470,18 @@ static struct rt6_info *rt6_make_pcpu_route(struct net *net, p = this_cpu_ptr(res->nh->rt6i_pcpu); prev = cmpxchg(p, NULL, pcpu_rt); - BUG_ON(prev); + if (unlikely(prev)) { + /* + * Another task on this CPU already installed a pcpu_rt. + * This can happen on PREEMPT_RT where preemption is possible. + * Free our allocation and return the existing one. + */ + BUG_ON(!IS_ENABLED(CONFIG_PREEMPT_RT)); + + dst_dev_put(&pcpu_rt->dst); + dst_release(&pcpu_rt->dst); + return prev; + } if (res->f6i->fib6_destroying) { struct fib6_info *from; -- 2.43.0