Re: [PATCH net 1/2] nexthop: take nh->lock for f6i_list walks in replace check and notify

fib6_check_nh_list() and __nexthop_replace_notify() walk nh->f6i_list
during an RTNL-serialized nexthop replace without holding nh->lock. IPv6
RTM_NEWROUTE/RTM_DELROUTE run without RTNL and mutate that list under
nh->lock (fib6_add_rt2node_nh(), fib6_purge_rt()), so both walks race a
concurrent route delete that unlinks and frees a fib6_info:

  BUG: KASAN: slab-use-after-free in rt6_fill_node.isra.0 (net/ipv6/route.c:5799)
  Read of size 4 at addr ffff888014607e64 by task exploit/143
   rt6_fill_node.isra.0 (net/ipv6/route.c:5799)
   fib6_rt_update (net/ipv6/route.c:6412)
   __nexthop_replace_notify (net/ipv4/nexthop.c:2542)
   rtm_new_nexthop (net/ipv4/nexthop.c:2554)
   rtnetlink_rcv_msg (net/core/rtnetlink.c:7076)

  BUG: KASAN: slab-use-after-free in fib6_check_nh_list (net/ipv4/nexthop.c:1605)
  Read of size 8 at addr ffff888014a7d068 by task exploit/142
   fib6_check_nh_list (net/ipv4/nexthop.c:1605)
   rtm_new_nexthop (net/ipv4/nexthop.c:2575)
   rtnetlink_rcv_msg (net/core/rtnetlink.c:7076)

Both walks only read the entries and take no tb6_lock, so protect them
with nh->lock; fib6_rt_update() uses gfp_any(), which returns GFP_ATOMIC
under the lock.

Fixes: 081efd18326e ("ipv6: Protect nh->f6i_list with spinlock and flag.")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <redacted>
---
 net/ipv4/nexthop.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 6205bd57aa85..c1d3b7d7100d 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c

@@ -1597,14 +1597,21 @@ static int fib6_check_nh_list(struct nexthop *old, struct nexthop *new,
 			      struct netlink_ext_ack *extack)
 {
 	struct fib6_info *f6i;
+	int err = 0;
 
 	if (list_empty(&old->f6i_list))
 		return 0;
 
+	spin_lock_bh(&old->lock);
 	list_for_each_entry(f6i, &old->f6i_list, nh_list) {
-		if (check_src_addr(&f6i->fib6_src.addr, extack) < 0)
-			return -EINVAL;
+		err = check_src_addr(&f6i->fib6_src.addr, extack);
+		if (err)
+			break;
 	}
+	spin_unlock_bh(&old->lock);
+
+	if (err)
+		return err;
 
 	return fib6_check_nexthop(new, NULL, extack);
 }

@@ -2538,8 +2545,10 @@ static void __nexthop_replace_notify(struct net *net, struct nexthop *nh,
 			fi->nh_updated = false;
 	}
 
+	spin_lock_bh(&nh->lock);
 	list_for_each_entry(f6i, &nh->f6i_list, nh_list)
 		fib6_rt_update(net, f6i, info);
+	spin_unlock_bh(&nh->lock);