[PATCH net v1 1/2] nexthop: fix IPv6 route referencing IPv4 nexthop

[Jiayuan Chen <jiayuan.chen@linux.dev>]

syzbot reported a panic [1] [2].

When an IPv6 nexthop is replaced with an IPv4 nexthop, the has_v4 flag
of all groups containing this nexthop is not updated. This is because
nh_group_v4_update is only called when replacing AF_INET to AF_INET6,
but the reverse direction (AF_INET6 to AF_INET) is missed.

This allows a stale has_v4=false to bypass fib6_check_nexthop, causing
IPv6 routes to be attached to groups that effectively contain only AF_INET
members. Subsequent route lookups then call nexthop_fib6_nh() which
returns NULL for the AF_INET member, leading to a NULL pointer
dereference.

Fix by calling nh_group_v4_update whenever the family changes, not just
AF_INET to AF_INET6.

Reproducer:
	# AF_INET6 blackhole
	ip -6 nexthop add id 1 blackhole
	# group with has_v4=false
	ip nexthop add id 100 group 1
	# replace with AF_INET (no -6), has_v4 stays false
	ip nexthop replace id 1 blackhole
	# pass stale has_v4 check
	ip -6 route add 2001:db8::/64 nhid 100
	# panic
	ping -6 2001:db8::1

[1] https://syzkaller.appspot.com/bug?id=e17283eb2f8dcf3dd9b47fe6f67a95f71faadad0
[2] https://syzkaller.appspot.com/bug?id=8699b6ae54c9f35837d925686208402949e12ef3
Fixes: 7bf4796dd099 ("nexthops: add support for replace")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/ipv4/nexthop.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 2c9036c719b6..11a763cbc848 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -2466,10 +2466,10 @@ static int replace_nexthop_single(struct net *net, struct nexthop *old,
 			goto err_notify;
 	}
 
-	/* When replacing an IPv4 nexthop with an IPv6 nexthop, potentially
+	/* When replacing a nexthop with one of a different family, potentially
 	 * update IPv4 indication in all the groups using the nexthop.
 	 */
-	if (oldi->family == AF_INET && newi->family == AF_INET6) {
+	if (oldi->family != newi->family) {
 		list_for_each_entry(nhge, &old->grp_list, nh_list) {
 			struct nexthop *nhp = nhge->nh_parent;
 			struct nh_group *nhg;
-- 
2.43.0