net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop

When a standalone IPv6 nexthop object is created with a loopback device
(e.g., "ip -6 nexthop add id 100 dev lo"), fib6_nh_init() misclassifies
it as a reject route. This is because nexthop objects have no destination
prefix (fc_dst=::), causing fib6_is_reject() to match any loopback
nexthop. The reject path skips fib_nh_common_init(), leaving
nhc_pcpu_rth_output unallocated. If an IPv4 route later references this
nexthop, __mkroute_output() dereferences NULL nhc_pcpu_rth_output and
panics.

Simplify the check in fib6_nh_init() to only match explicit reject
routes (RTF_REJECT) instead of using fib6_is_reject(). The loopback
promotion heuristic in fib6_is_reject() is handled separately by
ip6_route_info_create_nh(). After this change, the three cases behave
as follows:

1. Explicit reject route ("ip -6 route add unreachable 2001:db8::/64"):
   RTF_REJECT is set, enters reject path, skips fib_nh_common_init().
   No behavior change.

2. Implicit loopback reject route ("ip -6 route add 2001:db8::/32 dev lo"):
   RTF_REJECT is not set, takes normal path, fib_nh_common_init() is
   called. ip6_route_info_create_nh() still promotes it to reject
   afterward. nhc_pcpu_rth_output is allocated but unused, which is
   harmless.

3. Standalone nexthop object ("ip -6 nexthop add id 100 dev lo"):
   RTF_REJECT is not set, takes normal path, fib_nh_common_init() is
   called. nhc_pcpu_rth_output is properly allocated, fixing the crash
   when IPv4 routes reference this nexthop.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Fixes: 493ced1ac47c ("ipv4: Allow routes to use nexthop objects")
Reported-by: syzbot+334190e097a98a1b81bb@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@google.com/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260304113817.294966-2-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 7db0c837196c2dd79fb4d770f743ed5911c3282e..08cd86f49bf96383e3c37dbe1e662b42859afe90 100644 (file)
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3583,7 +3583,6 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
        netdevice_tracker *dev_tracker = &fib6_nh->fib_nh_dev_tracker;
        struct net_device *dev = NULL;
        struct inet6_dev *idev = NULL;
-       int addr_type;
        int err;
 
        fib6_nh->fib_nh_family = AF_INET6;
@@ -3625,11 +3624,10 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
 
        fib6_nh->fib_nh_weight = 1;
 
-       /* We cannot add true routes via loopback here,
-        * they would result in kernel looping; promote them to reject routes
+       /* Reset the nexthop device to the loopback device in case of reject
+        * routes.
         */
-       addr_type = ipv6_addr_type(&cfg->fc_dst);
-       if (fib6_is_reject(cfg->fc_flags, dev, addr_type)) {
+       if (cfg->fc_flags & RTF_REJECT) {
                /* hold loopback dev/idev if we haven't done so. */
                if (dev != net->loopback_dev) {
                        if (dev) {