Code looks fine, but see a few comments below
On Tue, Mar 03, 2026 at 11:13:14AM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <[email protected]>
>
> 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(). After this change, the three cases behave as
> follows:
s/ip6_route_info_create/ip6_route_info_create_nh/
>
> 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() still promotes it to reject afterward.
Same here
> 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 <[email protected]>
> Fixes: 7dd73168e273 ("ipv6: Always allocate pcpu memory in a fib6_nh")
AFAICT, even before this commit fib_nh_common_init() would be skipped for
nexthop objects that use the loopback device as their nexthop device. I
suggest blaming the commit that allowed user space to configure IPv4
routes with nexthop objects:
493ced1ac47c ("ipv4: Allow routes to use nexthop objects")
> Reported-by: [email protected]
> Closes:
> https://lore.kernel.org/all/[email protected]/T/
> Signed-off-by: Jiayuan Chen <[email protected]>
> ---
> net/ipv6/route.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index c0350d97307e..fb588a351609 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -3582,7 +3582,6 @@ 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;
> @@ -3624,11 +3623,10 @@ 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
> + /* Only check RTF_REJECT, not fib6_is_reject(): the loopback
> + * promotion heuristic is handled by ip6_route_info_create().
Same here (FTR, I suggested a different comment in [1])
> */
> - 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) {
> --
> 2.43.0
>
[1] https://lore.kernel.org/netdev/20260302082551.GA814377@shredder/
