On 3/4/26 4:38 AM, 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_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 <[email protected]> > Fixes: 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..08b4a324987e 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 > + /* 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) {
Reviewed-by: David Ahern <[email protected]>
