On Wed, Mar 04, 2026 at 07:38:13PM +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_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]>
Reviewed-by: Ido Schimmel <[email protected]>
