CVE-2025-39980

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> nexthop: Forbid FDB status change while nexthop is in a group<br /> <br /> The kernel forbids the creation of non-FDB nexthop groups with FDB<br /> nexthops:<br /> <br /> # ip nexthop add id 1 via 192.0.2.1 fdb<br /> # ip nexthop add id 2 group 1<br /> Error: Non FDB nexthop group cannot have fdb nexthops.<br /> <br /> And vice versa:<br /> <br /> # ip nexthop add id 3 via 192.0.2.2 dev dummy1<br /> # ip nexthop add id 4 group 3 fdb<br /> Error: FDB nexthop group can only have fdb nexthops.<br /> <br /> However, as long as no routes are pointing to a non-FDB nexthop group,<br /> the kernel allows changing the type of a nexthop from FDB to non-FDB and<br /> vice versa:<br /> <br /> # ip nexthop add id 5 via 192.0.2.2 dev dummy1<br /> # ip nexthop add id 6 group 5<br /> # ip nexthop replace id 5 via 192.0.2.2 fdb<br /> # echo $?<br /> 0<br /> <br /> This configuration is invalid and can result in a NPD [1] since FDB<br /> nexthops are not associated with a nexthop device:<br /> <br /> # ip route add 198.51.100.1/32 nhid 6<br /> # ping 198.51.100.1<br /> <br /> Fix by preventing nexthop FDB status change while the nexthop is in a<br /> group:<br /> <br /> # ip nexthop add id 7 via 192.0.2.2 dev dummy1<br /> # ip nexthop add id 8 group 7<br /> # ip nexthop replace id 7 via 192.0.2.2 fdb<br /> Error: Cannot change nexthop FDB status while in a group.<br /> <br /> [1]<br /> BUG: kernel NULL pointer dereference, address: 00000000000003c0<br /> [...]<br /> Oops: Oops: 0000 [#1] SMP<br /> CPU: 6 UID: 0 PID: 367 Comm: ping Not tainted 6.17.0-rc6-virtme-gb65678cacc03 #1 PREEMPT(voluntary)<br /> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014<br /> RIP: 0010:fib_lookup_good_nhc+0x1e/0x80<br /> [...]<br /> Call Trace:<br /> <br /> fib_table_lookup+0x541/0x650<br /> ip_route_output_key_hash_rcu+0x2ea/0x970<br /> ip_route_output_key_hash+0x55/0x80<br /> __ip4_datagram_connect+0x250/0x330<br /> udp_connect+0x2b/0x60<br /> __sys_connect+0x9c/0xd0<br /> __x64_sys_connect+0x18/0x20<br /> do_syscall_64+0xa4/0x2a0<br /> entry_SYSCALL_64_after_hwframe+0x4b/0x53