Linux vulnerability

What happened

In the Linux kernel, the following vulnerability has been resolved: nexthop: fix IPv6 route referencing IPv4 nexthop syzbot reported a panic [1] [2]. When an IPv6 nexthop is replaced with an IPv4 nexthop, the has_v4 flag of all groups containing this nexthop is not updated. This is because nh_group_v4_update is only called when replacing AF_INET to AF_INET6, but the reverse direction (AF_INET6 to AF_INET) is missed. This allows a stale has_v4=false to bypass fib6_check_nexthop, causing IPv6 routes to be attached to groups that effectively contain only AF_INET members. Subsequent route lookups then call nexthop_fib6_nh() which returns NULL for the AF_INET member, leading to a NULL pointer dereference. Fix by calling nh_group_v4_update whenever the family changes, not just AF_INET to AF_INET6. Reproducer: # AF_INET6 blackhole ip -6 nexthop add id 1 blackhole # group with has_v4=false ip nexthop add id 100 group 1 # replace with AF_INET (no -6), has_v4 stays false ip nexthop replace id 1 blackhole # pass stale has_v4 check ip -6 route add 2001:db8::/64 nhid 100 # panic ping -6 2001:db8::1 [1] https://syzkaller.appspot.com/bug?id=e17283eb2f8dcf3dd9b47fe6f67a95f71faadad0 [2] https://syzkaller.appspot.com/bug?id=8699b6ae54c9f35837d925686208402949e12ef3

Affected versions

Linux: 7bf4796dd09984ad1612877a82d0d139c70ae27f through before ceffe81a0be92afc0cd1340bc8ca46559cce9bb4 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before 9c2d6770a5f4545a307eb66979bef7656a34d621 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before 6275796f22bb382f3e9aa58ed0b4ef7bdad78cb8 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before aaac3bed034239e1d75732211d9b05f30b0b4f35 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before ad85961004fd4bd2f31209ac4b07612c6cefb9e7 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before 613c8f4a501421dd258b07ea614205d4e16ec845 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before b3b7e850e1541f0520c4a12ec884255c30427ff6 (git); 7bf4796dd09984ad1612877a82d0d139c70ae27f through before 29c95185ba32b621fbc3800fb86e7dc3edf5c2be (git); 5.3; V3.1.6 through before * (custom) Fixed: See vendor advisory.

Why it matters

Review the vendor advisory and exposure of the affected product to determine operational impact.

Detection & mitigation

• Apply vendor-provided updates or mitigations.
• Review affected product exposure and access logs.