net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop

Overview

In the Linux kernel, the following vulnerability has been resolved:

net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop

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.

Affected

• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < 607e68c1b7c5a30c795571be1906d716e989a644
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < c11d7c56c2076ee9cd72004f1976fe0734df2ae9
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < b5062fc2150614c9ea8a611c2e0cb6e047ebfa3a
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < b299121e7453d23faddf464087dff513a495b4fc
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < f7c9f8e3607440fe39300efbaf46cf7b5eecb23f
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < b3b5a037d520afe3d5276e653bc0ff516bbda34c
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < 8650db85b4259d2885d2a80fbc2317ce24194133
• Linux >= 493ced1ac47c48bb86d9d4e8e87df8592be85a0e < 21ec92774d1536f71bdc90b0e3d052eff99cf093
• Linux 5.3

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.