Re: [PATCH net v4 1/2] net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop

[Ido Schimmel]

On Wed, Mar 04, 2026 at 07:38:13PM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> 
> 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 <idosch@xxxxxxxxxx>
> Fixes: 493ced1ac47c ("ipv4: Allow routes to use nexthop objects")
> Reported-by: syzbot+334190e097a98a1b81bb@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@xxxxxxxxxx/T/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>