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

[David Ahern]

On 3/4/26 4:38 AM, 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>
> ---
>  net/ipv6/route.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index c0350d97307e..08b4a324987e 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -3582,7 +3582,6 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
>  	netdevice_tracker *dev_tracker = &fib6_nh->fib_nh_dev_tracker;
>  	struct net_device *dev = NULL;
>  	struct inet6_dev *idev = NULL;
> -	int addr_type;
>  	int err;
>  
>  	fib6_nh->fib_nh_family = AF_INET6;
> @@ -3624,11 +3623,10 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
>  
>  	fib6_nh->fib_nh_weight = 1;
>  
> -	/* We cannot add true routes via loopback here,
> -	 * they would result in kernel looping; promote them to reject routes
> +	/* Reset the nexthop device to the loopback device in case of reject
> +	 * routes.
>  	 */
> -	addr_type = ipv6_addr_type(&cfg->fc_dst);
> -	if (fib6_is_reject(cfg->fc_flags, dev, addr_type)) {
> +	if (cfg->fc_flags & RTF_REJECT) {
>  		/* hold loopback dev/idev if we haven't done so. */
>  		if (dev != net->loopback_dev) {
>  			if (dev) {


Reviewed-by: David Ahern <dsahern@xxxxxxxxxx>