Re: [PATCH net v3 1/2] net: ipv6: fix panic when IPv4 route references loopback IPv6 nexthop

[Ido Schimmel <idosch@nvidia.com>]

Code looks fine, but see a few comments below

On Tue, Mar 03, 2026 at 11:13:14AM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> 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(). After this change, the three cases behave as
> follows:

s/ip6_route_info_create/ip6_route_info_create_nh/

> 
> 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() still promotes it to reject afterward.

Same here

>    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@nvidia.com>
> Fixes: 7dd73168e273 ("ipv6: Always allocate pcpu memory in a fib6_nh")

AFAICT, even before this commit fib_nh_common_init() would be skipped for
nexthop objects that use the loopback device as their nexthop device. I
suggest blaming the commit that allowed user space to configure IPv4
routes with nexthop objects:

493ced1ac47c ("ipv4: Allow routes to use nexthop objects")

> Reported-by: syzbot+334190e097a98a1b81bb@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@google.com/T/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
>  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..fb588a351609 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
> +	/* Only check RTF_REJECT, not fib6_is_reject(): the loopback
> +	 * promotion heuristic is handled by ip6_route_info_create().

Same here (FTR, I suggested a different comment in [1])

>  	 */
> -	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) {
> -- 
> 2.43.0
> 

[1] https://lore.kernel.org/netdev/20260302082551.GA814377@shredder/