Re: [PATCH net 1/2] nexthop: take nh->lock for f6i_list walks in replace check and notify

[Ido Schimmel]

On Wed, Jul 22, 2026 at 12:29:50AM +0000, Xiang Mei (Microsoft) wrote:
> fib6_check_nh_list() and __nexthop_replace_notify() walk nh->f6i_list
> during an RTNL-serialized nexthop replace without holding nh->lock. IPv6
> RTM_NEWROUTE/RTM_DELROUTE run without RTNL and mutate that list under
> nh->lock (fib6_add_rt2node_nh(), fib6_purge_rt()), so both walks race a
> concurrent route delete that unlinks and frees a fib6_info:
> 
>   BUG: KASAN: slab-use-after-free in rt6_fill_node.isra.0 (net/ipv6/route.c:5799)
>   Read of size 4 at addr ffff888014607e64 by task exploit/143
>    rt6_fill_node.isra.0 (net/ipv6/route.c:5799)
>    fib6_rt_update (net/ipv6/route.c:6412)
>    __nexthop_replace_notify (net/ipv4/nexthop.c:2542)
>    rtm_new_nexthop (net/ipv4/nexthop.c:2554)
>    rtnetlink_rcv_msg (net/core/rtnetlink.c:7076)
> 
>   BUG: KASAN: slab-use-after-free in fib6_check_nh_list (net/ipv4/nexthop.c:1605)
>   Read of size 8 at addr ffff888014a7d068 by task exploit/142
>    fib6_check_nh_list (net/ipv4/nexthop.c:1605)
>    rtm_new_nexthop (net/ipv4/nexthop.c:2575)
>    rtnetlink_rcv_msg (net/core/rtnetlink.c:7076)
> 
> Both walks only read the entries and take no tb6_lock, so protect them
> with nh->lock; fib6_rt_update() uses gfp_any(), which returns GFP_ATOMIC
> under the lock.
> 
> Fixes: 081efd18326e ("ipv6: Protect nh->f6i_list with spinlock and flag.")
> Reported-by: AutonomousCodeSecurity@xxxxxxxxxxxxx
> Signed-off-by: Xiang Mei (Microsoft) <xmei5@xxxxxxx>
> ---
>  net/ipv4/nexthop.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
> index 6205bd57aa85..c1d3b7d7100d 100644
> --- a/net/ipv4/nexthop.c
> +++ b/net/ipv4/nexthop.c
> @@ -1597,14 +1597,21 @@ static int fib6_check_nh_list(struct nexthop *old, struct nexthop *new,
>  			      struct netlink_ext_ack *extack)
>  {
>  	struct fib6_info *f6i;
> +	int err = 0;
>  
>  	if (list_empty(&old->f6i_list))
>  		return 0;
>  
> +	spin_lock_bh(&old->lock);
>  	list_for_each_entry(f6i, &old->f6i_list, nh_list) {
> -		if (check_src_addr(&f6i->fib6_src.addr, extack) < 0)
> -			return -EINVAL;
> +		err = check_src_addr(&f6i->fib6_src.addr, extack);
> +		if (err)
> +			break;
>  	}
> +	spin_unlock_bh(&old->lock);
> +
> +	if (err)
> +		return err;
>  
>  	return fib6_check_nexthop(new, NULL, extack);
>  }
> @@ -2538,8 +2545,10 @@ static void __nexthop_replace_notify(struct net *net, struct nexthop *nh,
>  			fi->nh_updated = false;
>  	}
>  
> +	spin_lock_bh(&nh->lock);
>  	list_for_each_entry(f6i, &nh->f6i_list, nh_list)
>  		fib6_rt_update(net, f6i, info);
> +	spin_unlock_bh(&nh->lock);

I read the feedback from Sashiko, but even if we convert this list to
RCU we would need GFP_ATOMIC. I don't find __nexthop_replace_notify()
too interesting given that modern systems unset
net.ipv4.nexthop_compat_mode, which means that this function is never
called.

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>

>  }
>  
>  /* send RTM_NEWROUTE with REPLACE flag set for all FIB entries
> -- 
> 2.43.0
>