Subject: Re: [PATCH net v3] ipv6: flowlabel: enforce per-netns limit for unprivileged callers

From: Willem de Bruijn <[email-masked]>
Date: Thu, 30 Apr 2026 09:42:32 -0400

Maoyi Xie wrote:
> From: Maoyi Xie <[email-masked]>
> 
> fl_size, fl_ht and ip6_fl_lock in net/ipv6/ip6_flowlabel.c are file
> scope and shared across netns. mem_check() reads fl_size to decide
> whether to deny non-CAP_NET_ADMIN callers; capable() runs against
> init_user_ns, so an unprivileged user in any non-init userns can
> push fl_size past FL_MAX_SIZE - FL_MAX_SIZE/4 and starve every
> other unprivileged userns on the host.
> 
> Add struct netns_ipv6::flowlabel_count, bumped and decremented next
> to fl_size in fl_intern, ip6_fl_gc and ip6_fl_purge. Place it near
> ipmr_seq rather than next to flowlabel_has_excl: flowlabel_has_excl
> is read on every flowlabel lookup, and a counter written on every
> alloc would dirty its cacheline.

The cacheline point is more about truly ipv6 hot path fields. This
entire explicit flowlabel mgmt is not that.

Did this new location fill a 4B hole? (on 64b builds)

> 
> mem_check() folds an extra FL_MAX_SIZE/8 ceiling into the existing
> non-CAP_NET_ADMIN conditional.
> 
> Bump FL_MAX_SIZE from 4096 to 8192. It has been 4096 since the file
> was added; machines and connection counts have grown. The new
> per-netns ceiling is then 1024 flowlabels, half of FL_MAX_SIZE/4.
> 
> CAP_NET_ADMIN against init_user_ns still bypasses both caps.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Suggested-by: Willem de Bruijn <[email-masked]>
> Cc: [email-masked] # v5.15+
> Signed-off-by: Maoyi Xie <[email-masked]>
> ---
> v3 (this submission, netdev): addressed Willem's review on the
>     private security@ thread:
>     - merged the FL_MAX_SIZE doubling into this patch
>     - dropped the test data block from the commit body
>     - moved flowlabel_count to a 4-byte hole next to ipmr_seq, off
>       the flowlabel_has_excl cacheline
>     - inlined fl->fl_net in ip6_fl_gc (no local var)
> v2: per-netns counter + cap, sent to security@ as a 2-patch series
> v1: fix-shape sketch in original disclosure
> 
>  include/net/netns/ipv6.h |  1 +
>  net/ipv6/ip6_flowlabel.c | 10 ++++++++--
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
> index 34bdb1308..329482373 100644
> --- a/include/net/netns/ipv6.h
> +++ b/include/net/netns/ipv6.h
> @@ -119,6 +119,7 @@ struct netns_ipv6 {
>  	struct fib_notifier_ops	*notifier_ops;
>  	struct fib_notifier_ops	*ip6mr_notifier_ops;
>  	unsigned int ipmr_seq; /* protected by rtnl_mutex */
> +	atomic_t		flowlabel_count;
>  	struct {
>  		struct hlist_head head;
>  		spinlock_t	lock;
> diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
> index c92f98c6f..4a5219356 100644
> --- a/net/ipv6/ip6_flowlabel.c
> +++ b/net/ipv6/ip6_flowlabel.c
> @@ -36,7 +36,7 @@
>  /* FL hash table */
>  
>  #define FL_MAX_PER_SOCK	32
> -#define FL_MAX_SIZE	4096
> +#define FL_MAX_SIZE	8192
>  #define FL_HASH_MASK	255
>  #define FL_HASH(l)	(ntohl(l)&FL_HASH_MASK)
>  
> @@ -162,6 +162,7 @@ static void ip6_fl_gc(struct timer_list *unused)
>  				ttd = fl->expires;
>  				if (time_after_eq(now, ttd)) {
>  					*flp = fl->next;
> +					atomic_dec(&fl->fl_net->ipv6.flowlabel_count);
>  					fl_free(fl);
>  					atomic_dec(&fl_size);

nit: can you place these consistently immediately after the fl_size
operations, to make clear that they are paired.

>  					continue;
> @@ -195,6 +196,7 @@ static void __net_exit ip6_fl_purge(struct net *net)
>  			if (net_eq(fl->fl_net, net) &&
>  			    atomic_read(&fl->users) == 0) {
>  				*flp = fl->next;
> +				atomic_dec(&net->ipv6.flowlabel_count);
>  				fl_free(fl);
>  				atomic_dec(&fl_size);
>  				continue;
> @@ -245,6 +247,7 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
>  	fl->next = fl_ht[FL_HASH(fl->label)];
>  	rcu_assign_pointer(fl_ht[FL_HASH(fl->label)], fl);
>  	atomic_inc(&fl_size);
> +	atomic_inc(&net->ipv6.flowlabel_count);
>  	spin_unlock_bh(&ip6_fl_lock);
>  	rcu_read_unlock();
>  	return NULL;
> @@ -464,6 +467,7 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
>  
>  static int mem_check(struct sock *sk)
>  {
> +	struct net *net = sock_net(sk);
>  	int room = FL_MAX_SIZE - atomic_read(&fl_size);
>  	struct ipv6_fl_socklist *sfl;
>  	int count = 0;
> @@ -478,7 +482,9 @@ static int mem_check(struct sock *sk)
>  
>  	if (room <= 0 ||
>  	    ((count >= FL_MAX_PER_SOCK ||
> -	      (count > 0 && room < FL_MAX_SIZE/2) || room < FL_MAX_SIZE/4) &&
> +	      (count > 0 && room < FL_MAX_SIZE/2) ||
> +	      room < FL_MAX_SIZE/4 ||
> +	      atomic_read(&net->ipv6.flowlabel_count) >= FL_MAX_SIZE/8) &&
>  	     !capable(CAP_NET_ADMIN)))
>  		return -ENOBUFS;
>  
> -- 
> 2.34.1
>