Subject: Re: [PATCH net v6] ipv6: flowlabel: enforce per-netns limit for unprivileged callers

From: Willem de Bruijn <[email-masked]>
Date: Sun, 03 May 2026 16:43:37 -0400

Maoyi Xie wrote:
> 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. The new field
> is placed in the existing 4-byte hole after ipmr_seq, so struct
> netns_ipv6 stays the same size on 64-bit builds.
> 
> Bump FL_MAX_SIZE from 4096 to 8192. It has been 4096 since the file
> was added; machines and connection counts have grown.
> 
> mem_check() folds an extra per-netns ceiling into the existing
> non-CAP_NET_ADMIN conditional. The ceiling is half of the total
> budget that unprivileged callers have ever been able to use, i.e.
> (FL_MAX_SIZE - FL_MAX_SIZE/4) / 2 = 3072 entries. With FL_MAX_SIZE
> doubled, this preserves the original per-user reach (~3K, what an
> unprivileged caller could already obtain before this change) while
> forcing an attacker to spread allocations across at least two
> netns to exhaust the global non-CAP_NET_ADMIN budget.
> 
> 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]>
> ---
> v6 (this submission, addressing v5 review by Willem):
>     - Rebased onto current net (resolves the conflict on
>       include/net/netns/ipv6.h that v5 hit. ipmr_seq is now
>       atomic_t but remains 4 bytes, so flowlabel_count still
>       fills the 4-byte hole after it).
>     - Restored fl_free() to its original position in both
>       ip6_fl_gc() and ip6_fl_purge(). v5 had moved fl_free()
>       after the new atomic_dec() to avoid the use-after-free
>       on fl->fl_net. v6 instead caches fl->fl_net into a
>       local before fl_free() in ip6_fl_gc(), and uses the
>       net argument already in scope in ip6_fl_purge().
> v5: replaced the per-netns ceiling FL_MAX_SIZE/8 with the
>     computed unpriv_user_limit = (FL_MAX_SIZE - FL_MAX_SIZE/4)/2,
>     which evaluates to 3072. v4's FL_MAX_SIZE/8 = 1024 would
>     have reduced the per-user budget below the ~3K an
>     unprivileged caller could already obtain before any of
>     this work, defeating the reason FL_MAX_SIZE was doubled
>     in the first place.
> v4: addressed Willem's v3 review on netdev. Dropped the
>     flowlabel_has_excl cacheline argument in favour of "fills
>     the existing 4-byte hole after ipmr_seq", and reordered
>     atomic_dec(&...flowlabel_count) to sit immediately after
>     atomic_dec(&fl_size) in ip6_fl_gc and ip6_fl_purge.
> v3: addressed Willem's review on the private security@ thread.
>     Merged FL_MAX_SIZE doubling, dropped test data, moved
>     flowlabel_count near ipmr_seq, inlined fl->fl_net in
>     ip6_fl_gc.
> 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 | 14 ++++++++++++--
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
> index 499e42881..ef698f5fa 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;
>  	atomic_t		ipmr_seq;
> +	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..28e43718d 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)
>  
> @@ -161,9 +161,12 @@ static void ip6_fl_gc(struct timer_list *unused)
>  					fl->expires = ttd;
>  				ttd = fl->expires;
>  				if (time_after_eq(now, ttd)) {
> +					struct net *net = fl->fl_net;
> +
>  					*flp = fl->next;
>  					fl_free(fl);
>  					atomic_dec(&fl_size);
> +					atomic_dec(&net->ipv6.flowlabel_count);

If resubmitting, moving fl_free here makes sense (only the second case
was entirely unnecessary).

>  					continue;
>  				}
>  				if (!sched || time_before(ttd, sched))
> @@ -197,6 +200,7 @@ static void __net_exit ip6_fl_purge(struct net *net)
>  				*flp = fl->next;
>  				fl_free(fl);
>  				atomic_dec(&fl_size);
> +				atomic_dec(&net->ipv6.flowlabel_count);
>  				continue;
>  			}
>  			flp = &fl->next;
> @@ -245,6 +249,7 @@ 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 +469,9 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
>  
>  static int mem_check(struct sock *sk)
>  {
> +	const int unpriv_total_limit = FL_MAX_SIZE - (FL_MAX_SIZE / 4);
> +	const int unpriv_user_limit = unpriv_total_limit / 2;
> +	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 +486,9 @@ 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 ||

And here make checkpatch happy and add spaces around the division
operator.

> +	      atomic_read(&net->ipv6.flowlabel_count) >= unpriv_user_limit) &&
>  	     !capable(CAP_NET_ADMIN)))
>  		return -ENOBUFS;
>  
> -- 
> 2.34.1
>