[PATCH net v8 0/2] ipv6: flowlabel: per-netns budget for unprivileged callers

[PATCH net v8 0/2] ipv6: flowlabel: per-netns budget for unprivileged callers

[Maoyi Xie]

From: Maoyi Xie <maoyi.xie@ntu.edu.sg>

This series fixes the cross-tenant DoS in net/ipv6/ip6_flowlabel.c.
v1 through v6 were single-patch postings, each in its own thread.
v6 review pointed out that the existing fl_size read in
mem_check() and the corresponding write in fl_intern() are not in
the same critical section. v7 split the work into 2 patches.

Patch 1/2 is a prerequisite. It moves spin_lock_bh(&ip6_fl_lock)
and the matching unlock from fl_intern() into its only caller
ipv6_flowlabel_get(), so the mem_check() call runs under the same
critical section as the fl_intern() insert. With all writers and
the read of fl_size under the lock, fl_size is converted from
atomic_t to plain int. This is independent of the per-netns
budget. It also makes 2/2 backportable without conflicts.

Patch 2/2 is the v6 patch, rebased on 1/2.

  - flowlabel_count is plain int rather than atomic_t, since the
    previous patch put all writers and readers under ip6_fl_lock.
  - In ip6_fl_gc(), fl_free() is now placed below the fl_size
    and flowlabel_count decrements, removing the v6 cache of
    fl->fl_net.
  - In ip6_fl_purge(), fl_free() stays in its original position.
    The function argument net is used for flowlabel_count.
  - mem_check() uses spaces around the / operator on all four
    expressions, addressing the checkpatch note in v6 review.

Numeric budget (preserved from v6):

  pre-patch:
    global non-CAP_NET_ADMIN budget = FL_MAX_SIZE - FL_MAX_SIZE/4
                                    = 4096 - 1024 = 3072
    per-actor reach                 = 3072

  post-patch:
    FL_MAX_SIZE doubled to 8192
    global non-CAP_NET_ADMIN budget = 8192 - 2048 = 6144
    per-netns ceiling               = 6144 / 2 = 3072
    per-actor reach                 = 3072 (preserved)

CAP_NET_ADMIN against init_user_ns still bypasses both caps.

Reproducer (KASAN VM, 4 cores, qemu): unprivileged netns A holds
3072 flowlabels via 100 procs. Fresh unprivileged netns B then
allocates 32 flowlabels (the FL_MAX_PER_SOCK ceiling for one
socket), the same as a clean baseline. Without the per-netns
ceiling, netns A could push fl_size past FL_MAX_SIZE - FL_MAX_SIZE
/ 4 and netns B would see allocations denied.

v8:
  - 1/2: replaced the "Caller must hold ip6_fl_lock" comment in
    fl_intern() with lockdep_assert_held(&ip6_fl_lock), matching
    the runtime check already used in mem_check(), per Willem's
    review.
  - 1/2: added Fixes: 1da177e4c3f4 trailer to match 2/2, per
    Willem's review.
  - Carried forward Reviewed-by: Willem de Bruijn on both
    patches.
  - No code change beyond the lockdep_assert_held swap.
v7:
  - 2-patch series: 1/2 (lock prep) and 2/2 (v6 rebased on 1/2).
  - 2/2: flowlabel_count int, fl_free() reorder removed in
    ip6_fl_purge(), checkpatch / spacing in mem_check() fixed.
v6: rebased onto current net (resolves the conflict on
    include/net/netns/ipv6.h that v5 hit). fl_free() restored
    to its pre-series position, with fl->fl_net cached locally
    in ip6_fl_gc().
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: 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".
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.

Maoyi Xie (2):
  ipv6: flowlabel: take ip6_fl_lock across mem_check and fl_intern
  ipv6: flowlabel: enforce per-netns limit for unprivileged callers

 include/net/netns/ipv6.h |  1 +
 net/ipv6/ip6_flowlabel.c | 46 +++++++++++++++++++++++++++-------------
 2 files changed, 32 insertions(+), 15 deletions(-)


base-commit: ebb639024ebd47a13a511cce6ae630c15e4b3126
-- 
2.34.1

[PATCH net v8 1/2] ipv6: flowlabel: take ip6_fl_lock across mem_check and fl_intern

[Maoyi Xie]

From: Maoyi Xie <maoyi.xie@ntu.edu.sg>

mem_check() in net/ipv6/ip6_flowlabel.c reads fl_size without
holding ip6_fl_lock. fl_intern() takes the lock immediately
afterwards. The two checks therefore race against concurrent
fl_intern, ip6_fl_gc and ip6_fl_purge writers, which makes the
mem_check budget check approximate.

Move spin_lock_bh(&ip6_fl_lock) and the matching unlock from
fl_intern() into its only caller ipv6_flowlabel_get(). The
mem_check() call now runs under the same critical section as the
fl_intern() insert, so the budget check is exact.

With all writers and the read of fl_size under ip6_fl_lock,
convert fl_size from atomic_t to plain int. The four sites that
update or read fl_size are fl_intern (insert path), ip6_fl_gc
(garbage collector, the !sched check and the per-entry decrement),
ip6_fl_purge (per-netns purge), and mem_check (budget check), and
all four now run under ip6_fl_lock.

This is a prerequisite for adding a per-netns budget alongside
fl_size. The follow-up patch adds netns_ipv6::flowlabel_count and
folds it into mem_check().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Suggested-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
---
 net/ipv6/ip6_flowlabel.c | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
index c92f98c6f..a89746431 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c
@@ -40,7 +40,7 @@
 #define FL_HASH_MASK	255
 #define FL_HASH(l)	(ntohl(l)&FL_HASH_MASK)
 
-static atomic_t fl_size = ATOMIC_INIT(0);
+static int fl_size;
 static struct ip6_flowlabel __rcu *fl_ht[FL_HASH_MASK+1];
 
 static void ip6_fl_gc(struct timer_list *unused);
@@ -163,7 +163,7 @@ static void ip6_fl_gc(struct timer_list *unused)
 				if (time_after_eq(now, ttd)) {
 					*flp = fl->next;
 					fl_free(fl);
-					atomic_dec(&fl_size);
+					fl_size--;
 					continue;
 				}
 				if (!sched || time_before(ttd, sched))
@@ -172,7 +172,7 @@ static void ip6_fl_gc(struct timer_list *unused)
 			flp = &fl->next;
 		}
 	}
-	if (!sched && atomic_read(&fl_size))
+	if (!sched && fl_size)
 		sched = now + FL_MAX_LINGER;
 	if (sched) {
 		mod_timer(&ip6_fl_gc_timer, sched);
@@ -196,7 +196,7 @@ static void __net_exit ip6_fl_purge(struct net *net)
 			    atomic_read(&fl->users) == 0) {
 				*flp = fl->next;
 				fl_free(fl);
-				atomic_dec(&fl_size);
+				fl_size--;
 				continue;
 			}
 			flp = &fl->next;
@@ -210,10 +210,10 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 {
 	struct ip6_flowlabel *lfl;
 
+	lockdep_assert_held(&ip6_fl_lock);
+
 	fl->label = label & IPV6_FLOWLABEL_MASK;
 
-	rcu_read_lock();
-	spin_lock_bh(&ip6_fl_lock);
 	if (label == 0) {
 		for (;;) {
 			fl->label = htonl(get_random_u32())&IPV6_FLOWLABEL_MASK;
@@ -235,8 +235,6 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 		lfl = __fl_lookup(net, fl->label);
 		if (lfl) {
 			atomic_inc(&lfl->users);
-			spin_unlock_bh(&ip6_fl_lock);
-			rcu_read_unlock();
 			return lfl;
 		}
 	}
@@ -244,9 +242,7 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 	fl->lastuse = jiffies;
 	fl->next = fl_ht[FL_HASH(fl->label)];
 	rcu_assign_pointer(fl_ht[FL_HASH(fl->label)], fl);
-	atomic_inc(&fl_size);
-	spin_unlock_bh(&ip6_fl_lock);
-	rcu_read_unlock();
+	fl_size++;
 	return NULL;
 }
 
@@ -464,10 +460,14 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
 
 static int mem_check(struct sock *sk)
 {
-	int room = FL_MAX_SIZE - atomic_read(&fl_size);
+	int room;
 	struct ipv6_fl_socklist *sfl;
 	int count = 0;
 
+	lockdep_assert_held(&ip6_fl_lock);
+
+	room = FL_MAX_SIZE - fl_size;
+
 	if (room > FL_MAX_SIZE - FL_MAX_PER_SOCK)
 		return 0;
 
@@ -692,11 +692,19 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq,
 	if (!sfl1)
 		goto done;
 
+	rcu_read_lock();
+	spin_lock_bh(&ip6_fl_lock);
 	err = mem_check(sk);
+	if (err == 0)
+		fl1 = fl_intern(net, fl, freq->flr_label);
+	else
+		fl1 = NULL;
+	spin_unlock_bh(&ip6_fl_lock);
+	rcu_read_unlock();
+
 	if (err != 0)
 		goto done;
 
-	fl1 = fl_intern(net, fl, freq->flr_label);
 	if (fl1)
 		goto recheck;
 
-- 
2.34.1

[PATCH net v8 2/2] ipv6: flowlabel: enforce per-netns limit for unprivileged callers

[Maoyi Xie]

From: Maoyi Xie <maoyi.xie@ntu.edu.sg>

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 fills 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
of 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.

The previous patch took ip6_fl_lock across mem_check and
fl_intern, so the new flowlabel_count read in mem_check and the
new flowlabel_count++ in fl_intern run under the same critical
section. flowlabel_count is therefore plain int, like fl_size.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Suggested-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
---
 include/net/netns/ipv6.h |  1 +
 net/ipv6/ip6_flowlabel.c | 14 +++++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
index 499e42881..875916d60 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;
+	int			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 a89746431..b1ccdf0dc 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)
 
@@ -162,8 +162,9 @@ static void ip6_fl_gc(struct timer_list *unused)
 				ttd = fl->expires;
 				if (time_after_eq(now, ttd)) {
 					*flp = fl->next;
-					fl_free(fl);
 					fl_size--;
+					fl->fl_net->ipv6.flowlabel_count--;
+					fl_free(fl);
 					continue;
 				}
 				if (!sched || time_before(ttd, sched))
@@ -197,6 +198,7 @@ static void __net_exit ip6_fl_purge(struct net *net)
 				*flp = fl->next;
 				fl_free(fl);
 				fl_size--;
+				net->ipv6.flowlabel_count--;
 				continue;
 			}
 			flp = &fl->next;
@@ -243,6 +245,7 @@ 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);
 	fl_size++;
+	net->ipv6.flowlabel_count++;
 	return NULL;
 }
 
@@ -460,6 +463,9 @@ 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;
 	struct ipv6_fl_socklist *sfl;
 	int count = 0;
@@ -478,7 +484,9 @@ 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 ||
+	      net->ipv6.flowlabel_count >= unpriv_user_limit) &&
 	     !capable(CAP_NET_ADMIN)))
 		return -ENOBUFS;
 
-- 
2.34.1

Re: [PATCH net v8 0/2] ipv6: flowlabel: per-netns budget for unprivileged callers

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed,  6 May 2026 16:24:14 +0800 you wrote:
> From: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> 
> This series fixes the cross-tenant DoS in net/ipv6/ip6_flowlabel.c.
> v1 through v6 were single-patch postings, each in its own thread.
> v6 review pointed out that the existing fl_size read in
> mem_check() and the corresponding write in fl_intern() are not in
> the same critical section. v7 split the work into 2 patches.
> 
> [...]

Here is the summary with links:
  - [net,v8,1/2] ipv6: flowlabel: take ip6_fl_lock across mem_check and fl_intern
    https://git.kernel.org/netdev/net/c/7ce5556f255a
  - [net,v8,2/2] ipv6: flowlabel: enforce per-netns limit for unprivileged callers
    https://git.kernel.org/netdev/net/c/e68eadffb724

You are awesome, thank you!
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