[PATCH net 0/2] ipv6: tunnel changelink: use cached netns pointer

[PATCH net 0/2] ipv6: tunnel changelink: use cached netns pointer

[Maoyi Xie]

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

This series addresses two slab-use-after-free reports against the IPv6
tunnel changelink callbacks vti6_changelink() and ip6erspan_changelink(),
both reachable from an unprivileged user namespace and verified on
Linux v7.0 with KASAN.

Both bugs are sibling misses of commit 5e72ce3e3980 ("net: ipv6: Use
link netns in newlink() of rtnl_link_ops"), which migrated the
*_newlink callbacks for vti6, ip6_gre, ip6_tunnel, sit and ip_tunnel
from dev_net() to link_net but did not convert the corresponding
*_changelink callbacks. As a result, after a device is migrated via
IFLA_NET_NS_FD, the changelink path looks up the per-netns hash in the
wrong namespace, leaving a stale hash entry in the original creation
netns. The next cleanup_net() of that netns walks freed memory.

Patch 1/2 was authored by Kuniyuki Iwashima during the security
disclosure thread; it converts vti6_changelink() and vti6_update() to
use the cached t->net.

Patch 2/2 applies the equivalent conversion to ip6erspan_changelink().
The non-erspan sibling ip6gre_changelink() in the same file already
uses the cached t->net correctly.

Both bugs were originally reported on security@kernel.org on
2026-04-26 and triaged with Kuniyuki Iwashima and Xiao Liang. Posting
publicly per standard practice once the technical fix shape is
settled.

The bugs are present on all maintained LTS branches (v5.15, v6.1, v6.6,
v6.12, v6.18) with byte-identical source, hence Cc: stable@.

Tested with KASAN reproducers (unshare --user --map-root-user --net,
RTM_NEWLINK + IFLA_NET_NS_FD migration, RTM_NEWLINK changelink in
the migrated netns, then teardown of the original netns); without the
patches both reports trip within ~2 seconds, with the patches the
reproducers complete cleanly.

Kuniyuki Iwashima (1):
  ip6: vti: Use ip6_tnl.net in vti6_changelink().

Maoyi Xie (1):
  ip6_gre: Use cached t->net in ip6erspan_changelink().

 net/ipv6/ip6_gre.c |  3 ++-
 net/ipv6/ip6_vti.c | 12 +++++++-----
 2 files changed, 9 insertions(+), 6 deletions(-)

--
2.34.1

[PATCH net 1/2] ip6: vti: Use ip6_tnl.net in vti6_changelink().

[Maoyi Xie]

From: Kuniyuki Iwashima <kuniyu@google.com>

ip netns add ns1
ip netns add ns2
ip -n ns1 link add vti6_test type vti6 remote ::1 local ::2 key 7
ip -n ns1 link set vti6_test netns ns2
ip -n ns2 link set vti6_test type vti6 remote ::3 local ::4 key 9
ip netns del ns2
ip netns del ns1
[  132.495484] ------------[ cut here ]------------
[  132.497609] kernel BUG at net/core/dev.c:12376!

After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
rtnl_link_ops"), vti6_newlink() correctly resolves the per-netns vti6
hash via link_net. vti6_changelink() and vti6_update() were not
converted in that series and still read dev_net(dev) /
dev_net(t->dev), which diverge from the device's creation netns
after IFLA_NET_NS_FD migration. The result is a stale per-netns hash
entry; cleanup_net() of the original netns then walks freed memory.

Reachable from an unprivileged user namespace ("unshare --user
--map-root-user --net"); cross-tenant scope on container hosts.

Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv6/ip6_vti.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
index ad5290be4..dcb257411 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -722,10 +722,11 @@ vti6_tnl_change(struct ip6_tnl *t, const struct __ip6_tnl_parm *p,
 static int vti6_update(struct ip6_tnl *t, struct __ip6_tnl_parm *p,
 		       bool keep_mtu)
 {
-	struct net *net = dev_net(t->dev);
-	struct vti6_net *ip6n = net_generic(net, vti6_net_id);
+	struct net *net = t->net;
+	struct vti6_net *ip6n;
 	int err;
 
+	ip6n = net_generic(net, vti6_net_id);
 	vti6_tnl_unlink(ip6n, t);
 	synchronize_net();
 	err = vti6_tnl_change(t, p, keep_mtu);
@@ -1031,11 +1032,12 @@ static int vti6_changelink(struct net_device *dev, struct nlattr *tb[],
 			   struct nlattr *data[],
 			   struct netlink_ext_ack *extack)
 {
-	struct ip6_tnl *t;
+	struct ip6_tnl *t = netdev_priv(dev);
+	struct net *net = t->net;
 	struct __ip6_tnl_parm p;
-	struct net *net = dev_net(dev);
-	struct vti6_net *ip6n = net_generic(net, vti6_net_id);
+	struct vti6_net *ip6n;
 
+	ip6n = net_generic(net, vti6_net_id);
 	if (dev == ip6n->fb_tnl_dev)
 		return -EINVAL;
 
-- 
2.34.1

Re: [PATCH net 1/2] ip6: vti: Use ip6_tnl.net in vti6_changelink().

[Eric Dumazet]

On Tue, Apr 28, 2026 at 4:07 AM Maoyi Xie <maoyixie.tju@gmail.com> wrote:
>
> From: Kuniyuki Iwashima <kuniyu@google.com>
>
> ip netns add ns1
> ip netns add ns2
> ip -n ns1 link add vti6_test type vti6 remote ::1 local ::2 key 7
> ip -n ns1 link set vti6_test netns ns2
> ip -n ns2 link set vti6_test type vti6 remote ::3 local ::4 key 9
> ip netns del ns2
> ip netns del ns1
> [  132.495484] ------------[ cut here ]------------
> [  132.497609] kernel BUG at net/core/dev.c:12376!
>
> After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
> rtnl_link_ops"), vti6_newlink() correctly resolves the per-netns vti6
> hash via link_net. vti6_changelink() and vti6_update() were not
> converted in that series and still read dev_net(dev) /
> dev_net(t->dev), which diverge from the device's creation netns
> after IFLA_NET_NS_FD migration. The result is a stale per-netns hash
> entry; cleanup_net() of the original netns then walks freed memory.
>
> Reachable from an unprivileged user namespace ("unshare --user
> --map-root-user --net"); cross-tenant scope on container hosts.
>
> Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
> Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> Cc: stable@vger.kernel.org # v5.15+
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Eric Dumazet <edumazet@google.com>

[PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Maoyi Xie]

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

After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
ip6gre hash via link_net. ip6erspan_changelink() was not converted in
that series and still uses dev_net(dev), which diverges from the
device's creation netns after IFLA_NET_NS_FD migration.

This re-inserts the tunnel into the wrong per-netns hash, leaving a
stale entry in the original creation netns. When that netns is later
destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
slab-use-after-free reported by KASAN, followed by a kernel BUG at
net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().

Reachable from an unprivileged user namespace ("unshare --user
--map-root-user --net"); cross-tenant scope on container hosts.

Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
file) already uses the cached t->net correctly. The bug is specific
to ip6erspan_changelink() copying the wrong shape.

Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
---
 net/ipv6/ip6_gre.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index dafcc0dcd..38ac14cc0 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -2261,7 +2261,8 @@ static int ip6erspan_changelink(struct net_device *dev, struct nlattr *tb[],
 				struct nlattr *data[],
 				struct netlink_ext_ack *extack)
 {
-	struct ip6gre_net *ign = net_generic(dev_net(dev), ip6gre_net_id);
+	struct ip6_tnl *nt = netdev_priv(dev);
+	struct ip6gre_net *ign = net_generic(nt->net, ip6gre_net_id);
 	struct __ip6_tnl_parm p;
 	struct ip6_tnl *t;
 
-- 
2.34.1

Re: [PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Eric Dumazet]

On Tue, Apr 28, 2026 at 4:07 AM Maoyi Xie <maoyixie.tju@gmail.com> wrote:
>
> From: Maoyi Xie <maoyi.xie@ntu.edu.sg>
>
> After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
> rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
> ip6gre hash via link_net. ip6erspan_changelink() was not converted in
> that series and still uses dev_net(dev), which diverges from the
> device's creation netns after IFLA_NET_NS_FD migration.
>
> This re-inserts the tunnel into the wrong per-netns hash, leaving a
> stale entry in the original creation netns. When that netns is later
> destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
> slab-use-after-free reported by KASAN, followed by a kernel BUG at
> net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().
>
> Reachable from an unprivileged user namespace ("unshare --user
> --map-root-user --net"); cross-tenant scope on container hosts.
>
> Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
> file) already uses the cached t->net correctly. The bug is specific
> to ip6erspan_changelink() copying the wrong shape.
>
> Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
> Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> Cc: stable@vger.kernel.org # v5.15+
> Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> ---
>  net/ipv6/ip6_gre.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index dafcc0dcd..38ac14cc0 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -2261,7 +2261,8 @@ static int ip6erspan_changelink(struct net_device *dev, struct nlattr *tb[],
>                                 struct nlattr *data[],
>                                 struct netlink_ext_ack *extack)
>  {
> -       struct ip6gre_net *ign = net_generic(dev_net(dev), ip6gre_net_id);
> +       struct ip6_tnl *nt = netdev_priv(dev);
> +       struct ip6gre_net *ign = net_generic(nt->net, ip6gre_net_id);
>         struct __ip6_tnl_parm p;
>         struct ip6_tnl *t;
>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Thanks.

Re: [PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Kuniyuki Iwashima]

On Tue, Apr 28, 2026 at 4:07 AM Maoyi Xie <maoyixie.tju@gmail.com> wrote:
>
> From: Maoyi Xie <maoyi.xie@ntu.edu.sg>
>
> After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
> rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
> ip6gre hash via link_net. ip6erspan_changelink() was not converted in
> that series and still uses dev_net(dev), which diverges from the
> device's creation netns after IFLA_NET_NS_FD migration.
>
> This re-inserts the tunnel into the wrong per-netns hash, leaving a
> stale entry in the original creation netns. When that netns is later
> destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
> slab-use-after-free reported by KASAN, followed by a kernel BUG at
> net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().
>
> Reachable from an unprivileged user namespace ("unshare --user
> --map-root-user --net"); cross-tenant scope on container hosts.
>
> Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
> file) already uses the cached t->net correctly. The bug is specific
> to ip6erspan_changelink() copying the wrong shape.
>
> Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
> Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>

nit: Reported-by is not needed if it's same with SOB.

> Cc: stable@vger.kernel.org # v5.15+
> Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> ---
>  net/ipv6/ip6_gre.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index dafcc0dcd..38ac14cc0 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -2261,7 +2261,8 @@ static int ip6erspan_changelink(struct net_device *dev, struct nlattr *tb[],
>                                 struct nlattr *data[],
>                                 struct netlink_ext_ack *extack)
>  {
> -       struct ip6gre_net *ign = net_generic(dev_net(dev), ip6gre_net_id);
> +       struct ip6_tnl *nt = netdev_priv(dev);
> +       struct ip6gre_net *ign = net_generic(nt->net, ip6gre_net_id);

nit: Please keep reverse xmas tree order, and you can
reuse *t below.
https://docs.kernel.org/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs

  struct ip6_tnl *t = netdev_priv(dev);
  struct ip6_tnl *nt;
  ...

  ign = net_generic(nt->net, ip6gre_net_id);


Otherwise looks good.

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Thanks

>         struct __ip6_tnl_parm p;
>         struct ip6_tnl *t;
>
> --
> 2.34.1
>

Re: [PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Xiao Liang]

On Tue, Apr 28, 2026 at 7:07 PM Maoyi Xie <maoyixie.tju@gmail.com> wrote:
>
> From: Maoyi Xie <maoyi.xie@ntu.edu.sg>
>
> After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
> rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
> ip6gre hash via link_net. ip6erspan_changelink() was not converted in
> that series and still uses dev_net(dev), which diverges from the
> device's creation netns after IFLA_NET_NS_FD migration.
>
> This re-inserts the tunnel into the wrong per-netns hash, leaving a
> stale entry in the original creation netns. When that netns is later
> destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
> slab-use-after-free reported by KASAN, followed by a kernel BUG at
> net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().
>
> Reachable from an unprivileged user namespace ("unshare --user
> --map-root-user --net"); cross-tenant scope on container hosts.
>
> Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
> file) already uses the cached t->net correctly. The bug is specific
> to ip6erspan_changelink() copying the wrong shape.
>
> Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")

The changes look good to me. But why is 5e72ce3e3980 mentioned
here? It neither introduced nor was intended to fix this bug.

Thanks.

> Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> Cc: stable@vger.kernel.org # v5.15+
> Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> ---
>  net/ipv6/ip6_gre.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index dafcc0dcd..38ac14cc0 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -2261,7 +2261,8 @@ static int ip6erspan_changelink(struct net_device *dev, struct nlattr *tb[],
>                                 struct nlattr *data[],
>                                 struct netlink_ext_ack *extack)
>  {
> -       struct ip6gre_net *ign = net_generic(dev_net(dev), ip6gre_net_id);
> +       struct ip6_tnl *nt = netdev_priv(dev);
> +       struct ip6gre_net *ign = net_generic(nt->net, ip6gre_net_id);
>         struct __ip6_tnl_parm p;
>         struct ip6_tnl *t;
>
> --
> 2.34.1
>

Re: [PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Eric Dumazet]

On Tue, Apr 28, 2026 at 6:58 PM Xiao Liang <shaw.leon@gmail.com> wrote:
>
> On Tue, Apr 28, 2026 at 7:07 PM Maoyi Xie <maoyixie.tju@gmail.com> wrote:
> >
> > From: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> >
> > After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
> > rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
> > ip6gre hash via link_net. ip6erspan_changelink() was not converted in
> > that series and still uses dev_net(dev), which diverges from the
> > device's creation netns after IFLA_NET_NS_FD migration.
> >
> > This re-inserts the tunnel into the wrong per-netns hash, leaving a
> > stale entry in the original creation netns. When that netns is later
> > destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
> > slab-use-after-free reported by KASAN, followed by a kernel BUG at
> > net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().
> >
> > Reachable from an unprivileged user namespace ("unshare --user
> > --map-root-user --net"); cross-tenant scope on container hosts.
> >
> > Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
> > file) already uses the cached t->net correctly. The bug is specific
> > to ip6erspan_changelink() copying the wrong shape.
> >
> > Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
>
> The changes look good to me. But why is 5e72ce3e3980 mentioned
> here? It neither introduced nor was intended to fix this bug.

Which patch added the bug then in your opinion?

Re: [PATCH net 2/2] ip6_gre: Use cached t->net in ip6erspan_changelink().

[Xiao Liang]

On Wed, Apr 29, 2026 at 10:00 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Tue, Apr 28, 2026 at 6:58 PM Xiao Liang <shaw.leon@gmail.com> wrote:
> >
> > On Tue, Apr 28, 2026 at 7:07 PM Maoyi Xie <maoyixie.tju@gmail.com> wrote:
> > >
> > > From: Maoyi Xie <maoyi.xie@ntu.edu.sg>
> > >
> > > After commit 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of
> > > rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
> > > ip6gre hash via link_net. ip6erspan_changelink() was not converted in
> > > that series and still uses dev_net(dev), which diverges from the
> > > device's creation netns after IFLA_NET_NS_FD migration.
> > >
> > > This re-inserts the tunnel into the wrong per-netns hash, leaving a
> > > stale entry in the original creation netns. When that netns is later
> > > destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
> > > slab-use-after-free reported by KASAN, followed by a kernel BUG at
> > > net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().
> > >
> > > Reachable from an unprivileged user namespace ("unshare --user
> > > --map-root-user --net"); cross-tenant scope on container hosts.
> > >
> > > Note: ip6gre_changelink() (the non-erspan sibling earlier in the same
> > > file) already uses the cached t->net correctly. The bug is specific
> > > to ip6erspan_changelink() copying the wrong shape.
> > >
> > > Fixes: 5e72ce3e3980 ("net: ipv6: Use link netns in newlink() of rtnl_link_ops")
> >
> > The changes look good to me. But why is 5e72ce3e3980 mentioned
> > here? It neither introduced nor was intended to fix this bug.
>
> Which patch added the bug then in your opinion?

Maybe 2d665034f239 ("net: ip6_gre: Fix ip6erspan hlen calculation")
which initially introduced ip6erspan_changelink using the wrong
dev_net()?
And ab5098fa25b9 ("ip6_gre: fix tunnel list corruption for x-netns")
fixed this for ip6gre, but ip6erspan was left.
Anyway 5e72ce3e3980 doesn't exist before v6.15.