[PATCH net-next v2] ipv4: igmp: remove multicast group from hash table on device destruction

[PATCH net-next v2] ipv4: igmp: remove multicast group from hash table on device destruction

[Yuyang Huang]

When a device is destroyed under RTNL, ip_mc_destroy_dev() iterates through
the multicast list and calls ip_ma_put() on each membership, scheduling
them for RCU reclamation. However, they are not unlinked from the device's
multicast hash table (mc_hash).

Since the device remains published in dev->ip_ptr until after
ip_mc_destroy_dev() completes, concurrent RCU readers traversing mc_hash
can still locate and access the multicast group after its refcount is
decremented. If the RCU callback runs and frees the group while a reader is
accessing it, a use-after-free occurs.

Fix this by unlinking the multicast group from mc_hash using
ip_mc_hash_remove() before scheduling it for reclamation.

Fixes: e9897071350b ("igmp: hash a hash table to speedup ip_check_mc_rcu()")
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
---
v2:
- Add Fixes tag in the commit message.

 net/ipv4/igmp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index b6337a47c141..af38073a822d 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -1923,6 +1923,7 @@ void ip_mc_destroy_dev(struct in_device *in_dev)
 
 	while ((i = rtnl_dereference(in_dev->mc_list)) != NULL) {
 		in_dev->mc_list = i->next_rcu;
+		ip_mc_hash_remove(in_dev, i);
 		WRITE_ONCE(in_dev->mc_count, in_dev->mc_count - 1);
 		ip_mc_clear_src(i);
 		ip_ma_put(i);
-- 
2.55.0.rc0.799.gd6f94ed593-goog

RE: [PATCH net-next v2] ipv4: igmp: remove multicast group from hash table on device destruction

[Jagielski, Jedrzej]

From: Yuyang Huang <yuyanghuang@google.com> 
Sent: Tuesday, June 30, 2026 4:23 AM
>When a device is destroyed under RTNL, ip_mc_destroy_dev() iterates through
>the multicast list and calls ip_ma_put() on each membership, scheduling
>them for RCU reclamation. However, they are not unlinked from the device's
>multicast hash table (mc_hash).
>
>Since the device remains published in dev->ip_ptr until after
>ip_mc_destroy_dev() completes, concurrent RCU readers traversing mc_hash
>can still locate and access the multicast group after its refcount is
>decremented. If the RCU callback runs and frees the group while a reader is
>accessing it, a use-after-free occurs.
>
>Fix this by unlinking the multicast group from mc_hash using
>ip_mc_hash_remove() before scheduling it for reclamation.
>
>Fixes: e9897071350b ("igmp: hash a hash table to speedup ip_check_mc_rcu()")
>Signed-off-by: Yuyang Huang <yuyanghuang@google.com>

Hi,

why sending this to net-next not to net if that's a bug fix?

In the v1 thread it was said
>This is a long-standing bug, not a recent regression.

so why do not cc stable kernel to get rid of this bug from
stable kernels in such case?

Re: [PATCH net-next v2] ipv4: igmp: remove multicast group from hash table on device destruction

[Yuyang Huang]

> Hi,
>
> why sending this to net-next not to net if that's a bug fix?
>
> In the v1 thread it was said
> >This is a long-standing bug, not a recent regression.
>
> so why do not cc stable kernel to get rid of this bug from
> stable kernels in such case?

Thanks for the advise, will send this patch to stable kernel.

Re: [PATCH net-next v2] ipv4: igmp: remove multicast group from hash table on device destruction

[Ido Schimmel]

On Tue, Jun 30, 2026 at 04:55:22PM +0900, Yuyang Huang wrote:
> > Hi,
> >
> > why sending this to net-next not to net if that's a bug fix?
> >
> > In the v1 thread it was said
> > >This is a long-standing bug, not a recent regression.
> >
> > so why do not cc stable kernel to get rid of this bug from
> > stable kernels in such case?
> 
> Thanks for the advise, will send this patch to stable kernel.

Please target v3 at net and add a trace given you're claiming for a
use-after-free. That way we know that the problem is real and not a
false-positive from some tool. You can reproduce it by adding enough
delay in inetdev_destroy():

BUG: KASAN: slab-use-after-free in ip_check_mc_rcu+0x2cc/0x500
Read of size 4 at addr ffff88810c571208 by task mausezahn/419

CPU: 2 UID: 0 PID: 419 Comm: mausezahn Not tainted 7.1.0-virtme-g15d4a7c23bf6 #17 PREEMPT(lazy)
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Call Trace:
 <IRQ>
 dump_stack_lvl+0x4d/0x70
 print_report+0x153/0x4c2
 kasan_report+0xda/0x110
 ip_check_mc_rcu+0x2cc/0x500
 ip_route_input_rcu.part.0+0x13d/0xbc0
 ip_route_input_noref+0xb6/0x110
 ip_rcv_finish_core+0x41b/0x1d90
 ip_rcv_finish+0xea/0x1b0
 ip_rcv+0xb7/0x1b0
 __netif_receive_skb_one_core+0xfc/0x180
 process_backlog+0x1ea/0x5e0
 __napi_poll+0x97/0x480
 net_rx_action+0x97c/0xfa0
 handle_softirqs+0x18c/0x4f0
 do_softirq+0x42/0x60
 </IRQ>