Re: [PATCH net v2 1/1] ipv6: ip6mr: fix mr_table leak from MRT6_TABLE

[Ido Schimmel]

On Sat, Sep 05, 2026 at 04:30:07PM +0000, Zihan Xi wrote:
> MRT6_TABLE is supposed to select a multicast routing table id, but
> ip6_mroute_setsockopt() currently calls ip6mr_new_table() for every
> unseen id. The new mr_table is linked into mr6_tables and is only
> destroyed when the net namespace goes away.
> 
> A raw ICMPv6 socket with CAP_NET_ADMIN can therefore loop
> MRT6_TABLE(fresh id) without MRT6_INIT, close the socket, and still
> leave the tables allocated. Repeating this grows unreclaimable slab
> until the machine OOMs.

Isn't the established way to deal with this sort of issue to simply
account for the memory and assume that the admin put some kind of a
memory limit on the container?

See for example commit 6126891c6d4f ("memcg: enable accounting for IP
address and routing-related objects") and commit 5d26cff5bdbe ("net:
account alternate interface name memory").

This diff [1] accounts for the memory of both the multicast tables
and the multicast routes.

With your C reproducer and this diff I get:

# echo +memory > /sys/fs/cgroup/cgroup.subtree_control
# mkdir /sys/fs/cgroup/mrtest
# echo 64M > /sys/fs/cgroup/mrtest/memory.max
# echo $$ > /sys/fs/cgroup/mrtest/cgroup.procs
# unshare -Urn ./mrt_poc
allocated=1 current_table=1 maxrss_kb=1788
allocated=10001 current_table=10001 maxrss_kb=1788
Killed                     bash
# dmesg | tail -1
[   63.360047] Memory cgroup out of memory: Killed process 297 (mrt_poc) total-vm:2424kB, anon-rss:92kB, file-rss:1448kB, shmem-rss:0kB, UID:0 pgtables:44kB oom_score_adj:0

[1]
diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index e5f2b1c6150d..b9c544d48c45 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -3376,7 +3376,8 @@ int __init ip_mr_init(void)
 {
 	int err;
 
-	mrt_cachep = KMEM_CACHE(mfc_cache, SLAB_HWCACHE_ALIGN | SLAB_PANIC);
+	mrt_cachep = KMEM_CACHE(mfc_cache,
+				SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
 
 	err = register_pernet_subsys(&ipmr_net_ops);
 	if (err)
diff --git a/net/ipv4/ipmr_base.c b/net/ipv4/ipmr_base.c
index 867b24beded1..a0ec6d19a237 100644
--- a/net/ipv4/ipmr_base.c
+++ b/net/ipv4/ipmr_base.c
@@ -52,7 +52,7 @@ mr_table_alloc(struct net *net, u32 id,
 	struct mr_table *mrt;
 	int err;
 
-	mrt = kzalloc_obj(*mrt);
+	mrt = kzalloc_obj(*mrt, GFP_KERNEL_ACCOUNT);
 	if (!mrt)
 		return ERR_PTR(-ENOMEM);
 	mrt->id = id;
diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 3f2ed9b77deb..9d8116b5edb1 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1427,7 +1427,7 @@ int __init ip6_mr_init(void)
 {
 	int err;
 
-	mrt_cachep = KMEM_CACHE(mfc6_cache, SLAB_HWCACHE_ALIGN);
+	mrt_cachep = KMEM_CACHE(mfc6_cache, SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT);
 	if (!mrt_cachep)
 		return -ENOMEM;