net: defer final 'struct net' free in netns dismantle

Ilya reported a slab-use-after-free in dst_destroy [1]

Issue is in xfrm6_net_init() and xfrm4_net_init() :

They copy xfrm[46]_dst_ops_template into net->xfrm.xfrm[46]_dst_ops.

But net structure might be freed before all the dst callbacks are
called. So when dst_destroy() calls later :

if (dst->ops->destroy)
    dst->ops->destroy(dst);

dst->ops points to the old net->xfrm.xfrm[46]_dst_ops, which has been freed.

See a relevant issue fixed in :

ac888d5 ("net: do not delay dst_entries_add() in dst_release()")

A fix is to queue the 'struct net' to be freed after one
another cleanup_net() round (and existing rcu_barrier())

[1]

BUG: KASAN: slab-use-after-free in dst_destroy (net/core/dst.c:112)
Read of size 8 at addr ffff8882137ccab0 by task swapper/37/0
Dec 03 05:46:18 kernel:
CPU: 37 UID: 0 PID: 0 Comm: swapper/37 Kdump: loaded Not tainted 6.12.0 torvalds#67
Hardware name: Red Hat KVM/RHEL, BIOS 1.16.1-1.el9 04/01/2014
Call Trace:
 <IRQ>
dump_stack_lvl (lib/dump_stack.c:124)
print_address_description.constprop.0 (mm/kasan/report.c:378)
? dst_destroy (net/core/dst.c:112)
print_report (mm/kasan/report.c:489)
? dst_destroy (net/core/dst.c:112)
? kasan_addr_to_slab (mm/kasan/common.c:37)
kasan_report (mm/kasan/report.c:603)
? dst_destroy (net/core/dst.c:112)
? rcu_do_batch (kernel/rcu/tree.c:2567)
dst_destroy (net/core/dst.c:112)
rcu_do_batch (kernel/rcu/tree.c:2567)
? __pfx_rcu_do_batch (kernel/rcu/tree.c:2491)
? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4339 kernel/locking/lockdep.c:4406)
rcu_core (kernel/rcu/tree.c:2825)
handle_softirqs (kernel/softirq.c:554)
__irq_exit_rcu (kernel/softirq.c:589 kernel/softirq.c:428 kernel/softirq.c:637)
irq_exit_rcu (kernel/softirq.c:651)
sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1049 arch/x86/kernel/apic/apic.c:1049)
 </IRQ>
 <TASK>
asm_sysvec_apic_timer_interrupt (./arch/x86/include/asm/idtentry.h:702)
RIP: 0010:default_idle (./arch/x86/include/asm/irqflags.h:37 ./arch/x86/include/asm/irqflags.h:92 arch/x86/kernel/process.c:743)
Code: 00 4d 29 c8 4c 01 c7 4c 29 c2 e9 6e ff ff ff 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 90 0f 00 2d c7 c9 27 00 fb f4 <fa> c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 90
RSP: 0018:ffff888100d2fe00 EFLAGS: 00000246
RAX: 00000000001870ed RBX: 1ffff110201a5fc2 RCX: ffffffffb61a3e46
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffb3d4d123
RBP: 0000000000000000 R08: 0000000000000001 R09: ffffed11c7e1835d
R10: ffff888e3f0c1aeb R11: 0000000000000000 R12: 0000000000000000
R13: ffff888100d20000 R14: dffffc0000000000 R15: 0000000000000000
? ct_kernel_exit.constprop.0 (kernel/context_tracking.c:148)
? cpuidle_idle_call (kernel/sched/idle.c:186)
default_idle_call (./include/linux/cpuidle.h:143 kernel/sched/idle.c:118)
cpuidle_idle_call (kernel/sched/idle.c:186)
? __pfx_cpuidle_idle_call (kernel/sched/idle.c:168)
? lock_release (kernel/locking/lockdep.c:467 kernel/locking/lockdep.c:5848)
? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4347 kernel/locking/lockdep.c:4406)
? tsc_verify_tsc_adjust (arch/x86/kernel/tsc_sync.c:59)
do_idle (kernel/sched/idle.c:326)
cpu_startup_entry (kernel/sched/idle.c:423 (discriminator 1))
start_secondary (arch/x86/kernel/smpboot.c:202 arch/x86/kernel/smpboot.c:282)
? __pfx_start_secondary (arch/x86/kernel/smpboot.c:232)
? soft_restart_cpu (arch/x86/kernel/head_64.S:452)
common_startup_64 (arch/x86/kernel/head_64.S:414)
 </TASK>
Dec 03 05:46:18 kernel:
Allocated by task 12184:
kasan_save_stack (mm/kasan/common.c:48)
kasan_save_track (./arch/x86/include/asm/current.h:49 mm/kasan/common.c:60 mm/kasan/common.c:69)
__kasan_slab_alloc (mm/kasan/common.c:319 mm/kasan/common.c:345)
kmem_cache_alloc_noprof (mm/slub.c:4085 mm/slub.c:4134 mm/slub.c:4141)
copy_net_ns (net/core/net_namespace.c:421 net/core/net_namespace.c:480)
create_new_namespaces (kernel/nsproxy.c:110)
unshare_nsproxy_namespaces (kernel/nsproxy.c:228 (discriminator 4))
ksys_unshare (kernel/fork.c:3313)
__x64_sys_unshare (kernel/fork.c:3382)
do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
Dec 03 05:46:18 kernel:
Freed by task 11:
kasan_save_stack (mm/kasan/common.c:48)
kasan_save_track (./arch/x86/include/asm/current.h:49 mm/kasan/common.c:60 mm/kasan/common.c:69)
kasan_save_free_info (mm/kasan/generic.c:582)
__kasan_slab_free (mm/kasan/common.c:271)
kmem_cache_free (mm/slub.c:4579 mm/slub.c:4681)
cleanup_net (net/core/net_namespace.c:456 net/core/net_namespace.c:446 net/core/net_namespace.c:647)
process_one_work (kernel/workqueue.c:3229)
worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391)
kthread (kernel/kthread.c:389)
ret_from_fork (arch/x86/kernel/process.c:147)
ret_from_fork_asm (arch/x86/entry/entry_64.S:257)
Dec 03 05:46:18 kernel:
Last potentially related work creation:
kasan_save_stack (mm/kasan/common.c:48)
__kasan_record_aux_stack (mm/kasan/generic.c:541)
insert_work (./include/linux/instrumented.h:68 ./include/asm-generic/bitops/instrumented-non-atomic.h:141 kernel/workqueue.c:788 kernel/workqueue.c:795 kernel/workqueue.c:2186)
__queue_work (kernel/workqueue.c:2340)
queue_work_on (kernel/workqueue.c:2391)
xfrm_policy_insert (net/xfrm/xfrm_policy.c:1610)
xfrm_add_policy (net/xfrm/xfrm_user.c:2116)
xfrm_user_rcv_msg (net/xfrm/xfrm_user.c:3321)
netlink_rcv_skb (net/netlink/af_netlink.c:2536)
xfrm_netlink_rcv (net/xfrm/xfrm_user.c:3344)
netlink_unicast (net/netlink/af_netlink.c:1316 net/netlink/af_netlink.c:1342)
netlink_sendmsg (net/netlink/af_netlink.c:1886)
sock_write_iter (net/socket.c:729 net/socket.c:744 net/socket.c:1165)
vfs_write (fs/read_write.c:590 fs/read_write.c:683)
ksys_write (fs/read_write.c:736)
do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
Dec 03 05:46:18 kernel:
Second to last potentially related work creation:
kasan_save_stack (mm/kasan/common.c:48)
__kasan_record_aux_stack (mm/kasan/generic.c:541)
insert_work (./include/linux/instrumented.h:68 ./include/asm-generic/bitops/instrumented-non-atomic.h:141 kernel/workqueue.c:788 kernel/workqueue.c:795 kernel/workqueue.c:2186)
__queue_work (kernel/workqueue.c:2340)
queue_work_on (kernel/workqueue.c:2391)
__xfrm_state_insert (./include/linux/workqueue.h:723 net/xfrm/xfrm_state.c:1150 net/xfrm/xfrm_state.c:1145 net/xfrm/xfrm_state.c:1513)
xfrm_state_update (./include/linux/spinlock.h:396 net/xfrm/xfrm_state.c:1940)
xfrm_add_sa (net/xfrm/xfrm_user.c:912)
xfrm_user_rcv_msg (net/xfrm/xfrm_user.c:3321)
netlink_rcv_skb (net/netlink/af_netlink.c:2536)
xfrm_netlink_rcv (net/xfrm/xfrm_user.c:3344)
netlink_unicast (net/netlink/af_netlink.c:1316 net/netlink/af_netlink.c:1342)
netlink_sendmsg (net/netlink/af_netlink.c:1886)
sock_write_iter (net/socket.c:729 net/socket.c:744 net/socket.c:1165)
vfs_write (fs/read_write.c:590 fs/read_write.c:683)
ksys_write (fs/read_write.c:736)
do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

Fixes: a8a572a ("xfrm: dst_entries_init() per-net dst_ops")
Reported-by: Ilya Maximets <[email protected]>
Closes: https://lore.kernel.org/netdev/CANn89iKKYDVpB=MtmfH7nyv2p=rJWSLedO5k7wSZgtY_tO8WQg@mail.gmail.com/T/#m02c98c3009fe66382b73cfb4db9cf1df6fab3fbf
Signed-off-by: Eric Dumazet <[email protected]>
Signed-off-by: NipaLocal <nipa@local>

include/net/net_namespace.h

@@ -80,6 +80,7 @@ struct net {
 						 * or to unregister pernet ops
 						 * (pernet_ops_rwsem write locked).
 						 */
+	struct llist_node	defer_free_list;
 	struct llist_node	cleanup_list;	/* namespaces on death row */
 
 #ifdef CONFIG_KEYS

net/core/net_namespace.c

@@ -449,6 +449,21 @@ static struct net *net_alloc(void)
 	goto out;
 }
 
+static LLIST_HEAD(defer_free_list);
+
+static void net_complete_free(void)
+{
+	struct llist_node *kill_list;
+	struct net *net, *next;
+
+	/* Get the list of namespaces to free from last round. */
+	kill_list = llist_del_all(&defer_free_list);
+
+	llist_for_each_entry_safe(net, next, kill_list, defer_free_list)
+		kmem_cache_free(net_cachep, net);
+
+}
+
 static void net_free(struct net *net)
 {
 	if (refcount_dec_and_test(&net->passive)) {
@@ -457,7 +472,8 @@ static void net_free(struct net *net)
 		/* There should not be any trackers left there. */
 		ref_tracker_dir_exit(&net->notrefcnt_tracker);
 
-		kmem_cache_free(net_cachep, net);
+		/* Wait for an extra rcu_barrier() before final free. */
+		llist_add(&net->defer_free_list, &defer_free_list);
 	}
 }
 
@@ -642,6 +658,8 @@ static void cleanup_net(struct work_struct *work)
 	 */
 	rcu_barrier();
 
+	net_complete_free();
+
 	/* Finally it is safe to free my network namespace structure */
 	list_for_each_entry_safe(net, tmp, &net_exit_list, exit_list) {
 		list_del_init(&net->exit_list);