net: Fix wild-memory-access in __register_pernet_operations() when CONFIG_NET_NS=n.

kernel test robot reported the splat below. [0]

Before commit fed176b ("net: Add ops_undo_single for module
load/unload."), if CONFIG_NET_NS=n, ops was linked to pernet_list
only when init_net had not been initialised, and ops was unlinked
from pernet_list only under the same condition.

Let's say an ops is loaded before the init_net setup but unloaded
after that.  Then, the ops remains in pernet_list, which seems odd.

The cited commit added ops_undo_single(), which calls list_add() for
ops to link it to a temporary list, so a minor change was added to
__register_pernet_operations() and __unregister_pernet_operations()
under CONFIG_NET_NS=n to avoid the pernet_list corruption.

However, the corruption must have been left as is.

When CONFIG_NET_NS=n, pernet_list was used to keep ops registered
before the init_net setup, and after that, pernet_list was not used
at all.

This was because some ops annotated with __net_initdata are cleared
out of memory at some point during boot.

Then, such ops is initialised by POISON_FREE_INITMEM (0xcc), resulting
in that ops->list.{next,prev} suddenly switches from a valid pointer
to a weird value, 0xcccccccccccccccc.

To avoid such wild memory access, let's allow the pernet_list
corruption for CONFIG_NET_NS=n.

[0]:
Oops: general protection fault, probably for non-canonical address 0xf999959999999999: 0000 [#1] SMP KASAN NOPTI
KASAN: maybe wild-memory-access in range [0xccccccccccccccc8-0xcccccccccccccccf]
CPU: 2 UID: 0 PID: 346 Comm: modprobe Not tainted 6.15.0-rc1-00294-ga4cba7e98e35 #85 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
RIP: 0010:__list_add_valid_or_report (lib/list_debug.c:32)
Code: 48 c1 ea 03 80 3c 02 00 0f 85 5a 01 00 00 49 39 74 24 08 0f 85 83 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 1f 01 00 00 4c 39 26 0f 85 ab 00 00 00 4c 39 ee
RSP: 0018:ff11000135b87830 EFLAGS: 00010a07
RAX: dffffc0000000000 RBX: ffffffffc02223c0 RCX: ffffffff8406fcc2
RDX: 1999999999999999 RSI: cccccccccccccccc RDI: ffffffffc02223c0
RBP: ffffffff86064e40 R08: 0000000000000001 R09: fffffbfff0a9f5b5
R10: ffffffff854fadaf R11: 676552203a54454e R12: ffffffff86064e40
R13: ffffffffc02223c0 R14: ffffffff86064e48 R15: 0000000000000021
FS:  00007f6fb0d9e1c0(0000) GS:ff11000858ea0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6fb0eda580 CR3: 0000000122fec005 CR4: 0000000000771ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 <TASK>
 register_pernet_operations (./include/linux/list.h:150 (discriminator 5) ./include/linux/list.h:183 (discriminator 5) net/core/net_namespace.c:1315 (discriminator 5) net/core/net_namespace.c:1359 (discriminator 5))
 register_pernet_subsys (net/core/net_namespace.c:1401)
 inet6_init (net/ipv6/af_inet6.c:535) ipv6
 do_one_initcall (init/main.c:1257)
 do_init_module (kernel/module/main.c:2942)
 load_module (kernel/module/main.c:3409)
 init_module_from_file (kernel/module/main.c:3599)
 idempotent_init_module (kernel/module/main.c:3611)
 __x64_sys_finit_module (./include/linux/file.h:62 ./include/linux/file.h:83 kernel/module/main.c:3634 kernel/module/main.c:3621 kernel/module/main.c:3621)
 do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
 RIP: 0033:0x7f6fb0df7e5d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 9f 1b 00 f7 d8 64 89 01 48
RSP: 002b:00007fffdc6a8968 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 000055b535721b70 RCX: 00007f6fb0df7e5d
RDX: 0000000000000000 RSI: 000055b51e44aa2a RDI: 0000000000000004
RBP: 0000000000040000 R08: 0000000000000000 R09: 000055b535721b30
R10: 0000000000000004 R11: 0000000000000246 R12: 000055b51e44aa2a
R13: 000055b535721bf0 R14: 000055b5357220b0 R15: 0000000000000000
 </TASK>
Modules linked in: ipv6(+) crc_ccitt

Fixes: fed176b ("net: Add ops_undo_single for module load/unload.")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: q2ven

net/core/net_namespace.c

@@ -1314,19 +1314,19 @@ static void __unregister_pernet_operations(struct pernet_operations *ops)
 static int __register_pernet_operations(struct list_head *list,
 					struct pernet_operations *ops)
 {
-	list_add_tail(&ops->list, list);
-
-	if (!init_net_initialized)
+	if (!init_net_initialized) {
+		list_add_tail(&ops->list, list);
 		return 0;
+	}
 
 	return ops_init(ops, &init_net);
 }
 
 static void __unregister_pernet_operations(struct pernet_operations *ops)
 {
-	list_del(&ops->list);
-
-	if (init_net_initialized) {
+	if (!init_net_initialized) {
+		list_del(&ops->list);
+	} else {
 		LIST_HEAD(net_exit_list);
 
 		list_add(&init_net.exit_list, &net_exit_list);