[PATCH] Bluetooth: 6LoWPAN: Use RCU-safe device list removal

[Cen Zhang <zzzccc427@gmail.com>]

RCU readers of bt_6lowpan_devices must be able to follow an adapter's
forward link until their read-side critical section ends. The entries
are published with list_add_rcu(), but device_event() removes an entry
on NETDEV_UNREGISTER with list_del(), poisoning that link.

When an adapter is being unregistered and 6LoWPAN is enabled, a
6lowpan_control debugfs write with an existing LE L2CAP connection can
enter lookup_peer() after unregister's first synchronize_net(). If the
connection has no peer on that adapter, the reader must advance to the
next list entry. Neither RTNL nor devices_lock excludes this RCU reader.

The last peer's chan_close_cb() schedules delete_netdev(), which reaches
unregister_netdevice_many_notify() through unregister_dev() and
lowpan_unregister_netdev(). The following ordering is possible:

  Unregister worker                 Control writer
  -----------------                 --------------
  synchronize_net() returns
                                    lowpan_control_write()
                                    lookup_peer(): rcu_read_lock()
                                    reach adapter; no matching peer
  device_event(): devices_lock
  list_del(&entry->list)
  spin_unlock(&devices_lock)
                                    follow poisoned next link
                                    read invalid adapter address
  post-notifier synchronize_net()
    waits for the reader

This invalid pointer access can fault in lookup_peer(). The first grace
period drains existing readers but permits new ones to start, and the
later grace period cannot prevent the intervening poisoned-link access.

Use list_del_rcu() under the existing devices_lock to preserve the
forward link for readers that already reached the adapter. The existing
post-notifier synchronize_net() waits for those readers before netdevice
teardown frees the private storage containing lowpan_btle_dev.

KASAN report as below:

    [...]
    [   45.867526] Oops: general protection fault, probably for non-canonical address 0xfbd59c0000000023: 0000 [#1] SMP KASAN NOPTI
    [   45.869489] KASAN: maybe wild-memory-access in range [0xdead000000000118-0xdead00000000011f]
    [   45.870981] CPU: 1 UID: 0 PID: 646 Comm: sh Not tainted 7.2.0-rc6-pmb-bt-functional-v1+ #4 PREEMPT(lazy) 
    [   45.872609] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
    [   45.874380] RIP: 0010:lookup_peer+0x1ba/0x320
    [   45.875067] Code: 48 89 d9 ba 04 00 00 00 4c 89 fe bf a0 06 00 00 e8 2b e8 ad fb e9 56 ff ff ff e8 01 1e c3 fb 49 8d 7f 18 48 89 f8 48 c1 e8 03 <80> 3c 28 00 0f 85 4a 01 00 00 4d 8b 47 18 4c 89 f1 ba 04 00 00 00
    [   45.878206] RSP: 0018:ffff888114d67b90 EFLAGS: 00010212
    [   45.879106] RAX: 1bd5a00000000023 RBX: 0000000000000000 RCX: ffffffff85c1d19f
    [   45.880334] RDX: ffff88810dc35700 RSI: 0000000000000000 RDI: dead000000000118
    [   45.881455] RBP: dffffc0000000000 R08: 0000000000000001 R09: 0000000000000001
    [   45.882546] R10: ffffffff893d3057 R11: ffff88810dc35700 R12: ffff88810a690000
    [   45.883839] R13: ffff888115451038 R14: ffff88810a690000 R15: dead000000000100
    [   45.885186] FS:  00007fa889bac780(0000) GS:ffff8881fd852000(0000) knlGS:0000000000000000
    [   45.886539] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    [   45.887527] CR2: 00007ffe304f2f80 CR3: 0000000104f64005 CR4: 0000000000770ef0
    [   45.888687] PKRU: 55555554
    [   45.889183] Call Trace:
    [   45.889666]  <TASK>
    [   45.890061]  lowpan_control_write+0x445/0x9e0
    [   45.890854]  ? __pfx_lowpan_control_write+0x10/0x10
    [   45.891738]  ? srso_alias_return_thunk+0x5/0xfbef5
    [   45.892603]  ? srso_alias_return_thunk+0x5/0xfbef5
    [   45.893383]  ? __debugfs_file_get+0x1bd/0x7f0
    [   45.894057]  ? srso_alias_return_thunk+0x5/0xfbef5
    [   45.894902]  ? lock_acquire+0x17b/0x2f0
    [   45.895593]  ? ksys_write+0x111/0x200
    [   45.896290]  full_proxy_write+0x11f/0x180
    [   45.896991]  vfs_write+0x25a/0x1010
    [   45.897642]  ? __pfx_full_proxy_write+0x10/0x10
    [   45.898440]  ? __pfx_vfs_write+0x10/0x10
    [   45.899178]  ? lock_release+0x11f/0x280
    [   45.899855]  ksys_write+0x111/0x200
    [   45.900435]  ? __pfx_ksys_write+0x10/0x10
    [   45.901074]  ? irqentry_exit+0xbe/0x820
    [   45.901722]  ? srso_alias_return_thunk+0x5/0xfbef5
    [   45.902489]  do_syscall_64+0x115/0x6a0
    [   45.903064]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
    [   45.903813] RIP: 0033:0x7fa889c3e687
    [   45.904359] Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
    [   45.906981] RSP: 002b:00007ffd7651b960 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
    [   45.908070] RAX: ffffffffffffffda RBX: 00007fa889bac780 RCX: 00007fa889c3e687
    [   45.909102] RDX: 000000000000001c RSI: 00005570f25e8d10 RDI: 0000000000000001
    [   45.910133] RBP: 00005570f25e8d10 R08: 0000000000000000 R09: 0000000000000000
    [   45.911162] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000001
    [   45.912190] R13: 000000000000001c R14: 0000000000000000 R15: 00005570f25e8a78
    [   45.913208]  </TASK>
    [   45.913530] Modules linked in:
    [   45.914022] ---[ end trace 0000000000000000 ]---
    [   45.914709] RIP: 0010:lookup_peer+0x1ba/0x320
    [   45.915436] Code: 48 89 d9 ba 04 00 00 00 4c 89 fe bf a0 06 00 00 e8 2b e8 ad fb e9 56 ff ff ff e8 01 1e c3 fb 49 8d 7f 18 48 89 f8 48 c1 e8 03 <80> 3c 28 00 0f 85 4a 01 00 00 4d 8b 47 18 4c 89 f1 ba 04 00 00 00
    [   45.919191] RSP: 0018:ffff888114d67b90 EFLAGS: 00010212
    [   45.920046] RAX: 1bd5a00000000023 RBX: 0000000000000000 RCX: ffffffff85c1d19f
    [   45.921129] RDX: ffff88810dc35700 RSI: 0000000000000000 RDI: dead000000000118
    [   45.922553] RBP: dffffc0000000000 R08: 0000000000000001 R09: 0000000000000001
    [   45.923653] R10: ffffffff893d3057 R11: ffff88810dc35700 R12: ffff88810a690000
    [   45.925119] R13: ffff888115451038 R14: ffff88810a690000 R15: dead000000000100
    [   45.926180] FS:  00007fa889bac780(0000) GS:ffff8881fd852000(0000) knlGS:0000000000000000
    [   45.927817] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    [   45.928726] CR2: 00007ffe304f2f80 CR3: 0000000104f64005 CR4: 0000000000770ef0
    [   45.930069] PKRU: 55555554
    [   45.930456] Kernel panic - not syncing: Fatal exception
    [   45.931542] Kernel Offset: disabled
    [   45.932040] Rebooting in 1 seconds..

Fixes: 90305829635d ("Bluetooth: 6lowpan: Converting rwlocks to use RCU")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
index 836add41f5..4b05409929 100644
--- a/net/bluetooth/6lowpan.c
+++ b/net/bluetooth/6lowpan.c
@@ -1274,7 +1274,7 @@ static int device_event(struct notifier_block *unused,
 			if (entry->netdev == netdev) {
 				BT_DBG("Unregistered netdev %s %p",
 				       netdev->name, netdev);
-				list_del(&entry->list);
+				list_del_rcu(&entry->list);
 				break;
 			}
 		}