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Wed, 07 Oct 2026 21:11:41 -0700 (PDT) Received: from localhost ([111.228.63.84]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3a8a4dbaa8csm3396151a91.4.2026.10.07.21.11.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Oct 2026 21:11:41 -0700 (PDT) From: Cen Zhang To: marcel@holtmann.org, luiz.dentz@gmail.com Cc: linux-bluetooth@vger.kernel.org, linux-kernel@vger.kernel.org, baijiaju1990@gmail.com, jjzuming@gmail.com, zzzccc427@gmail.com Subject: [PATCH] Bluetooth: 6lowpan: Drain RCU readers on registration failure Date: Thu, 8 Oct 2026 12:11:35 +0800 Message-Id: X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-bluetooth@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit A lowpan_btle_dev is embedded in its net_device allocation and must remain alive until readers of bt_6lowpan_devices have left their RCU sections. setup_netdev() publishes it before registration, but its error path only unlinks it before calling free_netdev(). If a second controller's registration fails while multicast is sent on an existing 6LoWPAN interface, send_mcast_pkt() can already have selected the new entry. It reads entry->netdev before checking which interface owns it. An early failure, such as allocating the device's name node, leaves NETREG_UNINITIALIZED, so free_netdev() releases the storage immediately. The following ordering frees the selected entry before the reader loads its netdev field: Adapter creation (controller B) Multicast on controller A setup_netdev() list_add_rcu() rcu_read_lock() select B's entry lowpan_register_netdev() fails list_del_rcu() free_netdev() read entry->netdev rcu_read_unlock() Removing the list node does not wait for that reader, and devices_lock does not serialize the multicast reader. The subsequent field load is a use-after-free, reported by KASAN in bt_xmit(). Wait for an RCU grace period after dropping devices_lock and before free_netdev(). This keeps the failed adapter allocated until traversals that could have selected it complete. The callback is already sleepable, and the wait happens outside the list spinlock. KASAN report as below: ================================================================== BUG: KASAN: slab-use-after-free in bt_xmit+0x1d06/0x21e0 Read of size 8 at addr ffff88810ef45030 by task python3/556 Call Trace: dump_stack_lvl+0x93/0xd0 print_report+0xce/0x630 ? bt_xmit+0x1d06/0x21e0 ? srso_alias_return_thunk+0x5/0xfbef5 ? __virt_addr_valid+0x20d/0x410 ? bt_xmit+0x1d06/0x21e0 kasan_report+0xe0/0x110 ? bt_xmit+0x1d06/0x21e0 bt_xmit+0x1d06/0x21e0 ? __pfx_bt_xmit+0x10/0x10 ? srso_alias_return_thunk+0x5/0xfbef5 ? lock_acquire+0x17b/0x2f0 ? srso_alias_return_thunk+0x5/0xfbef5 ? lock_is_held_type+0x8f/0x100 ? dev_hard_start_xmit+0x187/0x660 ? __pfx_bt_xmit+0x10/0x10 dev_hard_start_xmit+0x187/0x660 sch_direct_xmit+0x153/0x850 ? srso_alias_return_thunk+0x5/0xfbef5 ? __pfx_sch_direct_xmit+0x10/0x10 ? do_raw_spin_lock+0x130/0x270 ? __pfx_do_raw_spin_lock+0x10/0x10 ? lock_is_held_type+0x8f/0x100 __dev_queue_xmit+0x3111/0x3aa0 ? srso_alias_return_thunk+0x5/0xfbef5 ? look_up_lock_class+0x59/0x130 ? register_lock_class+0x41/0x790 ? __pfx___dev_queue_xmit+0x10/0x10 ? __lock_acquire+0x466/0x2260 ? find_held_lock+0x2b/0x80 ? ___neigh_create+0x1690/0x2540 ? __local_bh_enable_ip+0xa6/0x120 ? srso_alias_return_thunk+0x5/0xfbef5 ? lockdep_hardirqs_on_prepare+0xea/0x1a0 ? srso_alias_return_thunk+0x5/0xfbef5 ? lock_acquire+0x17b/0x2f0 ? srso_alias_return_thunk+0x5/0xfbef5 ? find_held_lock+0x2b/0x80 ? ip6_finish_output2+0x8be/0x1840 ? srso_alias_return_thunk+0x5/0xfbef5 ? neigh_connected_output+0x39f/0x4e0 ? srso_alias_return_thunk+0x5/0xfbef5 neigh_connected_output+0x33f/0x4e0 ip6_finish_output2+0x8be/0x1840 ip6_finish_output+0x424/0xd50 ip6_output+0x210/0x640 ? __pfx_ip6_mr_output+0x10/0x10 ip6_local_out+0x193/0x1c0 ip6_send_skb+0xfb/0x3e0 udp_v6_send_skb+0x6e1/0x1180 udpv6_sendmsg+0x1f9b/0x2a00 ? get_random_u32+0xaf/0x640 ? __pfx_udpv6_sendmsg+0x10/0x10 ? lock_release+0xc8/0x280 ? srso_alias_return_thunk+0x5/0xfbef5 ? lockdep_hardirqs_on_prepare+0xea/0x1a0 ? srso_alias_return_thunk+0x5/0xfbef5 ? trace_hardirqs_on+0x18/0x160 ? srso_alias_return_thunk+0x5/0xfbef5 ? __local_bh_enable_ip+0xa6/0x120 ? srso_alias_return_thunk+0x5/0xfbef5 ? __pfx_udpv6_sendmsg+0x10/0x10 ? inet6_sendmsg+0xff/0x140 inet6_sendmsg+0xff/0x140 __sys_sendto+0x320/0x470 ? __pfx___sys_sendto+0x10/0x10 ? exc_page_fault+0x5c/0xc0 ? srso_alias_return_thunk+0x5/0xfbef5 ? lock_release+0xc8/0x280 ? handle_mm_fault+0x12e/0x580 __x64_sys_sendto+0xe5/0x1c0 ? lockdep_hardirqs_on_prepare+0xea/0x1a0 ? srso_alias_return_thunk+0x5/0xfbef5 ? trace_hardirqs_on+0x18/0x160 do_syscall_64+0x115/0x6a0 entry_SYSCALL_64_after_hwframe+0x77/0x7f Allocated by task 502: kasan_save_stack+0x33/0x60 kasan_save_track+0x14/0x30 __kasan_kmalloc+0xaa/0xb0 __kvmalloc_node_noprof+0x353/0x930 alloc_netdev_mqs+0xa0/0x1250 chan_ready_cb+0x198/0x1110 l2cap_recv_frame+0x6dc0/0x8830 l2cap_recv_acldata+0xe9f/0x10e0 hci_rx_work+0x5a6/0xf00 process_one_work+0x908/0x19c0 worker_thread+0x65c/0xe40 kthread+0x34f/0x460 ret_from_fork+0x659/0x940 ret_from_fork_asm+0x1a/0x30 Freed by task 502: kasan_save_stack+0x33/0x60 kasan_save_track+0x14/0x30 kasan_save_free_info+0x3b/0x60 __kasan_slab_free+0x5f/0x80 kfree+0x307/0x580 free_netdev+0x6ca/0x8f0 chan_ready_cb+0xd91/0x1110 l2cap_recv_frame+0x6dc0/0x8830 l2cap_recv_acldata+0xe9f/0x10e0 hci_rx_work+0x5a6/0xf00 process_one_work+0x908/0x19c0 worker_thread+0x65c/0xe40 kthread+0x34f/0x460 ret_from_fork+0x659/0x940 ret_from_fork_asm+0x1a/0x30 The buggy address belongs to the object at ffff88810ef44000 which belongs to the cache kmalloc-8k of size 8192 The buggy address is located 4144 bytes inside of freed 8192-byte region [ffff88810ef44000, ffff88810ef46000) The buggy address belongs to the physical page: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x10ef40 head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 flags: 0x200000000000040(head|node=0|zone=2) page_type: f5(slab) raw: 0200000000000040 ffff888100043180 dead000000000100 dead000000000122 raw: 0000000000000000 0000000000020002 00000000f5000000 0000000000000000 head: 0200000000000040 ffff888100043180 dead000000000100 dead000000000122 head: 0000000000000000 0000000000020002 00000000f5000000 0000000000000000 head: 0200000000000003 fffffffffffffe01 00000000ffffffff 00000000ffffffff head: ffff88810ef41a80 0000000000000000 00000000ffffffff 0000000000000000 page dumped because: kasan: bad access detected Memory state around the buggy address: ffff88810ef44f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ffff88810ef44f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb >ffff88810ef45000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ^ ffff88810ef45080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ffff88810ef45100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ================================================================== Disabling lock debugging due to kernel taint Fixes: 5857d1dbae7d ("Bluetooth: 6lowpan: Fix possible race") Assisted-by: LLM Signed-off-by: Cen Zhang --- diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c index 836add41f5d16f1df81042cbf9ba9b927e883eb8..0f6a45509d71c20bd556ad987e05f2fa1b0ab26b 100644 --- a/net/bluetooth/6lowpan.c +++ b/net/bluetooth/6lowpan.c @@ -706,6 +706,7 @@ static int setup_netdev(struct l2cap_chan *chan, struct lowpan_btle_dev **dev) spin_lock(&devices_lock); list_del_rcu(&(*dev)->list); spin_unlock(&devices_lock); + synchronize_rcu(); free_netdev(netdev); goto out; }