[PATCH] Bluetooth: hci_core: Keep RCU protection through LTK filtering

[Cen Zhang <zzzccc427@gmail.com>]

The LTK selected by hci_find_ltk() must remain allocated while its
value is checked against the blocked-key list. The finder releases RCU
before passing k->val to hci_is_blocked_key(). The helper's own RCU
section protects the blocked-key list, but the selected LTK may already
have been reclaimed before that section starts.

An LE ATT socket security request can reach smp_ltk_encrypt() without
holding hdev->lock. With a matching stored LTK and an LTK-type
blocked-key entry present, a concurrent MGMT_OP_LOAD_LONG_TERM_KEYS can
remove the selected LTK in the following order:

  Socket security                 Management / RCU reclamation
  hci_find_ltk()
    rcu_read_lock()
    select k
    rcu_read_unlock()
                                  load_long_term_keys()
                                    hci_dev_lock(hdev)
                                    hci_smp_ltks_clear(hdev)
                                      list_del_rcu(&k->list)
                                      kfree_rcu(k, rcu)
                                  grace period ends; k is freed
    hci_is_blocked_key(..., k->val)
      memcmp(b->val, val, ...)

The final comparison reads freed LTK bytes. Entering a new RCU section
in hci_is_blocked_key() cannot protect a pointer obtained before the
completed grace period.

Keep the finder's RCU read-side section active through the blocked-key
comparison and the warning that reads k->bdaddr. Release it before
returning on either matched-key path. This delays reclamation until the
finder has finished using the selected record, following the existing
IRK lookup pattern.

KFENCE report as below:

    ==================================================================
    BUG: KFENCE: use-after-free read in hci_is_blocked_key+0xdf/0x240
    
    Use-after-free read at 0x00000000b9ee38e4 (in kfence-#249):
     hci_is_blocked_key+0xdf/0x240
     blocked_key_wrapper+0xc5/0x140 [pmbd_ltk_probe]
     hci_find_ltk+0x1aa/0x3e0
     smp_ltk_encrypt.isra.0+0xad/0x340
     smp_conn_security+0x3b8/0x8c0
     l2cap_sock_setsockopt+0x1c4b/0x2270
     do_sock_setsockopt+0x1fb/0x430
     __sys_setsockopt+0x106/0x180
     __x64_sys_setsockopt+0xc2/0x160
     do_syscall_64+0x115/0x6a0
     entry_SYSCALL_64_after_hwframe+0x77/0x7f
    
    kfence-#249: 0x00000000121f2ad9-0x000000005b5bfb3d, size=72, cache=kmalloc-96
    
    allocated by task 495 on cpu 0 at 36.722380s (2.603973s ago):
     hci_add_ltk+0x1f5/0x3a0
     load_long_term_keys+0x6d0/0x860
     hci_sock_sendmsg+0x128b/0x22e0
     __sys_sendto+0x425/0x470
     __x64_sys_sendto+0xe5/0x1c0
     do_syscall_64+0x115/0x6a0
     entry_SYSCALL_64_after_hwframe+0x77/0x7f
    
    freed by task 533 on cpu 0 at 36.993097s (2.339769s ago):
     __rcu_free_sheaf_prepare+0x261/0x2a0
     rcu_free_sheaf+0x1f/0xe0
     rcu_core+0x661/0x1d10
     handle_softirqs+0x201/0x930
     __irq_exit_rcu+0x110/0x1e0
     irq_exit_rcu+0xe/0x20
     sysvec_apic_timer_interrupt+0x6c/0x80
     asm_sysvec_apic_timer_interrupt+0x1a/0x20
     srso_alias_return_thunk+0x0/0xfbef5
     __asan_memcpy+0x23/0x60
     create_monitor_ctrl_event+0x1a2/0x460
     mgmt_cmd_complete+0x2da/0x580
     load_long_term_keys+0x741/0x860
     hci_sock_sendmsg+0x128b/0x22e0
     __sys_sendto+0x425/0x470
     __x64_sys_sendto+0xe5/0x1c0
     do_syscall_64+0x115/0x6a0
     entry_SYSCALL_64_after_hwframe+0x77/0x7f
    
    CPU: 1 UID: 0 PID: 495 Comm: python3 Tainted: G           O        7.2.0-rc6-pmb-bt-functional-v1+ #1 PREEMPT(lazy) 
    Tainted: [O]=OOT_MODULE
    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
    ==================================================================

Fixes: 600a87490ff9 ("Bluetooth: Implementation of MGMT_OP_SET_BLOCKED_KEYS.")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
index 05d1ca25f4..90905dabf7 100644
--- a/net/bluetooth/hci_core.c
+++ b/net/bluetooth/hci_core.c
@@ -1153,16 +1153,16 @@ struct smp_ltk *hci_find_ltk(struct hci_dev *hdev, bdaddr_t *bdaddr,
 			continue;
 
 		if (smp_ltk_is_sc(k) || ltk_role(k->type) == role) {
-			rcu_read_unlock();
-
 			if (hci_is_blocked_key(hdev, HCI_BLOCKED_KEY_TYPE_LTK,
 					       k->val)) {
 				bt_dev_warn_ratelimited(hdev,
 							"LTK blocked for %pMR",
 							&k->bdaddr);
+				rcu_read_unlock();
 				return NULL;
 			}
 
+			rcu_read_unlock();
 			return k;
 		}
 	}