Re: [PATCH v4] Bluetooth: defer cleanup of resources in hci_unregister_dev()

syzbot is hitting might_sleep() warning at hci_sock_dev_event()
due to calling lock_sock() with rw spinlock held [1].

It seems that history of this locking problem is a trial and error.

Commit b40df5743ee8aed8 ("[PATCH] bluetooth: fix socket locking in
hci_sock_dev_event()") in 2.6.21-rc4 changed bh_lock_sock() to lock_sock()
as an attempt to fix lockdep warning.

Then, commit 4ce61d1c7a8ef4c1 ("[BLUETOOTH]: Fix locking in
hci_sock_dev_event().") in 2.6.22-rc2 changed lock_sock() to
local_bh_disable() + bh_lock_sock_nested() as an attempt to fix
sleep in atomic context warning.

Then, commit 4b5dd696f81b210c ("Bluetooth: Remove local_bh_disable() from
hci_sock.c") in 3.3-rc1 removed local_bh_disable().

Then, commit e305509e678b3a4a ("Bluetooth: use correct lock to prevent UAF
of hdev object") in 5.13-rc5 again changed bh_lock_sock_nested() to
lock_sock() as an attempt to fix CVE-2021-3573.

This difficulty comes from current implementation that
hci_sock_dev_event(HCI_DEV_UNREG) is responsible for dropping all
references from sockets because hci_unregister_dev() immediately reclaims
resources as soon as returning from hci_sock_dev_event(HCI_DEV_UNREG).
But the history suggests that hci_sock_dev_event(HCI_DEV_UNREG) was not
doing what it should do.

Therefore, instead of trying to detach sockets from device, let's accept
not detaching sockets from device at hci_sock_dev_event(HCI_DEV_UNREG),
by moving actual cleanup of resources from hci_unregister_dev() to
hci_cleanup_dev() which is called by bt_host_release() when all references
to this unregistered device (which is a kobject) are gone.

Since hci_sock_dev_event(HCI_DEV_UNREG) no longer resets hci_pi(sk)->hdev,
we need to check whether this device was unregistered and return an error
based on HCI_UNREGISTER flag. There might be subtle behavioral difference
in "monitor the hdev" functionality; please report if you found something
went wrong due to this patch.

Link: https://syzkaller.appspot.com/bug?extid=a5df189917e79d5e59c9 [1]
Reported-by: syzbot <redacted>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: e305509e678b3a4a ("Bluetooth: use correct lock to prevent UAF of hdev object")
---
v4: Don't touch "monitor the hdev" functions.

v3: Unexport hci_unregister_dev because only bt_host_release calls it
    and both hci_core.o and hci_sysfs.o are combined into bluetooth.o.

    Rename hci_release_dev to hci_cleanup_dev and call it only when
    HCI_UNREGISTER was set, for syzbot
    [off-list ref] is reporting
    that bt_host_release might be called without hci_register_dev and
    hci_unregister_dev.

    Return -EPIPE instead of 0 if HCI_UNREGISTER was set, in order to
    make sure that userspace finds that the device was unregistred.

v2: Add hci_release_dev and make bt_host_release call it instead of making
    bt_host_release public.

 include/net/bluetooth/hci_core.h |  1 +
 net/bluetooth/hci_core.c         | 16 +++++------
 net/bluetooth/hci_sock.c         | 49 +++++++++++++++++++++-----------
 net/bluetooth/hci_sysfs.c        |  3 ++
 4 files changed, 45 insertions(+), 24 deletions(-)

diff --git a/include/net/bluetooth/hci_core.h b/include/net/bluetooth/hci_core.h
index a53e94459ecd..db4312e44d47 100644
--- a/include/net/bluetooth/hci_core.h
+++ b/include/net/bluetooth/hci_core.h

@@ -1230,6 +1230,7 @@ struct hci_dev *hci_alloc_dev(void);
 void hci_free_dev(struct hci_dev *hdev);
 int hci_register_dev(struct hci_dev *hdev);
 void hci_unregister_dev(struct hci_dev *hdev);
+void hci_cleanup_dev(struct hci_dev *hdev);
 int hci_suspend_dev(struct hci_dev *hdev);
 int hci_resume_dev(struct hci_dev *hdev);
 int hci_reset_dev(struct hci_dev *hdev);

diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
index 2560ed2f144d..e1a545c8a69f 100644
--- a/net/bluetooth/hci_core.c
+++ b/net/bluetooth/hci_core.c

@@ -3996,14 +3996,10 @@ EXPORT_SYMBOL(hci_register_dev);
 /* Unregister HCI device */
 void hci_unregister_dev(struct hci_dev *hdev)
 {
-       int id;
-
        BT_DBG("%p name %s bus %d", hdev, hdev->name, hdev->bus);

        hci_dev_set_flag(hdev, HCI_UNREGISTER);

-       id = hdev->id;
-
        write_lock(&hci_dev_list_lock);
        list_del(&hdev->list);
        write_unlock(&hci_dev_list_lock);

@@ -4038,7 +4034,14 @@ void hci_unregister_dev(struct hci_dev *hdev)
        }

        device_del(&hdev->dev);
+       /* Actual cleanup is deferred until hci_cleanup_dev(). */
+       hci_dev_put(hdev);
+}
+EXPORT_SYMBOL(hci_unregister_dev);

+/* Cleanup HCI device */
+void hci_cleanup_dev(struct hci_dev *hdev)
+{
        debugfs_remove_recursive(hdev->debugfs);
        kfree_const(hdev->hw_info);
        kfree_const(hdev->fw_info);

@@ -4063,11 +4066,8 @@ void hci_unregister_dev(struct hci_dev *hdev)
        hci_blocked_keys_clear(hdev);
        hci_dev_unlock(hdev);

-       hci_dev_put(hdev);
-
-       ida_simple_remove(&hci_index_ida, id);
+       ida_simple_remove(&hci_index_ida, hdev->id);
 }
-EXPORT_SYMBOL(hci_unregister_dev);

 /* Suspend HCI device */
 int hci_suspend_dev(struct hci_dev *hdev)

diff --git a/net/bluetooth/hci_sock.c b/net/bluetooth/hci_sock.c
index b04a5a02ecf3..f1128c2134f0 100644
--- a/net/bluetooth/hci_sock.c
+++ b/net/bluetooth/hci_sock.c

@@ -59,6 +59,17 @@ struct hci_pinfo {
        char              comm[TASK_COMM_LEN];
 };

+static struct hci_dev *hci_hdev_from_sock(struct sock *sk)
+{
+       struct hci_dev *hdev = hci_pi(sk)->hdev;
+
+       if (!hdev)
+               return ERR_PTR(-EBADFD);
+       if (hci_dev_test_flag(hdev, HCI_UNREGISTER))
+               return ERR_PTR(-EPIPE);
+       return hdev;
+}
+
 void hci_sock_set_flag(struct sock *sk, int nr)
 {
        set_bit(nr, &hci_pi(sk)->flags);

@@ -759,19 +770,13 @@ void hci_sock_dev_event(struct hci_dev *hdev, int event)
        if (event == HCI_DEV_UNREG) {
                struct sock *sk;

-               /* Detach sockets from device */
+               /* Wake up sockets using this dead device */
                read_lock(&hci_sk_list.lock);
                sk_for_each(sk, &hci_sk_list.head) {
-                       lock_sock(sk);
                        if (hci_pi(sk)->hdev == hdev) {
-                               hci_pi(sk)->hdev = NULL;
                                sk->sk_err = EPIPE;
-                               sk->sk_state = BT_OPEN;
                                sk->sk_state_change(sk);
-
-                               hci_dev_put(hdev);
                        }
-                       release_sock(sk);
                }
                read_unlock(&hci_sk_list.lock);
        }

@@ -930,10 +935,10 @@ static int hci_sock_reject_list_del(struct hci_dev *hdev, void __user *arg)
 static int hci_sock_bound_ioctl(struct sock *sk, unsigned int cmd,
                                unsigned long arg)
 {
-       struct hci_dev *hdev = hci_pi(sk)->hdev;
+       struct hci_dev *hdev = hci_hdev_from_sock(sk);

-       if (!hdev)
-               return -EBADFD;
+       if (IS_ERR(hdev))
+               return PTR_ERR(hdev);

        if (hci_dev_test_flag(hdev, HCI_USER_CHANNEL))
                return -EBUSY;

@@ -1103,6 +1108,18 @@ static int hci_sock_bind(struct socket *sock, struct sockaddr *addr,

        lock_sock(sk);

+       /* Allow detaching from dead device and attaching to alive device, if
+        * the caller wants to re-bind (instead of close) this socket in
+        * response to hci_sock_dev_event(HCI_DEV_UNREG) notification.
+        */
+       hdev = hci_pi(sk)->hdev;
+       if (hdev && hci_dev_test_flag(hdev, HCI_UNREGISTER)) {
+               hci_pi(sk)->hdev = NULL;
+               sk->sk_state = BT_OPEN;
+               hci_dev_put(hdev);
+       }
+       hdev = NULL;
+
        if (sk->sk_state == BT_BOUND) {
                err = -EALREADY;
                goto done;

@@ -1379,9 +1396,9 @@ static int hci_sock_getname(struct socket *sock, struct sockaddr *addr,

        lock_sock(sk);

-       hdev = hci_pi(sk)->hdev;
-       if (!hdev) {
-               err = -EBADFD;
+       hdev = hci_hdev_from_sock(sk);
+       if (IS_ERR(hdev)) {
+               err = PTR_ERR(hdev);
                goto done;
        }

@@ -1743,9 +1760,9 @@ static int hci_sock_sendmsg(struct socket *sock, struct msghdr *msg,
                goto done;
        }

-       hdev = hci_pi(sk)->hdev;
-       if (!hdev) {
-               err = -EBADFD;
+       hdev = hci_hdev_from_sock(sk);
+       if (IS_ERR(hdev)) {
+               err = PTR_ERR(hdev);
                goto done;
        }

diff --git a/net/bluetooth/hci_sysfs.c b/net/bluetooth/hci_sysfs.c
index 9874844a95a9..b69d88b88d2e 100644
--- a/net/bluetooth/hci_sysfs.c
+++ b/net/bluetooth/hci_sysfs.c

@@ -83,6 +83,9 @@ void hci_conn_del_sysfs(struct hci_conn *conn)
 static void bt_host_release(struct device *dev)
 {
        struct hci_dev *hdev = to_hci_dev(dev);
+
+       if (hci_dev_test_flag(hdev, HCI_UNREGISTER))
+               hci_cleanup_dev(hdev);
        kfree(hdev);
        module_put(THIS_MODULE);
 }
--

2.18.4