Re: [PATCH] Bluetooth: fix use-after-free error in lock_sock_nested()

在 2021/7/15 5:50, Luiz Augusto von Dentz 写道:

Hi,

On Tue, Jul 13, 2021 at 8:20 PM Wang ShaoBo [off-list ref] wrote:

quoted

use-after-free error in lock_sock_nested() is reported:

[  179.140137][ T3731] =====================================================
[  179.142675][ T3731] BUG: KMSAN: use-after-free in lock_sock_nested+0x280/0x2c0
[  179.145494][ T3731] CPU: 4 PID: 3731 Comm: kworker/4:2 Not tainted 5.12.0-rc6+ #54
[  179.148432][ T3731] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
[  179.151806][ T3731] Workqueue: events l2cap_chan_timeout
[  179.152730][ T3731] Call Trace:
[  179.153301][ T3731]  dump_stack+0x24c/0x2e0
[  179.154063][ T3731]  kmsan_report+0xfb/0x1e0
[  179.154855][ T3731]  __msan_warning+0x5c/0xa0
[  179.155579][ T3731]  lock_sock_nested+0x280/0x2c0
[  179.156436][ T3731]  ? kmsan_get_metadata+0x116/0x180
[  179.157257][ T3731]  l2cap_sock_teardown_cb+0xb8/0x890
[  179.158154][ T3731]  ? __msan_metadata_ptr_for_load_8+0x10/0x20
[  179.159141][ T3731]  ? kmsan_get_metadata+0x116/0x180
[  179.159994][ T3731]  ? kmsan_get_shadow_origin_ptr+0x84/0xb0
[  179.160959][ T3731]  ? l2cap_sock_recv_cb+0x420/0x420
[  179.161834][ T3731]  l2cap_chan_del+0x3e1/0x1d50
[  179.162608][ T3731]  ? kmsan_get_metadata+0x116/0x180
[  179.163435][ T3731]  ? kmsan_get_shadow_origin_ptr+0x84/0xb0
[  179.164406][ T3731]  l2cap_chan_close+0xeea/0x1050
[  179.165189][ T3731]  ? kmsan_internal_unpoison_shadow+0x42/0x70
[  179.166180][ T3731]  l2cap_chan_timeout+0x1da/0x590
[  179.167066][ T3731]  ? __msan_metadata_ptr_for_load_8+0x10/0x20
[  179.168023][ T3731]  ? l2cap_chan_create+0x560/0x560
[  179.168818][ T3731]  process_one_work+0x121d/0x1ff0
[  179.169598][ T3731]  worker_thread+0x121b/0x2370
[  179.170346][ T3731]  kthread+0x4ef/0x610
[  179.171010][ T3731]  ? process_one_work+0x1ff0/0x1ff0
[  179.171828][ T3731]  ? kthread_blkcg+0x110/0x110
[  179.172587][ T3731]  ret_from_fork+0x1f/0x30
[  179.173348][ T3731]
[  179.173752][ T3731] Uninit was created at:
[  179.174409][ T3731]  kmsan_internal_poison_shadow+0x5c/0xf0
[  179.175373][ T3731]  kmsan_slab_free+0x76/0xc0
[  179.176060][ T3731]  kfree+0x3a5/0x1180
[  179.176664][ T3731]  __sk_destruct+0x8af/0xb80
[  179.177375][ T3731]  __sk_free+0x812/0x8c0
[  179.178032][ T3731]  sk_free+0x97/0x130
[  179.178686][ T3731]  l2cap_sock_release+0x3d5/0x4d0
[  179.179457][ T3731]  sock_close+0x150/0x450
[  179.180117][ T3731]  __fput+0x6bd/0xf00
[  179.180787][ T3731]  ____fput+0x37/0x40
[  179.181481][ T3731]  task_work_run+0x140/0x280
[  179.182219][ T3731]  do_exit+0xe51/0x3e60
[  179.182930][ T3731]  do_group_exit+0x20e/0x450
[  179.183656][ T3731]  get_signal+0x2dfb/0x38f0
[  179.184344][ T3731]  arch_do_signal_or_restart+0xaa/0xe10
[  179.185266][ T3731]  exit_to_user_mode_prepare+0x2d2/0x560
[  179.186136][ T3731]  syscall_exit_to_user_mode+0x35/0x60
[  179.186984][ T3731]  do_syscall_64+0xc5/0x140
[  179.187681][ T3731]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  179.188604][ T3731] =====================================================

In our case, there are two Thread A and B:

Context: Thread A:              Context: Thread B:

l2cap_chan_timeout()            __se_sys_shutdown()
   l2cap_chan_close()              l2cap_sock_shutdown()
     l2cap_chan_del()                l2cap_chan_close()
       l2cap_sock_teardown_cb()        l2cap_sock_teardown_cb()

Once l2cap_sock_teardown_cb() excuted, this sock will be marked as SOCK_ZAPPED,
and can be treated as killable in l2cap_sock_kill() if sock_orphan() has
excuted, at this time we close sock through sock_close() which end to call
l2cap_sock_kill() like Thread C:

Context: Thread C:

sock_close()
   l2cap_sock_release()
     sock_orphan()
     l2cap_sock_kill()  #free sock if refcnt is 1

If C completed, Once A or B reaches l2cap_sock_teardown_cb() again,
use-after-free happened.

We should set chan->data to NULL if sock is freed, for telling teardown
operation is not allowed in l2cap_sock_teardown_cb(), and also we should
avoid killing an already killed socket in l2cap_sock_close_cb().

Signed-off-by: Wang ShaoBo <redacted>
---
  net/bluetooth/l2cap_sock.c | 14 ++++++++++++--
  1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index c99d65ef13b1..ddc6a692b237 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c

@@ -1215,14 +1215,18 @@ static int l2cap_sock_recvmsg(struct socket *sock, struct msghdr *msg,
   */
  static void l2cap_sock_kill(struct sock *sk)
  {
+       struct l2cap_chan *chan;
+
         if (!sock_flag(sk, SOCK_ZAPPED) || sk->sk_socket)
                 return;

         BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));

         /* Kill poor orphan */
-
-       l2cap_chan_put(l2cap_pi(sk)->chan);
+       chan = l2cap_pi(sk)->chan;
+       l2cap_chan_put(chan);

There is a problem here, the above sentence `l2cap_chan_put(chan)` 
should put after

following sentence.

quoted

+       if (refcount_read(&sk->sk_refcnt) == 1)
+               chan->data = NULL;

Instead of checking if it is the last reference here, wouldn't it be
better to reset the chan->data to NULL on l2cap_sock_destruct?

Hi,

In my case it looks OK, this is the diff:

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index f1b1edd0b697..32ef3328ab49 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c

@@ -1500,6 +1500,9 @@ static void l2cap_sock_close_cb(struct l2cap_chan 

*chan)
  {
         struct sock *sk = chan->data;

+       if (!sk)
+               return;
+
         l2cap_sock_kill(sk);
  }

@@ -1508,6 +1511,9 @@ static void l2cap_sock_teardown_cb(struct 

l2cap_chan *chan, int err)
         struct sock *sk = chan->data;
         struct sock *parent;

+       if (!sk)
+               return;
+
         BT_DBG("chan %p state %s", chan, state_to_string(chan->state));

         /* This callback can be called both for server (BT_LISTEN)

@@ -1700,6 +1706,7 @@ static void l2cap_sock_destruct(struct sock *sk)
         BT_DBG("sk %p", sk);

         if (l2cap_pi(sk)->chan)
+              l2cap_pi(sk)->chan->data = NULL;
                  l2cap_chan_put(l2cap_pi(sk)->chan);

But if it has potential risk if l2cap_sock_destruct() can not be excuted 
in time ?

sk_free():

         if (refcount_dec_and_test(&sk->sk_wmem_alloc)) //is possible 
this condition false ?

               __sk_free(sk)   -> ... l2cap_sock_destruct()