[PATCH] Bluetooth: RFCOMM: serialize session teardown

[Chengfeng Ye]

rfcomm_kill_listener() walks session_list and deletes every session
without holding rfcomm_mutex.  A connect task holds that mutex while
rfcomm_session_create() adds a session and while its error path deletes
the session, but the unlocked teardown can still observe the object
between those operations.

The race can proceed as follows:

  connect task                    krfcommd
  ------------                    --------
  rfcomm_lock()
  rfcomm_session_add()
                                  fetch session from session_list
  kernel_connect() fails
  rfcomm_session_del()
    remove and free session
                                  rfcomm_session_del(session)

The final call then reads the freed session and may corrupt the list.

KASAN reported:

  BUG: KASAN: slab-use-after-free in rfcomm_session_del+0x15f/0x170
  Read of size 8 at addr ffff8881019e9b40 by task krfcommd/87
  Call Trace:
   rfcomm_session_del+0x15f/0x170
   rfcomm_run+0x16d5/0x3de0
   kthread+0x2c6/0x3b0
   ret_from_fork+0x36e/0x5a0
  Allocated by task 96:
   rfcomm_session_add+0x9e/0x300
   rfcomm_dlc_open+0x8b1/0xdf0
   rfcomm_sock_connect+0x34c/0x530
  Freed by task 96:
   kfree+0x131/0x3c0
   rfcomm_session_del+0x109/0x170
   rfcomm_dlc_open+0x9eb/0xdf0
   rfcomm_sock_connect+0x34c/0x530

Hold rfcomm_mutex across the teardown traversal, matching the locking
used by normal session processing and connect error cleanup.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
---
 net/bluetooth/rfcomm/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
index 9cdfea666a2c..5fe2758e8c47 100644
--- a/net/bluetooth/rfcomm/core.c
+++ b/net/bluetooth/rfcomm/core.c
@@ -2178,8 +2178,10 @@ static void rfcomm_kill_listener(void)
 
 	BT_DBG("");
 
+	rfcomm_lock();
 	list_for_each_entry_safe(s, n, &session_list, list)
 		rfcomm_session_del(s);
+	rfcomm_unlock();
 }
 
 static int rfcomm_run(void *unused)
-- 
2.43.0