[PATCH v2] Bluetooth: RFCOMM: serialize session teardown

rfcomm_kill_listener() walks session_list and deletes every session
without holding rfcomm_mutex, unlike the normal session processing and
connect error paths.

Under normal operation, an open RFCOMM socket pins rfcomm.ko, so
rfcomm_kill_listener() does not run concurrently with rfcomm_dlc_open().
However, forced module unload via delete_module(O_TRUNC) can stop
krfcommd while a failed connect is still unwinding.

  connect task                    forced unload / krfcommd
  ------------                    ------------------------
  rfcomm_lock()
  rfcomm_session_add()
                                  delete_module("rfcomm", O_TRUNC)
                                  rfcomm_kill_listener()
                                    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 with mdelay() to enlarge critical window:

  BUG: KASAN: slab-use-after-free in rfcomm_run+0x3802/0x3f00 [rfcomm]
  Read of size 8 at addr ffff888111058d40 by task krfcommd/79
  Tainted: [R]=FORCED_RMMOD
  Allocated by task 86:
   rfcomm_session_add+0xa1/0x300 [rfcomm]
   rfcomm_dlc_open+0x8b2/0xf30 [rfcomm]
   rfcomm_sock_connect+0x34c/0x530 [rfcomm]
  Freed by task 86:
   kfree+0x121/0x3c0
   rfcomm_dlc_open+0xab7/0xf30 [rfcomm]
   rfcomm_sock_connect+0x34c/0x530 [rfcomm]

Hold rfcomm_mutex across the teardown traversal so every reachable
session_list walk uses the same serialization.

Reviewed-by: Ali Ahmet Memis <[email protected]>
Tested-by: Ali Ahmet Memis <[email protected]>
Reviewed-by: Pauli Virtanen <[email protected]>
Signed-off-by: Chengfeng Ye <[email protected]>
---
Changes in v2:
- Rewrite the changelog to describe make clear trigger: forced module
  unload with delete_module(O_TRUNC) and temporary delay widening.
- Drop the Fixes tag.
- Drop Cc: stable.
- Add Ali Ahmet Memis's Reviewed-by and Tested-by tags.
- Add Pauli Virtanen's Reviewed-by tag.

Link: https://lkml.iu.edu/2608.2/10891.html [v1]

 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