[PATCH] Bluetooth: HIDP: guard session->conn in hidp_connection_del

[Michael Bommarito <michael.bommarito@gmail.com>]

commit dbf666e4fc9b ("Bluetooth: HIDP: Fix possible UAF") changed
hidp_session_remove() to drop the L2CAP reference and set
session->conn = NULL once the session is considered removed, and
added an if (session->conn) guard around the l2cap_unregister_user()
call at the kthread-exit site in hidp_session_thread().

The sibling call site in hidp_connection_del() still invokes
l2cap_unregister_user(session->conn, &session->user) unconditionally.
hidp_session_find() takes the session refcount under
down_read(&hidp_session_sem) and returns; between the find() and the
call at :1421, hidp_session_remove() can run on another thread
(driven by the remote peer disconnecting or local teardown), take
down_write(&hidp_session_sem), set session->conn to NULL, and return.
The HIDPCONNDEL ioctl path then dereferences a NULL l2cap_conn inside
l2cap_unregister_user(), which acquires conn->lock without a NULL
check.  Result: kernel NULL-pointer dereference.

Apply the same if (session->conn) guard used at the twin site.  No
functional change when session->conn is non-NULL.

Discovery and verification:

- Found via static audit of every session->conn read in hidp/core.c
  after the referenced commit landed.  The other reads are safe
  (creation-time in hidp_session_dev_init, already-guarded in
  session_free / hidp_session_thread / hidp_session_remove; the other
  hidp_session_find callers do not touch session->conn at all), so
  :1421 is the only remaining unguarded site.
- Runtime A/B confirmed in UML with CONFIG_BT_HIDP=y + CONFIG_KASAN=y:
  a late_initcall stub that injects a fake hidp_session with
  conn=NULL into hidp_session_list and invokes hidp_connection_del()
  panics on the pre-fix tree at __mutex_lock from
  l2cap_unregister_user+0x2d, and returns cleanly on the post-fix
  tree with the new guard short-circuiting before the deref.

Fixes: dbf666e4fc9b ("Bluetooth: HIDP: Fix possible UAF")
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-7
---
 net/bluetooth/hidp/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/hidp/core.c b/net/bluetooth/hidp/core.c
index 7bcf8c5ceaee..9192efd1b156 100644
--- a/net/bluetooth/hidp/core.c
+++ b/net/bluetooth/hidp/core.c
@@ -1417,7 +1417,7 @@ int hidp_connection_del(struct hidp_conndel_req *req)
 				       HIDP_TRANS_HID_CONTROL |
 				         HIDP_CTRL_VIRTUAL_CABLE_UNPLUG,
 				       NULL, 0);
-	else
+	else if (session->conn)
 		l2cap_unregister_user(session->conn, &session->user);
 
 	hidp_session_put(session);
-- 
2.53.0