[PATCH v2] Bluetooth: ISO: fix use-after-free of listener socket in iso_conn_ready

iso_conn_ready() looks up the BIS listener socket with iso_get_sock(),
which takes a reference, and then, without re-checking its state,
creates a child socket from it:

    parent = iso_get_sock(hdev, ...);
    if (!parent)
        return;

    lock_sock(parent);
    sk = iso_sock_alloc(sock_net(parent), NULL, BTPROTO_ISO, ...);
    ...
    iso_chan_add(conn, sk, parent);
    ...
    release_sock(parent);
    sock_put(parent);

If the listener socket is closed concurrently, between iso_get_sock()
and lock_sock(), the reference taken by iso_get_sock() may be the last
one: the close path drops the link-list reference, and once
iso_conn_ready() drops its own reference at the end of the function the
socket is freed.  The child socket, however, is already linked to the
freed parent, and a later disconnect of the child runs iso_chan_del()
-> bt_accept_unlink(), which dereferences the dangling parent pointer
into the freed accept queue (a use-after-free).  The same dangling
pointer is also dereferenced through parent->***() in
iso_chan_del().

Fix it the same way the connected (non-BIS) path was fixed in commit
0d255e63fcf3 ("Bluetooth: ISO: hold sk properly in iso_conn_ready"):
after taking the socket lock, re-check that the parent is still a
listening, alive socket, and bail out otherwise.

Fixes: ccf74f2390d60 ("Bluetooth: Add BTPROTO_ISO socket type")
Cc: [email protected]

Changes in v2:
- Fix GitLint B3: replace hard tabs with spaces in the commit message
  code snippet (no functional change)

Signed-off-by: Hang Nan <[email protected]>
---
 net/bluetooth/iso.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/net/bluetooth/iso.c b/net/bluetooth/iso.c
index aa2ce78f56a2..069fc87a4e18 100644
--- a/net/bluetooth/iso.c
+++ b/net/bluetooth/iso.c
@@ -2277,6 +2277,21 @@ static void iso_conn_ready(struct iso_conn *conn)
 
 		lock_sock(parent);
 
+		/* The listener socket may have been closed concurrently
+		 * between iso_get_sock() and lock_sock(): the reference
+		 * taken by iso_get_sock() may be the last one, in which
+		 * case the socket is freed as soon as we drop it at the
+		 * end of this function.  Recheck that the parent is still
+		 * a valid, listening socket before creating a child
+		 * socket from it.
+		 */
+		if (parent->sk_state != BT_LISTEN ||
+		    sock_flag(parent, SOCK_ZAPPED)) {
+			release_sock(parent);
+			sock_put(parent);
+			return;
+		}
+
 		sk = iso_sock_alloc(sock_net(parent), NULL,
 				    BTPROTO_ISO, GFP_ATOMIC, 0);
 		if (!sk) {