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Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept

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rfcomm_sock_recvmsg() completes a deferred setup by calling
rfcomm_dlc_accept() without holding any RFCOMM lock:

	if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) {
		rfcomm_dlc_accept(d);
		return 0;
	}

and rfcomm_dlc_accept() dereferences the session on its first line:

	struct sock *sk = d->session->sock->sk;

Every other path that touches d->session runs under rfcomm_mutex:
rfcomm_dlc_open(), rfcomm_dlc_close(), rfcomm_dlc_exists(),
rfcomm_dlc_send_rpn(), and the RFCOMM thread through
rfcomm_process_sessions(). rfcomm_connect_ind() is even documented as
"called under rfcomm_lock()". This call site is the only one that skips
it.

The RFCOMM_DEFER_SETUP bit looks like it serialises the accept against
teardown, since __rfcomm_dlc_close() returns early when it wins the
test_and_clear. But rfcomm_recv_disc() forces the state first:

	d->state = BT_CLOSED;
	__rfcomm_dlc_close(d, err);

and the early return only covers BT_CONNECT, BT_CONFIG, BT_OPEN and
BT_CONNECT2. With the state already BT_CLOSED that switch does not
match, the bit is never consulted, and __rfcomm_dlc_close() falls
through to rfcomm_dlc_unlink(), which sets d->session = NULL.

So a remote DISC on a deferred dlc clears the session while leaving
RFCOMM_DEFER_SETUP set. The next recvmsg() then passes the
test_and_clear and dereferences a NULL session. No timing window is
needed: once the DISC has been processed, the dereference is
unconditional.

Give rfcomm_dlc_accept() the same shape as rfcomm_dlc_open() and
rfcomm_dlc_close(): an exported wrapper that takes rfcomm_mutex and
re-checks the session, around a __rfcomm_dlc_accept() that the two
in-core callers, which already hold the mutex, keep using.

Reproduced on a KASAN + PROVE_LOCKING kernel with a BR/EDR peer emulated
over /dev/vhci: the peer brings up an ACL link, opens L2CAP on the
RFCOMM PSM, starts a session, opens a dlc on a channel bound with
BT_DEFER_SETUP, and sends DISC after the socket is accepted. recv() on
the accepted socket then hits:

  Oops: general protection fault
  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:rfcomm_dlc_accept+0x54/0x350
  Call Trace:
    rfcomm_sock_recvmsg+0x1cd/0x230
    sock_recvmsg+0x166/0x1c0
    __sys_recvfrom+0x20d/0x300

0x10 is the offset of sock in struct rfcomm_session. With this patch the
same run completes with recv() returning 0 and no report, and lockdep
stays quiet, confirming rfcomm_mutex is still taken before lock_sock on
this path as it is on the thread side.

Fixes: bb23c0ab82 ("Bluetooth: Add support for deferring RFCOMM connection setup")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
This commit is contained in:
Ali Ahmet Memis
2026-08-07 02:03:44 +00:00
committed by Luiz Augusto von Dentz
parent 0079e1a944
commit 43a556b2fd
1 changed files with 21 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

24
net/bluetooth/rfcomm/core.c
View File

@@ -1331,7 +1331,10 @@ static struct rfcomm_session *rfcomm_recv_disc(struct rfcomm_session *s,
return s;
}
void rfcomm_dlc_accept(struct rfcomm_dlc *d)
/* Must be called with rfcomm_mutex held, so that the session cannot be
* unlinked from under us.
*/
static void __rfcomm_dlc_accept(struct rfcomm_dlc *d)
{
struct sock *sk = d->session->sock->sk;
struct l2cap_conn *conn = l2cap_pi(sk)->chan->conn;
@@ -1353,6 +1356,21 @@ void rfcomm_dlc_accept(struct rfcomm_dlc *d)
rfcomm_send_msc(d->session, 1, d->dlci, d->v24_sig);
}
void rfcomm_dlc_accept(struct rfcomm_dlc *d)
{
rfcomm_lock();
/* rfcomm_recv_disc() sets the dlc state to BT_CLOSED before calling
* __rfcomm_dlc_close(), so the RFCOMM_DEFER_SETUP handshake there is
* skipped and the session can already be unlinked by the time the
* deferred accept runs from rfcomm_sock_recvmsg().
*/
if (d->session)
__rfcomm_dlc_accept(d);
rfcomm_unlock();
}
static void rfcomm_check_accept(struct rfcomm_dlc *d)
{
if (rfcomm_check_security(d)) {
@@ -1365,7 +1383,7 @@ static void rfcomm_check_accept(struct rfcomm_dlc *d)
d->state_change(d, 0);
rfcomm_dlc_unlock(d);
} else
rfcomm_dlc_accept(d);
__rfcomm_dlc_accept(d);
} else {
set_bit(RFCOMM_AUTH_PENDING, &d->flags);
rfcomm_dlc_set_timer(d, RFCOMM_AUTH_TIMEOUT);
@@ -1958,7 +1976,7 @@ static void rfcomm_process_dlcs(struct rfcomm_session *s)
d->state_change(d, 0);
rfcomm_dlc_unlock(d);
} else
rfcomm_dlc_accept(d);
__rfcomm_dlc_accept(d);
}
continue;
} else if (test_and_clear_bit(RFCOMM_AUTH_REJECT, &d->flags)) {