Revert "tty: n_gsm: fix UAF in gsm_cleanup_mux"

This reverts commit 9b9c8195f3f0d74a826077fc1c01b9ee74907239.

The commit above is reverted as it did not solve the original issue.

gsm_cleanup_mux() tries to free up the virtual ttys by calling
gsm_dlci_release() for each available DLCI. There, dlci_put() is called to
decrease the reference counter for the DLCI via tty_port_put() which
finally calls gsm_dlci_free(). This already clears the pointer which is
being checked in gsm_cleanup_mux() before calling gsm_dlci_release().
Therefore, it is not necessary to clear this pointer in gsm_cleanup_mux()
as done in the reverted commit. The commit introduces a null pointer
dereference:
 <TASK>
 ? __die+0x1f/0x70
 ? page_fault_oops+0x156/0x420
 ? search_exception_tables+0x37/0x50
 ? fixup_exception+0x21/0x310
 ? exc_page_fault+0x69/0x150
 ? asm_exc_page_fault+0x26/0x30
 ? tty_port_put+0x19/0xa0
 gsmtty_cleanup+0x29/0x80 [n_gsm]
 release_one_tty+0x37/0xe0
 process_one_work+0x1e6/0x3e0
 worker_thread+0x4c/0x3d0
 ? __pfx_worker_thread+0x10/0x10
 kthread+0xe1/0x110
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x2f/0x50
 ? __pfx_kthread+0x10/0x10
 ret_from_fork_asm+0x1b/0x30
 </TASK>

The actual issue is that nothing guards dlci_put() from being called
multiple times while the tty driver was triggered but did not yet finished
calling gsm_dlci_free().

Fixes: 9b9c8195f3f0 ("tty: n_gsm: fix UAF in gsm_cleanup_mux")
Cc: stable <stable@kernel.org>
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230914051507.3240-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index b3550ff..1f3aba6 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3097,10 +3097,8 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
                gsm->has_devices = false;
        }
        for (i = NUM_DLCI - 1; i >= 0; i--)
                gsm->has_devices = false;
        }
        for (i = NUM_DLCI - 1; i >= 0; i--)

-               if (gsm->dlci[i]) {
+               if (gsm->dlci[i])

                        gsm_dlci_release(gsm->dlci[i]);
                        gsm_dlci_release(gsm->dlci[i]);

-                       gsm->dlci[i] = NULL;
-               }

        mutex_unlock(&gsm->mutex);
        /* Now wipe the queues */
        tty_ldisc_flush(gsm->tty);
        mutex_unlock(&gsm->mutex);
        /* Now wipe the queues */
        tty_ldisc_flush(gsm->tty);